Matthieu Chauvat
email: matthieu.chauvat@univ-rouen.fr

Estelle Forey

Temperature modifies the magnitude of a plant response to Collembola presence

Keywords: belowground interactions, soil fauna, plant growth, facilitation

Soil properties and the growth and maturation of herbaceous plants are known to be influenced by the soil fauna (Collembola) and by the temperature. But little is known about how these two factors interact. We hypothesized that the vegetative and reproductive responses of Poa annua L. to the presence of soil Collembola density will change according to temperature. P. annua was grown in microcosms with or without Collembola at two different air temperature conditions, 16°C as a Low T° vs. 19°C as a High T°. Both factors "Collembola" and "Temperature" significantly influenced several plant and soil variables. The number of flowers, the root biomass of P. annua were higher either in presence of Collