Mouna Chambon
email: ✉mouna.chambon@gmail.com

Edmond Roger
email: ✉rogeredmond1@yahoo.fr

Miadama Faramalala

Joro Rakotoarinivo

Solofo Rakotondraompiana
email: ✉srakotondraompiana@gmail.com

Camélias Rasoamanantenaniaina
email: rasoamanantenaniaina@gmail.com

Tony Arison

Bernard Riera
email: ✉bernard.riera@mnhn.fr

Estimating the Carbon Sequestration Ecosystem Services of the Savanna: the Case of the Bismarck Palm Bismarckia Nobilis Hildebr. &H.Wendl. in the Protected Area of Antrema, Madagascar

Keywords: Arecaceae, climate mitigation, conservation biology, nature-based solutions, participatory approach, sustainability, tropical climate

Sustainable management of natural resources has become a critical issue in the context of climate change. Although savannas represent a significant carbon sink, they have long been neglected in carbon inventories. Here, we addressed the potential win-wins of biodiversity conservation and climate change mitigation in savanna through a particular focus on the endemic palm species Bismarckia nobilis Hildebr. &H.Wendl. in the New Protected Area (NPA) of Antrema, North West of Madagascar. The aim of the present study was to assess the sustainability of the current management of Bismarckia nobilis populations in the NPA by drawing up a prospective scenario and to determine the middle-term potential carbon sink strength of the savanna. We collected ecological data in six survey sites. Then, we quantified the carbon sequestration levels in Bismarckia nobilis by using allometric relationships and spatial analysis. The data allowed us to build a model of change in Bismarckia nobilis population dynamics for the next 50 years. Our findings demonstrated the importance of carbon storage capacity of the species, with 90,000 tonnes of carbon currently stored in the total surface area of the savanna. Furthermore, we found that anthropogenic activities do not have a negative impact on the population size of the species in the NPA and rather, contribute to its expansion through regulated burnings. These results indicate the role of the savanna of the NPA as an important carbon sink in the middle term.

La gestion durable des ressources naturelles est devenue un enjeu primordial dans le contexte du changement climatique. Bien que les savanes représentent un puit de carbone important, elles ont longtemps été négligées dans les inventaires de carbone. Ici, nous avons abordé la question des possibles co-bénéfices de la conservation de la biodiversité et de l'atténuation aux changements climatiques dans la savane au travers du cas particulier de l'espèce de palmier endémique, Bismarckia nobilis Hildebr. & H.Wendl., dans la Nouvelle Aire Protégée (NAP) d'Antrema au nord-ouest de Madagascar. L'objectif de la présente étude était d'évaluer la durabilité de la gestion actuelle des populations de Bismarckia nobilis dans la NAP et d'estimer le potentiel de la savane en tant que puit de carbone à moyen terme. Dans le cadre d'une mission de terrain, nous avons quantifié les niveaux de séquestration de carbone du palmier Bismarckia nobilis sur la base de relations allométriques et d'une analyse spatiale. Ces données nous ont permis de construire un modèle d'évolution de la dynamique des populations de Bismarckia nobilis pour les 50 prochaines années. Nos résultats montrent que l'espèce Bismarckia nobilis présente une capacité de stockage de carbone importante, estimée à 90 000 tonnes pour la surface totale de la savane à Antrema. En outre, notre modèle d'évolution prévoit une augmentation des populations de Bismarckia nobilis pour les 50 prochaines années. Les résultats de cette étude mettent en avant l'importance de la savane d'Antrema en matière de séquestration de carbone à moyen terme. In tropical areas, deforestation and forest degradation have reached alarming rates and cause substantial damages to biodiversity and its related resources (Food and Agriculture Organisation [FAO], 2016) (Table S1). Stopping deforestation is not only essential for biodiversity conservation purpose, but also for climatic reasons given that the decrease in forest cover has a major impact on ecosystem services such as carbon storage [START_REF] Olander | Reference scenarios for deforestation and forest degradation in support of REDD: a review of data and methods[END_REF]. The Paris Agreement, which was adopted at the 21 st Conference of the Parties (COP21) in 2015, reaffirmed the world's commitment to tackling climate change. The process of quantifying carbon sequestration in terrestrial ecosystems has, therefore, emerged as a new tool to developing climate mitigation strategies (Vieilledent et al., 2012). Constituting two major crises of the contemporary epoch, also referred as the "anthropocene" [START_REF] Crutzen | Geology of mankind -The Anthropocene[END_REF], both climate change and biodiversity erosion are causing systematic changes all over the world. As a result, a synergic and integrated approach to tackling these two crises would achieve mutual co-benefits (Millenium Ecosystem Assessment [MEA], 2005).

Mots clés:

Significance of savannas in the global carbon budget

Although savannas are acting as an important carbon sink, there is a gap in the literature on their role in climate change mitigation [START_REF] Grace | Productivity and carbon fluxes of tropical savannas[END_REF][START_REF] Razanajatovo | Evaluation de stock de carbone dans la savane arborée à Bismarckia nobilis hildebrandt & h.wendl. de la Nouvelle Aire Protégée Antrema. (Master's thesis in tropical ecology -BEVT)[END_REF]. Broadly speaking, savannas are defined as "any tropical ecosystem containing grasses, including woodland and grassland types" [START_REF] Eiten | The cerrado vegetation of Brazil[END_REF], which encompasses a large range of savanna types. This biome covers one-sixth of the land surface, including 50% of the African continent [START_REF] Campbell | The miombo in transition, woodlands and welfare in Africa[END_REF].

Due to its large area of occupancy and combined with the frequency of burnings, savannas have a direct impact on the global carbon budget. It is estimated that up to 8 GtC of CO 2 is emitted annually from savanna burning (Seiler [START_REF] Seiler | Estimates of gross and net fluxes of carbon between the biosphere and the atmosphere from biomass burning[END_REF]. Furthermore, land-use change, through grazing and crop cultivation, contributes to increasing fire events and climate change is likely to increase the frequency of burning in the coming years [START_REF] Grace | Productivity and carbon fluxes of tropical savannas[END_REF]. Therefore, managing savannas has emerged as a crucial concern for tackling climate change.

1.1.3 Participatory management of natural resources in Madagascar: case of Antrema NPA In Madagascar, savannas cover a substantial part of the total land surface, about 70% [START_REF] Razanajatovo | Evaluation de stock de carbone dans la savane arborée à Bismarckia nobilis hildebrandt & h.wendl. de la Nouvelle Aire Protégée Antrema. (Master's thesis in tropical ecology -BEVT)[END_REF]. Madagascar has one of the greatest endemism rates in the world with notably 90% endemic vascular plants [START_REF] Goodman | The Natural History of Madagascar[END_REF]. However, this unique biodiversity is becoming more and more threatened [START_REF] Blanc-Pamard | Madagascar: les enjeux environnementaux[END_REF]. In [AFD], 2015). Among these new protected areas, the NPA of Antrema is one of the first community protected area in Madagascar, which means that it is completely managed by a bottom-up logic [START_REF] Pioch | La forêt chez les Sakalava de la Station Forestière à Usages Multiples d'Antrema (Nord-Ouest de Madagascar): Etat des lieux et transformations des usages et des représentations (Master's thesis in ethno-ecology)[END_REF]. In this NPA, woody savanna is dominated by the palm species Bismarckia nobilis Hildebr. & H.Wendl. [START_REF] Dransfield | The palms of Madagascar. Royal Botanic Gardens; First Edition[END_REF][START_REF] Gauthier | Pré-projet pilote Bio culturel d'Antrema Aranta[END_REF]. Palm species have been reported to be particularly relevant for assessing the resilience of ecosystems at a species level in tropical America [START_REF] Montufar | Disturbance and Resilience in tropical American palm populations and communities[END_REF] (Table S2).

Here, we apply a similar approach by assessing the resilience of the savanna of the NPA and its climatic implications through the particular case of B. nobilis.

Objectives

The present study aims to assess the sustainability of current management of B. nobilis populations in the NPA of Antrema by drawing up a prospective scenario for the next 50 years and therefore conclude the middle-term potential carbon sink strength of the savanna ecosystem.

The specific objectives of this study were to: (a) quantify the carbon sequestration levels of B. nobilis in the savanna of the NPA; (b) determine the key factors that affect the growth of the studied species and estimate their respective value.

Study site

The NPA of Antrema is located on the north-western coast of Madagascar (Figure S1). It covers 20, 620 ha of which 1, 000 ha is of the marine park. Throughout the year, temperatures remain high, with an average annual temperature of around 26°C. The annual average precipitation is quite high, with a value of 1, 498 mm per year. The station is characterised by two very distinct seasons: a warm and wet season (November to April) and a dry and fresh season (May to October). The NPA of Antrema is part of the "Western Region" of Madagascar [START_REF] Humbert | Les territoires phytogéographiques de Madagascar: leur cartographie[END_REF].

It is included in the Western eco-floristic zone from 0 to 800 m altitude characterised by a climatic vegetation [START_REF] Pioch | La forêt chez les Sakalava de la Station Forestière à Usages Multiples d'Antrema (Nord-Ouest de Madagascar): Etat des lieux et transformations des usages et des représentations (Master's thesis in ethno-ecology)[END_REF].

METHODS

Study species

Madagascar is famous for its remarkably diverse palm flora with 171 species of palms distributed in 16 genera and representative of 4 out of the 6 recognised subfamilies in the world [START_REF] Dransfield | Genera Palmarum: the evolution and classification of palms[END_REF]. The majority of palm species in Madagascar occur in the rainforests of the Eastern and Northern part of the Island. However, there are four palms species, such as B. nobilis, that have adapted to secondary habitats like savannas [START_REF] Dransfield | The biogeography of Madagascar Palms[END_REF]. B. nobilis is a monotypic palm species, one of the three representatives of the subtribe Hyphaeninae of the Arecaceae family [START_REF] Dransfield | Field guide to the palms of Madagascar[END_REF]. It is a solitary, robust palm tree that is endemic to the Red Island. It is one of the most common palm trees in the north and west of Madagascar and occurs as the dominant tree species on savannas. Although it is widespread on the west coast of the country, little is known about its life-history [START_REF] Dransfield | The biogeography of Madagascar Palms[END_REF]. B. nobilis has the ability to survive fire, which explains its success in invading such anthropic habitats over time [START_REF] Koechlin | Flore et végétation de Madagascar[END_REF].

Fieldwork in the NPA of Antrema

During one month in the NPA of Antrema, we conducted an ecological survey in order to determine key environmental factors of the populations of B. nobilis in the area. The choice of the survey sites was defined based on field prospecting. A total of six survey sites were selected (Table 1). The Braun-Blanquet method was applied in each of the six survey sites [START_REF] Braun-Blanquet | Pflanzensociologie: Grundzuge der Vegetationskund[END_REF]. In each site, two surveys were conducted, totalling 12 survey plots. Each survey plot respected three criteria: floristic homogeneity, structural and physiognomic homogeneity and uniformity of the ecological conditions. Each plot consisted of a 20 m × 50 m rectangle set perpendicular to sea level. They were then subdivided by fluorescent flags into ten parcels of 10 m × 10 m. The delineation of the plots and their subdivisions was done with string. Within each plot, we recorded the following environmental factors for all B. nobilis individuals of a height greater than 10 cm: Diameter at Breast Height (DBH) with Breast Height set at 130 cm, maximum height, phenological state and number of leaves. The DBH was measured along the stipe using tape measure. We estimated the height based on trigonometric methods using an Abney's level [START_REF] Philip | Measuring trees and forests[END_REF]. Finally, we classified each recorded individual according to its development state (Table S3). The geographical location of each plot was obtained by using a GPS and the topography with a clinometer

Analysis of data

Spatial analysis

In this study, we used a moderate spatial resolution image (15 m) from the Landsat 8 satellite (launched February 11, 2013) from the U.S Geological Survey. This image was acquired over the NPA of Antrema in March 2017 and comprised 11 spectral bands in total. We pre-processed the satellite image (virtual raster building; colour calibration; polygon clipping; area delimitation) and then applied a supervised classification involving seven Region of Interests (ROIs). All analysis was performed on QGIS software (version 2.14.3-Essen). The classification output enabled us to conduct further analysis based on the estimation of the surface area of the class of savanna with B. nobilis.

Quantifying carbon sequestration in Bismarckia nobilis

For each survey site, we quantified the above-ground carbon of B. nobilis using allometric relationships. Two equations were explored in order to first determine the above-ground biomass (AGB) of B. nobilis (Table S4). Equation 1 is an equation proper to the study species [START_REF] Razanajatovo | Evaluation de stock de carbone dans la savane arborée à Bismarckia nobilis hildebrandt & h.wendl. de la Nouvelle Aire Protégée Antrema. (Master's thesis in tropical ecology -BEVT)[END_REF], whereas equation 2 is employed as a general equation for the Sabal genus [START_REF] Delaney | Carbon-Offset Report for the Noel Kempff Climate Action[END_REF][START_REF] Brown | Carbon monitoring, analysis, and status report for the Rio Bravo[END_REF]. These two equations express the AGB as a function of the maximal height of individuals. Comparison of results based on these two equations is expected to confirm the relevance of using specific allometric equations. In this study, we did not take account the herbaceous biomass because we assumed that changes in carbon pools are greater in palms than herbaceous species [START_REF] Pellegrini | Trade-offs between savanna woody plant diversity and carbon storage in the Brazilian Cerrado[END_REF]. Finally, above-ground carbon per site was deduced from the AGB values, assuming that biomass is 50% carbon [START_REF] Chave | Tree allometry and improved estimation of carbon stocks and balance in tropical forests[END_REF]. Total carbon stocks per site were obtained by summing up the values of carbon stocks of the three following compartments: aboveground biomass, roots, and litter. Owing to time and logistic constraints, we could solely measure above-ground biomass, so we used estimations from previous studies for roots (1.9 -6 t/ha) and litter (7.13 -4 t/ha) [START_REF] Razanajatovo | Evaluation de stock de carbone dans la savane arborée à Bismarckia nobilis hildebrandt & h.wendl. de la Nouvelle Aire Protégée Antrema. (Master's thesis in tropical ecology -BEVT)[END_REF]. Finally, the overall potential for carbon sequestration of B. nobilis in the NPA of Antrema was calculated on the basis of the previous spatial analysis by multiplying the total surface area of the savanna with the average carbon stock of the species per hectare.

RESULTS

Carbon stocks by site

The average carbon stocks by survey site according to the two allometric equation types are summarised in Table 3. These results show that the average values are similar in the two cases.

Results based on the general equation for the Sabal genus show slightly higher values (X= 13.6

+/-7 SD t/ha) than those based on the equation proper to the species B. nobilis (X=12.215 +/-6 SD t/ha). But both cases indicate the same general distribution of carbon stocks between sites.

The highest average value is observed in North-West of Sahariaka Lake with 21.3 t/ha (Equation 1) and 24.4 t/ha (Equation 2) (Figure 1). By contrast, the sites of South-east and South -west Antrema are characterised by the lowest average values: respectively ranging from 4.5 t/ha to 4.71 t/ha and from 6.39 t/ha to 6.89 t/ha.

Carbon storage in the palm savanna

Results from the spatial analysis are displayed on the map of the NPA below (Figure 2), which has been classified in seven categories. For the purpose of this study, we focused on the class "Savanna and palm" only, without considerations to the other classes which might be a matter of further discussions. The total surface area of the palm savanna is valued at 7,417.4 ha out of the 20, 620 ha of the NPA, which represents 35.9 % of the total area. Based on this calculation and taking account the species density per survey site (Table S5), we estimated the total population size of B. nobilis in the palm savanna of the NPA 4 million individuals, as showed in Table 4.

Finally, the total value of carbon stored in B. nobilis in the savanna of the NPA is higher according to the equation for the Sabal genus than if we use the equation proper to the target species: respectively 100, 877 and 90, 604 t of carbon.

A prospective study on the sustainable dimension of the exploitation of

DISCUSSION

Potential carbon sink strength of the savanna with B. nobilis in the NPA

In this study, we applied two allometric equations for estimating the carbon sequestration potential of the savanna in the NPA. In the two cases, our results show similar values. We preferentially chose to base our interpretations on the first equation, since it is for B. nobilis specifically and reflects a higher accuracy. In this study, we estimated the value of total carbon stocks of B. nobilis in the savanna at 12.215 t/ha and demonstrated the variability in terms of carbon stocks among the six survey sites. Carbon stocks in North-West Sahariaka Lake are the highest, whereas the lowest carbon stocks are found in South-east and South-west Antrema.

These differences may be related to palm density in each site since there is a direct link between biomass accumulation and carbon stocks [START_REF] Nizami | Evaluation of forest structure, biomass and carbon sequestration in subtropical pristine forests of SW China[END_REF]. Regarding the outcome of our spatial analysis, we estimated the total surface area of the savanna to be 7,417.4 ha, which represents 35.9 % of the NPA. These findings are congruent with a previous research, which estimated a value of 35. 78 % of the NPA [START_REF] Razanajatovo | Evaluation de stock de carbone dans la savane arborée à Bismarckia nobilis hildebrandt & h.wendl. de la Nouvelle Aire Protégée Antrema. (Master's thesis in tropical ecology -BEVT)[END_REF]. In complementing field measurements, spatial analysis turned out to be of a great utility for estimating carbon sequestration levels in B. nobilis within the NPA. A better outcome could have been produced

with higher resolution satellite imagery, but this methodological choice would have exceeded our budget. As reported in several academic studies, satellite data now has a great potential for identifying co-benefits between biodiversity conservation and carbon storage [START_REF] Olander | Reference scenarios for deforestation and forest degradation in support of REDD: a review of data and methods[END_REF][START_REF] Hansen | High-resolution global maps of 21st-century forest cover change[END_REF][START_REF] Pfeifer | Mapping the structure of Borneo's tropical forests across a degradation gradient[END_REF]. According to similar studies that were carried out on other vegetal formation, B. nobilis savanna shows an intermediate carbon storage capacity, lower than pristine forest, which is of utmost importance in terms of carbon sequestration, but much higher than herbaceous savanna (Table S6). Although falling in the same order of magnitude, our findings are slightly lower than that of a previous research that was conducted in the savanna of the NPA as well [START_REF] Razanajatovo | Evaluation de stock de carbone dans la savane arborée à Bismarckia nobilis hildebrandt & h.wendl. de la Nouvelle Aire Protégée Antrema. (Master's thesis in tropical ecology -BEVT)[END_REF]. The fact that we did not include the carbon stocks of herbaceous species in the savanna, but focused on the B. nobilis species only, might explain this contrast. Moreover, our calculations do not include the carbon amount that is stored in palm leaves and the leaves contain about 31.7% of the total biomass in the case of the palm species Attalea speciosa Mart. [START_REF] Gehring | Allometry of the babassu palm growing on slash-and-burn agroecosystem of the eastern periphery of Amazonia[END_REF]. By taking account the leaf biomass of B. nobilis, we could therefore have increased its carbon storage capacity.

Overall, the species B. nobilis shows a great carbon storage capacity since its total carbon stock in the savanna is about 90,604 t without considering the leaf biomass. Comparing to previous studies that were conducted in the NPA, this storage capacity is lower than that of the dry dense forests but represents double of that of the mangrove formation (Table 5). Therefore, the abundance of B. nobilis in the savanna of the NPA makes it a critical carbon sink in the long term. Although savannas are not a priority ecosystem in REDD+ mechanisms, this study falls within a substantial academic corpus that shows their potential in terms of carbon storage [START_REF] Grace | Productivity and carbon fluxes of tropical savannas[END_REF]Pelligrini et al., 2016). According to [START_REF] Scurlock | The global carbon sink, a grassland perspective[END_REF], the global carbon sink strength of savannas might even be valued up to 0.5Gt. Besides biomass accumulation, the carbon storage capacity of savannas is influenced by the frequency of fires and grazing worldwide. Therefore, protecting savannas from anthropogenic pressures would considerably increase their carbon sink potential [START_REF] Grace | Productivity and carbon fluxes of tropical savannas[END_REF].

Middle-term availability of the studied natural resource

In this study, we tested different estimations of mortality rates of B. nobilis related to fire, roofing, and handicrafts activities as well as its natural mortality and regeneration rate.

According to these estimations, we came up with three prospective scenarios on the availability of the resource over the next 50 years. Scenario n°1 points to a rapid and substantial decrease in the population size of B. nobilis. This result reflects that the natural regeneration rate is too low to catch up with mortality rate related to anthropogenic activities. By contrast, scenario n°3

shows an extensive increase in the population size. In this situation, the related natural regeneration rate is two times higher than in scenario n°1 and exceeds the losses due to anthropogenic activities. Finally, scenario n°2 is an intermediate scenario that excludes fire events but considers a mortality rate from roofing activities higher than the two other scenarios, whereas the natural regeneration rate is lowered to 1% only. As a result of this disequilibrium, the scenario shows a significant decrease of the population size. Natural regeneration and anthropogenic activities, which include fire and local uses, seem to act as two driving forces on the population dynamics of B. nobilis species. Based on evidence from the field assessment and local perceptions, we conclude that scenario n°3 is the most appropriate scenario to describe the current situation within the NPA. On the middle-term, the overall population of Antrema is not under a risk of depletion but will rather experience an extension phase. Anthropogenic activities do not have a negative impact on the population size and rather contribute to its expansion through fire that stimulates seed germination. Given the substantial carbon storage capacity of B.

nobilis, as stated previously, we can cautiously predict an increase in the carbon sink potential of the savanna of the NPA over the next 50 years. These findings contrast with two previous studies that were carried out in other parts of Madagascar (Table S7). Their conclusions indicate a severe depletion of B. nobilis populations, either on the short-term [START_REF] Ratoavimbahoaka | Etude biologique et analyse de l'exploitation abusive du palmier endémique "satrabe" (Bismarckia nobilis) dans la commune rurale de Betanatanana-Maintirano en vue d'une conservation pérenne[END_REF] or the middle-term [START_REF] Rabefarihy | Valorisation économique de Bismarckia nobilis dans le Menabe central (Master's thesis in vegetal biology[END_REF]. These assessments differ from that of our study because the two studied areas are not part of any protection scheme like in the case of Antrema, which is a category VI Protected Area according to the IUCN classification [START_REF] Pioch | La forêt chez les Sakalava de la Station Forestière à Usages Multiples d'Antrema (Nord-Ouest de Madagascar): Etat des lieux et transformations des usages et des représentations (Master's thesis in ethno-ecology)[END_REF]. Moreover, the cultural usages that characterise these regions are associated with a more intensive exploitation of the species (Table S7). These two alarming scenarios do stress the necessity to continue monitoring the population dynamics of B. nobilis because some threats are common to all regions, and therefore likely to challenge populations in Antrema in the long-term.

CONCLUSION

This research contributed to highlight the significance of savannas in carbon sequestration. In this case study, we found that the average value of carbon that is stored in B. nobilis in the savanna of Antrema is 12.215 t/ha, based on an allometric equation proper to the species. Given that the total surface area of the savanna was estimated at 7, 417 ha, which represents 35.9% of the NPA, we estimated an amount of 90, 604 t of carbon stored in B. nobilis in the study area, without considering its leaf biomass. Although lower than the carbon sequestration levels in the dry dense forests, this value represents double of that of the mangrove formation in the NPA [START_REF] Rakotovao | Essai d'évaluation du stockage de carbone des mangroves de la station forestière d'Antrema, Katsepy, Région Boeny-Madagascar[END_REF]. Furthermore, B. nobilis turned out to be a good indicator of the resilience of the savanna ecosystem at a species level. Scenario n°3 was chosen as the most appropriate scenario for describing changes in the palm population over the next 50 years. Natural regeneration and anthropogenic activities, which include fire and local uses, seem to act as two driving forces on population dynamics of the B. nobilis species. Anthropogenic activities do not have a negative impact on the overall population but, rather, contribute to its expansion through fire that stimulates seed germination. In the middle term the resource is not under risk of depletion but will rather experience an extension phase. Therefore, our findings highlight the mutual co-benefits of sustainable use of natural resources and climate change mitigation.

TABLES

  Approche participative, Arecaceae, atténuation du changement climatique, biologie de la conservation, climat tropical, durabilité, solutions fondées sur la nature 1. Potential win-wins of biodiversity conservation and carbon storage incentives

  2003, at the Durban World Parks Congress in Durban, the Government of Madagascar made the commitment of tripling the surface area of its protected areas and launching a new Protected Area System (SAPM), which covers both the extension of the network of existing Protected Areas and the creation of New Protected Areas (NPAs) (Agence Française de Développement

  2.3.3 Prospective model of Bismarckia nobilis population dynamics in the NPAWe created a prospective model of the population dynamics of B. nobilis in the NPA using a linear regression. First, we compiled data on the excel software (version 15.2) and subsequently applied the equation below (Equation3). We considered the six following factors: the current population size of B. nobilis in the NPA (Pn); the annual mortality rates of the species related to roofing (m1), handicraft activities (m2), and fire (m3); the natural mortality (m4) and regeneration (r) rate of the species. The current population size of the species was estimated by multiplying the average palm density per hectare (564 indiv /ha) with the total surface area of the savanna. In addition, we used results from a complementary study on the local uses of B.nobilis in the NPA, which estimates that 87, 333 palm individuals are harvested per year for roofing and 700,000 palms for handicrafts. The reiteration period covers 50 years. The final model comprises three prospective scenarios that differ in the estimation of the mortality rates as given in Table2. Equation 3: Y = Pn -(87,333×m1 + 700,000×m2 + Pn×m3 + Pn×m4) + Pn× r Where: Pn is the population size of B. nobilis at t 0 Y is the population size of B. nobilis at t+1 (Pn+1)

  The comparison of the three scenarios are illustrated on Figure3. None of these three scenarios indicate a total depletion of populations over the next 50 years. However, scenarios n°1 and n°2 show a significant degradation of the resource, whereas scenario n°3 rather suggests an increase of populations. Scenario n°1 points to a rapid and substantial decrease in the population size of B. nobilis. It indicates a drop of 60% of populations by 2067: from 4 million to 1.6 million of individuals. Although less critical, scenario n°2 also indicates a drop of the population size of 22% by 2067. By contrast, scenario n°3 shows an extensive increase in the population size. From 4 million, we reach almost 6 millions of individuals in 2067.

FIGURE LEGENDS Figure 1 .

 LEGENDS1 FIGURE LEGENDS

Figure 2 .

 2 Figure 2. Classified map of the New Protected Area of Antrema. We used a satellite image from

Figure 3 .

 3 Figure 3. Changes in the population size of Bismarckia nobilis in the NPA over the next 50

Table 1 .

 1 Location of the six survey sites in Antrema NPA with their GPS coordinates

	Location of the site	GPS coordinates
	North-west of Sahariaka Lake	Plot 1: 15° 44,658' S, 046° 07,492' E
		Plot 2: 15° 44,698' S, 046° 07,312' E
	West Kapahazo	Plot 3: 15° 45,675' S, 046° 06,634' E
		Plot 4: 15° 45,696' S, 046° 06,666' E
	Ankokoala	Plot 5: 15° 45, 974' S, 046° 09, 504' E
		Plot 6: 15° 45, 917' S, 046° 09, 538' E
	Maskoamena	Plot 7: 15° 45, 623' S, 046° 10, 621' E
		Plot 8 :15° 45, 606' S, 046° 10, 656' E
	South-east Antrema	Plot 9: 15° 43, 400' S, 046° 10, 350' E
		Plot 10:15° 43, 442' S, 046° 10, 341' E
	South-west Antrema	Plot 11:15° 43, 126' S, 046° 09, 738' E
		Plot 12:15° 43, 099' S, 046° 09, 813' E

Table 2 .

 2 Mortality and regeneration rates (%) applied to the species Bismarckia nobilis according to the three scenarios of this prospective study

	Rates (%)	Scenario n°1	Scenario n°2	Scenario n°3
	Mortality related to roofing (m1) 2	5	2
	Mortality related to harvest (m2)	1	2	1
	Mortality related to fire (m3)	2	0	2
	Natural mortality (m4)	1	1	1
	Regeneration (r)	2	1	4

Table 3 .

 3 Biomass and carbon stocks (t/ha) by survey site according to the two allometric equations. Equation 1 (AGB = 12.72H + 12.90) is proper to Bismarckia nobilis[START_REF] Razanajatovo | Evaluation de stock de carbone dans la savane arborée à Bismarckia nobilis hildebrandt & h.wendl. de la Nouvelle Aire Protégée Antrema. (Master's thesis in tropical ecology -BEVT)[END_REF] and Equation2(AGB = 24.553 + 4.921H + 1.017H 2 ) is generally used for the Sabal genus[START_REF] Delaney | Carbon-Offset Report for the Noel Kempff Climate Action[END_REF][START_REF] Brown | Carbon monitoring, analysis, and status report for the Rio Bravo[END_REF]

	Survey site	Biomass	Carbon stocks	Biomass	Carbon stocks
		Equation 1	Equation 1	Equation 2	Equation 2
		(t/ha)	(t/ha)	(t/ha)	(t/ha)
	North-west of	42.6	21.3	48.8	24.4
	Sahariaka Lake
	West Kapahazo	26.2	13.1	30.8	15.4
	Ankokoala	29.8	14.9	33.2	16.6
	Maskoamena	26.2	13.1	27.4	13.7
	South-east	9	4.5	9.42	4.71
	Antrema
	South-west	12.78	6.39	13.78	6.89
	Antrema
	Mean	24.430 (+/-6 SD) 12.215 (+/-6 SD) 27.2(+/-7SD) 13.6 (+/-7SD)

Table 4 .

 4 Population size of Bismarckia. nobilis in the NPA of Antrema and related carbon stocks for the total surface area of the savanna, calculated based on the classification output resulting from the spatial analysis

	Average density in	Surface area of the	Total population	Total carbon
	B.nobilis	palm savanna (ha)	size in the NPA	stocks of B.nobilis
	(average number of		(number of	in the palm
	individuals/ha)		individuals)	savanna (t)
	564	7,417.4	4,183,413 ~ 4	90,604	100,877
			million	(Eq.1)	(Eq.2)

Table 5 .

 5 Comparison of carbon stock estimates in different vegetal formations of the NPA

	Reference	Rasoamanantenaniaina (2016) Chambon (2017) Rakotovao (2013)
	Vegetal	Dry dense forest	Savanna with	Mangroves
	formation		B.nobilis
	Surface area (ha) 4,379	7,417.4	1,764.46
	Carbon stock	54.6	12.215	5.471
	(t/ha)
	Total carbon	115,182	90,604	9,653
	stock (t)

ACKNOWLEDGEMENTS

I would first like to thank my thesis supervisor Dr. Bernard Riéra of the Adaptive Mechanisms and Evolution department (UMR 7179) at the National Museum of Natural History of Paris. His help was precious throughout this research project. I would also like to thank my thesis cosupervisor Dr. Edmond Roger and Mrs. Amelie Faramalala of the Department of Ecology and Vegetal Biology at the University of Antananarivo. They were deeply involved during my time in Madagascar and my fieldwork could not have been successfully conducted without their inputs. I would also like to acknowledge Dr. Farid Dahdouh-Guebas of the Systems Ecology and Resource Management Department at ULB and Dr. Claude Marcel Hladik of the Man and Environment department (UMR 7206) at the National Museum of Natural History of Paris, as my two examiners. I am also gratefully indebted to the experts who have guided my work analysis with patience and strong support: Dr. Flora Pennec of the UMR 7206 at the National Museum of Natural History of Paris and Anoumou Kemavo, GIS and Forest Expert Project Officer at the National Agency of Forests (ONF). In addition, I would never have been able to conduct my fieldwork and collect my data without the support and guidance of my assistant Camelias Rasoamanantenaniaina. Many thanks to all people I met in Madagascar and who participated to my overall understanding of the region and research context.


Seyed Hamidreza Sadeghi
email: sadeghi@modares.ac.ir

Mahboobeh Kiani-Harchegani
email: mahboobeh.kiyani20@gmail.com

Zeinab Hazbavi
email: z.hazbavi@uma.ac.ir

Padidehsadat Sadeghi
email: pssadeghi1371@gmail.com

Rafael Angulo-Jaramillo

Laurent Lassabatere
email: laurent.lassabatere@entpe.fr

Habibollah Younesi
email: hunesi@modares.ac.ir

Field Measurement of Effects of Individual and Combined Application of Biochar and Polyacrylamide on Erosion Variables in Loess and Marl Soils

Keywords: Natural condition, Soil conservation, Soil amendment, Splash erosion

Controlling soil erosion as one of the most important threats to soil quality and quantity, particularly in its initial stages, is greatly important in natural resources management.

However, performance evaluation of various soil and water conservation techniques under real circumstances are being rarely conducted. Consequently, the present research intended to control soil erosion caused by splash and interrill erosion in two soils (marl at Marzan-Abad and loess at Maraveh-Tapeh sites) susceptible to erosion by using various additives and under field conditions. We established 0.5 m × 0.5 m plots in the field and used the sugarcane byproduct biochar (BC), polyacrylamide (PAM) and BC+PAM additives together with control plots in three replications under a slope steepness of 25% at the two mentioned sites. We used a rainfall simulator to produce rainfall intensity of 50 mm h -1 in the experiments. Analysis of the results obtained from the variables of splash and interrill erosion during the rainfall-runoff process showed that the PAM additive significantly (p≤ 0.05) increased all study variables of splash erosion. However, in interrill erosion, it reduced the variables of soil loss and sediment concentration but not significantly (p>0.05) compared to the control plot and increased runoff compared to the control plots at the two mentioned sites. The plot treated with BC showed decreased runoff volume and coefficient and soil loss compared to the control plot at the Marzan-Abad site, but the differences were not significant statistically (p>0.05). However, the plot treated with BC in the loess soil at the Maraveh-Tapeh site considerably (p≤ 0.05) increased runoff and soil loss compared to the control plot. Finally, runoff volume and coefficient and soil loss increased in the loam and loess soils in the plots treated with BC+PAM compared to the control plot.

Introduction

Water and soil loss in different ecosystems, mainly in sloping agricultural lands, are major environmental threats that lead to instability and unsustainability of the ecosystems, particularly in terms of land degradation and reduced fertility and productivity. Soil erosion is accelerated as a result of increased runoff on soil surface that results in soil vulnerability with respect to reduced permeability, crusting, and slacking [START_REF] Liu | Land use and climate changes and their impacts on runoff in the Yarlung Zangbo river basin, China[END_REF][START_REF] Gholami | Soil and water conservation using biochar and various soil moisture in laboratory conditions[END_REF]. Approximately 60% of the total runoff and sediment loss that happen in various ecosystems of watersheds are caused by natural and anthropogenic factors such as rainfall intensity, geomorphology and soil management practices [START_REF] Keesstra | Effects of soil management techniques on soil water erosion in apricot orchards[END_REF][START_REF] Zhao | Evidence and causes of spatiotemporal changes in runoff and sediment yield on the Chinese Loess Plateau[END_REF][START_REF] Wang | Effects of wheat stubble on runoff, infiltration, and erosion of farmland on the Loess Plateau, China, subjected to simulated rainfall[END_REF]. Therefore, soil conservation and erosion control are among the most important measures that need special attention in all countries. Various methods have been introduced to control soil erosion, but use of methods that can protect soil from the first impacts of erosion agents is a top priority in soil erosion control programs [START_REF] Blanco-Sepúlveda | An erosion control and soil conservation method for agrarian uses based on determining the erosion threshold[END_REF].

One of the strategies introduced in recent decades is the use of soil amendments. These are substances used to enhance soil productivity and improve soil structure quality and its biochemical performance [START_REF] Fangueiro | Strategies for soil protection and remediation[END_REF][START_REF] Maiti | Ecological restoration of coal mine degraded lands: topsoil management, pedogenesis, carbon sequestration, and Mine Pit Limnology[END_REF]. Organic and inorganic amendments have been studied for various purposes over the past several years, particularly with emphasis on reducing runoff and soil loss, increasing permeability and improving physical and chemical properties of soils with various properties. In this regard, use of different types of biomass such as Biochar (BC), which is produced from industrial and rural wastes and waste produced in agriculture and animal husbandry, forests and rangelands, fishing and aquaculture, and their optimal management and reproduction as eco-friendly amendments have received greater attention from researchers [START_REF] Sadeghi | Reducing sediment concentration and soil loss using organic and inorganic amendments at plot scale[END_REF]Sadeghi et al., 2016b;[START_REF] Głąb | Effect of biochar application on soil hydrological properties and physical quality of sandy soil[END_REF][START_REF] Wang | Effects of wheat stubble on runoff, infiltration, and erosion of farmland on the Loess Plateau, China, subjected to simulated rainfall[END_REF][START_REF] Yue | Slow pyrolysis as a measure for rapidly treating cow manure and the biochar characteristics[END_REF][START_REF] Sadeghi | Ameliorating some quality properties of an erosion-prone soil using biochar produced from dairy wastewater sludge[END_REF]Zhang et al., 2019;[START_REF] Khadem | Response of soil alkaline phosphatase to biochar amendments: Changes in kinetic and thermodynamic characteristics[END_REF][START_REF] Gholami | Soil and water conservation using biochar and various soil moisture in laboratory conditions[END_REF]. BC, carbon or pyrogenic carbon, is now widely used as a soil amendment due to its beneficial effects on soil quality [START_REF] Wang | Modeling simple experiments of biochar erosion from soil[END_REF]. BC is the product of thermal-chemical conversion of biomass in a pyrolysis reactor [START_REF] Verheijen | The influence of biochar particle size and concentration on bulk density and maximum water holding capacity of sandy vs. sandy loam soil in a column experiment[END_REF]. It has many benefits from the perspective of agriculture and natural resources that have been mentioned by various researchers. These benefits include improved soil water retention capacity [START_REF] Laird | Impact of biochar amendments on the quality of a typical Midwestern agricultural soil[END_REF], provision of plant nutrients and enhancement of plant growth (Graber et al., 2010), improvement of soil physical and chemical properties (Zhang et al., 2019) and reduced runoff and soil loss (Sadeghi et al., 2016b;[START_REF] Sadeghi | Ameliorating some quality properties of an erosion-prone soil using biochar produced from dairy wastewater sludge[END_REF][START_REF] Gholami | Soil and water conservation using biochar and various soil moisture in laboratory conditions[END_REF]. However, some researchers including [START_REF] Cheng | Ageing of black carbon along a temperature gradient[END_REF], [START_REF] Rumpel | Erosion budget and process selectivity of black carbon at meter scale[END_REF], [START_REF] Major | Fate of soil-applied black carbon: downward migration, leaching and soil respiration[END_REF], [START_REF] Nguyen | Temperature sensitivity of black carbon decomposition and oxidation[END_REF]Sadeghi et al. (2016b) have pointed at BC loss along with sediments down slopes during the rainfall-runoff process caused by factors such as lack of adhesion between particles. Some other soil amendments such as high molecular weight polymers, which are formed by covalent bonds between monomers, stabilize soil aggregates in the long term, particularly in soils susceptible to erosion such as marl soils [START_REF] Hamidi Nehrani | Effect of polyvinyl acetate on hydraulic conductivity, runoff and sediment production in a Marl Soil[END_REF] and thereby play an effective role in reducing soil erosion through binding soil particles together. Among these polymers is anionic-Polyacrylamide (PAM) that binds fine soil particles together thus reducing their floating duration, increases their deposition rate. Such an additive may strengthen the bounds between particles, meaning a higher stability of soil aggregate, a positive effect on soil structure and consequently on its capability to infiltrate water (Hamidi Sojka et al., 2005;[START_REF] Hamidi Nehrani | Effect of polyvinyl acetate on hydraulic conductivity, runoff and sediment production in a Marl Soil[END_REF][START_REF] Karimi | Variations of runoff generation during rainfall events with different levels of powder and liquid application of polyacrylamide[END_REF][START_REF] Sarkar | Agriculture: Polymers in Crop Production Mulch and Fertilizer[END_REF][START_REF] Biju | Biopolymer-Modified Soil: Prospects of a Promising Green Technology[END_REF]. Therefore, considering the research background, the present study investigated use of BC alone and in combination with PAM to reduce runoff and soil loss during the rainfall-runoff process. In this regard, effects of these amendments on splash and interril erosion control and on different variables such as upward-splash, downward-splash, total-splash, runoff, time of runoff generation, runoff coefficient, and soil loss and sediment concentration of runoff water were studied in two types of erosion-susceptible soil (loess and marl) found in the northern regions of Iran under natural conditions. The whole study has been conducted under field conditions, which is obviously differentiated from many existing literatures, as a crucial criterion to properly assess the performance of soil and water conservation techniques leading to propose efficient and effective measures. The work was formulated for two erosion prone soils located on Marl and Loess formations in two distant localities in North Iran.

Materials and methods

Test materials

In this research, plots 0.5 × 0.5-m 2 were installed in the two study regions of Marzan-Abad (in west of Mazandaran Province) with marl formation and Maraveh-Tapeh (in north of Golestan Province) with loess soil whose characteristics have been presented in Table 1 (Sadeghi et al., 2013;[START_REF] Karimi | Variations of runoff generation during rainfall events with different levels of powder and liquid application of polyacrylamide[END_REF]Sadeghi et al., 2017a). BC produced from sugarcane industries and anion PAM amendments with the properties listed in Table 2 were used in the experiments.

Rainfall and soil erosion simulation

In these experiments, well water with pH 7.27, electrical conductivity 5.13 mS m -1 , dissolved oxygen 30.7%, phosphorus phosphate 0.17 mg l -1 and nitrate 4.41 mg l -1 was used to simulate rainfall [START_REF] Kiani-Harchegani | Comparative analysis of the effects of rainfall intensity and experimental plot slope on Raindrop Impact Induced Erosion (RIIE)[END_REF]2019). In addition, the mean slope steepness and rainfall intensity were chosen in accordance with the general conditions in the native area of the soils and taking into account rainfalls with a return period of 25-30 years. Therefore, a 25% slope was selected for the experiments to conform to standard agricultural lands and the general and prevailing conditions in rain-fed agricultural lands and natural resources areas in Iran. Rainfall simulation experiments were also performed under soil moisture conditions similar to the general conditions prevailing in the two mentioned areas prior to rainfall and by measuring volumetric water content in those areas and in the laboratory using wet and dry sample weighing and maintaining relatively similar conditions in the predicted experiments.

Therefore, the experiments were run at rainfall intensity of about 50 ± 5 mm h -1 lasting for 30 min according to the Intensity, Duration, and Frequency (IDF) graphs prepared for the two sites. All rainfall simulations were performed on small plots with both length and width of 0.5 m in three replications (Sadeghi et al., 2016b) in the two regions of Marzan-Abad (in western Mazandaran Province) and Maraveh-Tapeh (in northern Golestan Province). Some views of field experiments have been shown in Fig. 2.

Experimental design for field inventory

In the present study, BC (800 gm -2 ) and PAM (2 gm -2 ) were used in individual and combination in treated plots with three replications. These levels were selected based on successful performance of the same rates in previous researches [START_REF] Sadeghi | Controllability of runoff and soil loss from small plots treated by vinasse-produced biochar[END_REF]Sadeghi et al. 2017a;[START_REF] Sadeghi | Ameliorating some quality properties of an erosion-prone soil using biochar produced from dairy wastewater sludge[END_REF]. BC was spread whilst PAM was just sprayed on the soil surface, individually. PAM was also sprayed on BC when they were concurrently used. Splash cups were installed in the plot to measure splash erosion (Fig. 2). The splashed sediment samples were collected from upward and downward parts of the splash cups during each run from the beginning of the rainfall [START_REF] Kiani-Harchegani | Comparative analysis of the effects of rainfall intensity and experimental plot slope on Raindrop Impact Induced Erosion (RIIE)[END_REF]Sadeghi et al. 2017b).

The time the first signs of runoff were observed at the outlet in the control and treated plots was recorded as the early stages of interrill erosion, runoff generation and combined impacts of rainfall and water flow on erosion using a stopwatch [START_REF] Kiani-Harchegani | Effect of rainfall intensity and slope on sediment particle size distribution during erosion using partial eta squared[END_REF]. The volume of runoff was then measured at two min intervals for six minutes, at three min intervals for nine minutes and at five min intervals for 15 minutes in three replications.

Data analysis

After measuring the different variables caused by splash and interrill erosions in the control plots and the plots treated by BC, PAM and combination of BC and PAM, the data from the experiments were classified in Excel and their database was created. Before any statistical analysis, data normality was checked using the Kolmogorov-Smirnov test. Levene's test of variance homogeneity was then used to evaluate homogeneity of variance in the different treatments. The paired samples t-test used to compare pairwise BC and PAM on different variables of splash and interrill erosion. In addition, one-way ANOVA and Tukey's HSD test at a probability level (P) < 0.05 was used to compare the means of the different variables in two sites of Marzan-Abad and Maraveh-Tapeh that are affected by combined and individual application BC and PAM [START_REF] Awad | Effects of polyacrylamide, biopolymer, and biochar on decomposition of soil organic matter and plant residues as determined by 14C and enzyme activities[END_REF][START_REF] Kiani-Harchegani | Comparative analysis of the effects of rainfall intensity and experimental plot slope on Raindrop Impact Induced Erosion (RIIE)[END_REF][START_REF] Wang | Effects of wheat stubble on runoff, infiltration, and erosion of farmland on the Loess Plateau, China, subjected to simulated rainfall[END_REF].

Results and Discussion

The present research studied effects of individual and combined application of BC and PAM on the various variables of splash and interrill erosion under field conditions for the two studied soils.

Differences in splash erosion variables between treatments for Loess and Marl soils

The variables of upward-splash and downward-splash erosion in the control plots and plots treated with BC, PAM, and BC+PAM at the Marzan-Abad and Maraveh-Tapeh sites were measured using splash cups at the start of rainfall (Sadeghi et al., 2017b). In this relation, the total-splash variable was the sum of the downward-splash and upward-splash variables.

The paired sample t-test compared the splash erosion variables at the Marzan-Abad and Maraveh-Tapeh sites. Fig. 3 presents the mean and standard deviation values of the splash erosion variables and the results of the test. These results show that the values of the variables at the Marzan-Abad site in the plots treated with BC and BC+PAM decreased significantly compared to the control plot (p ≤ 0.05). However, they increased significantly in the plot treated with PAM (p ≤ 0.05) compared to the control plot and the plots treated with BC and BC+PAM (p ≤ 0.05). Fig. 3 shows that the upward-splash, downward-splash and total splash erosion in the plot treated with PAM had the largest difference with those in the control plot and the plots treated with BC and BC +PAM at the Maraveh-Tapeh site. These results do not conform to those reported by [START_REF] Kavian | Polyacrylamide effects on splash erosion rate in different soils using rainfall simulator[END_REF] concerning reduced splash erosion in soils with various textures under the influence of PAM probably because of the more complicated natural conditions in the processes of rainfall-runoff compared to the laboratory conditions. It is remarkable that for the two soils, similar trends are obtained with a negative effect of PAM alone, resulting in an increase in splash.

Differences in splash erosion between Loess and Marl soils

To compare the variables resulting from splash erosion including upward-splash, downwardsplash and total splash erosion in the control plots and the treated plots at Marzan-Abad and Maraveh-Tapeh sites, one-way ANOVA was used the results of which are presented in Table 3. These results demonstrate that upward-splash and total-splash rates in the control plot and in the plot treated with BC+PAM were significantly different (p ≤ 0.05). The average values of upward-splash and total-splash erosion at the Marzan-Abad site were higher than the control plot and the plot treated with BC+PAM at the Maraveh-Tapeh site. However, their average rates in the plots treated with BC and PAM were not significantly different (p > 0.05) at the Marzan-Abad and Maraveh-Tapeh sites. In the same relation, the rates of downwardsplash erosion in the control plot and the treated plots at the Marzan-Abad and Maraveh-Tapeh sites were not significantly different (p> 0.05).

Table 3

Differences in interrill erosion variables between treatments for Loess and Marl soils

After observing the first runoff drops in the runoff collection system of the plot, we measured interrill erosion. A chronometer recorded the time to runoff generation in the control and treated plots. Fig. 4 presents the mean and standard deviation values of the time to runoff generation in the control and treated plots at the Marzan-Abad and Maraveh-Tapeh sites. Figs. 5 and6 shows the box plots of the various interrill erosion variables in each rainfall-runoff process for the control plots and the various treated plots at the Maraveh-Tapeh and Marzan-Abad sites. The results did not indicate any significant difference between the different treatments at these two sites (p > 0.05). However, at the Marzan-Abad site, the time to runoff generation was longer in the control plot than in the treated plots. This reduction in the time to runoff generation was probably due to the physical adhesion of the soil surface, the salts and the nature of the material that protected the soil mass (Hillet, 2010). These factors caused the soil surface pores to fill up sooner, the treated plot reached saturation earlier and the time to runoff generation arrived sooner compared to the control plot [START_REF] Sharifi Moghaddam | Small plot soil hydrologic components as affected by application of Vinasse organic residue[END_REF]. However, at the Maraveh-Tapeh site, the time to runoff generation was longer in the treated plots than in the control plot. This showed the protective materials were more effective in loess soils in which the presence of a greater quantity of clay particles having larger specific surface and greater ability to attach to the particles of the protective materials increased their permeability and decreased their erosivity compared to the marl soils [START_REF] Jafari Honar | The study on the variation of some physical indices of the loess soils quality along climatic transect in Golestan Province[END_REF]. These results show the contrasting sensitivity of soils to such treatment for the improvement of infiltration capability.

Table 4 lists the results related to the statistical analysis concerning pairwise comparison between treatments of interrill erosion variables including the time to runoff generation, runoff volume, pH, soil loss, and sediment concentration in runoff water using the paired sample t-test at the Marzan-Abad and Maraveh-Tapeh sites. In this relation, the results in Table 4 also demonstrate that there were no significant differences in the pairwise comparison of the time to runoff generation between the control and treated plots at the two sites (p > 0.05).

The results in Table 4 also suggest that there were no significant differences between the control and the treated plots in runoff volume and coefficient at the Marzan-Abad site.

However, the pairwise comparison of the pH and EC (Electrical Conductivity) values showed that the control and the treated plots differed significantly in pH and EC values (Table 4 and Figs. 5a andb). Moreover, at the Marzan-Abad site, the variables of soil loss and sediment concentration were significantly different only in the pairwise comparison of the plots treated with PAM and with BC+PAM (p ≤ 0.02). The pairwise comparison of the control plot and the other treated plots did not show any significant differences with respect to the values of the mentioned variables (p ≥ 0.05).

At the Maraveh-Tapeh site, the interrill erosion variables including runoff volume, runoff coefficient, pH, soil loss and sediment concentrations exhibited behaviors that were more complicated during the rainfall-runoff process. Fig. 4 demonstrates that there were significant differences between the control plot and the plots treated with BC, PAM and BC + PAM in runoff volume and coefficient (p ≤ 0.02). However, the various treated plots did not differ significantly with each other (p ≥ 0.1). The results shown in Figs. 5(c) and 5(d) also confirm these results. Soil aggregates adsorbed the additives sprayed on the soil surface in this research and thus they became more stable and more adhesive. This increase was much more evident in the loess soils of Maraveh-Tapeh than in the marl soils of Marzan-Abad because of the higher clay content and the larger specific surface of the particles in the loess soils [START_REF] Jafari Honar | The study on the variation of some physical indices of the loess soils quality along climatic transect in Golestan Province[END_REF]. Therefore, the additives were more effective in creating the hydrophobic layer during the rainfall-runoff and hence the runoff volume and coefficient were higher than the control plot at the Maraveh-Tapeh site. 4 also indicate that there were significant differences between the control plot and the treated plots at the Marzan-Abad site in the variables of pH, soil loss and sediment concentration. However, the control plot and the plot treated with PAM at this site did not differ significantly in EC values, soil loss and sediment concentration (p ≥ 0.16).

Results in Table

Differences in interrill erosion between Loess and Marl soils

Variables of interrill erosion including time to runoff generation, runoff volume, runoff coefficient, pH, soil loss and sediment concentration between the control and treated plots at the Marzan-Abad site with those at the Maraveh-Tapeh site compared by one-way ANOVA.

Table 5 lists the results, which shows there were no significant differences between the control plot and the treated plots at the two sites in the time to run off generation and soil loss (p ≥ 0.05). However, in general, the variables of runoff volume, runoff coefficient, pH, and sediment concentration differed significantly at the 95% level. Of course, there were no significant differences between the control plot and the plots treated with BC, BC+PAM in EC values and the measured runoff volume at the 2 sites.

As shown by the box plots in Figures 5 and6, we can see the intergroup and intragroup behavioral changes at the two sites for the various interrill erosion variables. Fig. 5(a BC with its high pH value is very probably able to cause the development of the mineral phases of hydroxides, phosphates and carbonates that lead to aggregate coalescence, particularly in the long term [START_REF] Czimczik | Controls on black carbon storage in soils[END_REF][START_REF] Omondi | Quantification of biochar effects on soil hydrological properties using meta-analysis of literature data[END_REF]. each other. This could result from the different nature and intrinsic structure and texture of loess and marl soils caused by their interplay with hydrological processes [START_REF] Glaser | Ameliorating physical and chemical properties of highly weathered soils in the tropics with charcoal-a review[END_REF].

Therefore, we must consider this behavioral difference in introducing protection and management strategies. Finally, Figs. 6(a) and 6(b) revealed that the plots treated with PAM at the Marzan-Abad and Maraveh-Tapeh sites decreased soil loss and sediment concentration but increased runoff volume compared to the control plots (Fig. 5c). These results conform to those reported by many researchers including [START_REF] Yu | Infiltration and erosion in soils treated with dry PAM and gypsum[END_REF], [START_REF] Lentz | Long-term polyacrylamide formulation effects on soil erosion, water infiltration, and yields of furrow-irrigated crops[END_REF], [START_REF] Hamidi Nehrani | Effect of polyvinyl acetate on hydraulic conductivity, runoff and sediment production in a Marl Soil[END_REF], Sadeghi et al. (2013) and [START_REF] Karimi | Variations of runoff generation during rainfall events with different levels of powder and liquid application of polyacrylamide[END_REF]. In this relation, the additive BC at the Marzan-Abad site decreased runoff volume and coefficient and reduced soil loss in the treated plot compared to the control plot. These results agree with the laboratory results that Sadeghi et al. (2017a) found in protecting marl soil using BC produced from dairy factory waste. They also are in agreement with results reported by [START_REF] Gholami | Soil and water conservation using biochar and various soil moisture in laboratory conditions[END_REF] that showed runoff volume and soil loss decreased in soils treated with BC. In general, BC decreases soil bulk density, increases soil aggregate stability and porosity and is more effective in coarse-textured soils that in soft-textured ones [START_REF] Omondi | Quantification of biochar effects on soil hydrological properties using meta-analysis of literature data[END_REF]. These effects increase soil permeability and reduce runoff. However, in loess soils treated with BC at Maraveh-Tapeh site soil loss increased compared to the control plot. These results conform to those in the study by Zhang et al. (2016) who reported that soil loss increased during interrill erosion in loess soils. Finally, results in Figs. 5(c) and 5(d) also indicate that runoff volume and coefficient increased in plots treated with BC+PAM compared to the control plots in loam and loess soils at Marzan-Abad and Maraveh-Tapeh sites, respectively. Moreover, Figs. 6(a) and 6(b) also demonstrated that soil loss and sediment concentration increased in plots treated with BC+PAM compared to the control plots at both sites. Therefore, we should not recommend these two additives in combination for reducing runoff and sediment during the interrill erosion process in erodible soils such as marl and loess.

Table 5 4. Conclusion Remarks

The present research intended to protect two types of soil (marl and loess soils) susceptible to splash and interrill erosion at the Marzan-Abad and Maraveh-Tapeh sites during the simulated rainfall-runoff process in small plots by using the BC, PAM and BC+PAM additives. In general, the results suggested that the loess and marl soils behaved similarly during splash erosion. The largest amount of soil loss that happened due to upward-splash, downwardsplash and total-splash variables was recorded in the plot treated with PAM compared to the control plot, and the quantities of soil loss caused by splash erosion in plots treated with BC and BC+PAM were lower than those in the control plots at the two sites. At the Marzan-Abad site, this reduction in soil loss was not significant compared to the control.

In interrill erosion, there were no significant differences between the treated and control plots in the time to runoff generation at the two sites. However, the time to runoff generation was shorter in the treated plots compared to the control plot at the Marzan-Abad site but longer at the Maraveh-Tappeh site. In the interrill erosion process, soil loss and sediment concentrations decreased in the plots treated with PAM compared to the control plots at the Marzan-Abad and Maraveh-Tapeh sites but runoff volume increased. However, runoff volume and coefficient and also soil loss decreased in the plot treated with BC compared to the control plot at the Marzan-Abad site, whereas soil loss increased in the plot treated with BC compared to the control plot in the loess soil at the Maraveh-Tapeh site. Finally, runoff volume and coefficient increased in the plots treated with BC+PAM compared with the control plots in the loam and loess soils at the Marzan-Abad and Maraveh-Tapeh sites, respectively. The addition PAM appears to be the best solution for the two soil regarding the limitation of soil erosion. However, the effect on the soil capability to infiltrate water and reduce runoff is mostly ensured by BC, mostly for loess soils. Differences in the hydrological behavior in the loam and loess soils treated with these protective materials indicated that, in order to use any of the additives, we must consider the purpose of soil conservation during each stage of the soil erosion process and then suggest management actions and apply them.

In conclusion, we believe that study of experiments conducted on various temporal and spatial scales and under natural rainfall (by taking into account the environmental and economic goals) enables us to attain a more comprehensive summation in this relation and introduce strategies that are more practical. Additional perspective insists on coupling of physicochemical processes (hydrophobicity) and geochemical processes (mineral precipitation, dissolution, etc.). A granular anionic PAM having a mean molecular mass of 10-15million gmol -1 was dissolved in deionized water and aged (25 °C, dark) for one week to obtain a uniform solution. The standard solutions (100mg l -1 ) was then used for the study (Sadeghi et al., 2016a).

Liquid PAM Table 5. Statically significant differences among interrill erosion results between the Marzan-Abad and Maraveh-Tapeh sites (one-way ANOVA).

Fig. 1

 1 Fig.1presents the locations where soil samples were taken in the mentioned Provinces.

Fig. 2

 2 Fig.2

Fig. 3

 3 Fig.3

  Fig.4

  ) demonstrates the maximum intergroup and intragroup changes in runoff pH values at the Marzan-Abad and Maraveh-Tapeh sites. The runoff pH values in the various treated plots at these two sites indicate increases compared to the control plots. The increase in runoff pH was due to the application of BC (which has a high pH value).

  Figs. 6(a) and 6(b) present intergroup and intragroup behavioral changes in the variables of

Fig. 1 .Fig. 2 .Fig. 3 .Fig. 4 .Fig. 5 .

 12345 Fig. 5

Fig. 6 .

 6 Fig. 6. Box plots of interrill erosion results. a: soil loss; b: sediment concentration, minimum,

Fig. 1 .

 1 Fig. 1. Location of Marzan-Abad (Marl Soil) and Maraveh-Tapeh (Loess Soil) sites in Iran

Fig. 2 .

 2 Fig. 2. Experimental setting for rainfall simulation in the Marzan-Abad and Maraveh-Tapeh sites, Iran

Fig. 3 .Fig. 4 .Fig. 5 .Fig. 6 .

 3456 Fig. 3. Comparing pairwise of splash erosion variables by paired-samples t-test

  c

Table 1

 1

Table 2 Fig. 1

 2

Table 1 .

 1 Physical and chemical properties of the soils tested

				OM	EC		BD	Particle Size Distribution (%)
	Study soils	E. Longitude	N. Latitude			pH
				(%)	(ds/m)		(gr/cm 3 )	Clay	Silt	Sand
	Marl	51º ׳32	36º ׳82	1.63	0.21	7.68	1.12	42	43	15
	Loess	55º ׳62	37º ׳53	0.155	137.3	8.2		57	36	7

Table 2 .

 2 Some properties of the BC (%) and PAM prepared and used for the study

	Amendments	Form	Ash	Na2O	MgO	Al2O3	SiO2	P2O5	SO3	Cl	K2O	CaO	TiO2	Cr	MnO	Fe2O3	Ni
	BC	Powder	29.78	1.87	2.787	3.377	15.502	0.604	8.592	4.883	11.007	18.578	0.306	0.013	0.04	2.469	0.01

Acknowledgements

The authors would like to give their thanks to Tarbiat Modares University for providing facilities at the Rainfall Simulation and Soil Erosion Laboratory of the Faculty of Natural Resources Noor, Iran, where all lab works have been conducted. This research has been collaboratively supported by the Iranian Ministry of Science, Research and Technology, Center for International Scientific Studies and Collaboration (CISSC), and the French PHC-Gundishapur Project (No42534UA) whose valuable assistances are greatly appreciated. The valuable cooperation of Dr. F. Soleimani in providing raw vinasse, and En. A. Jafarpour, En.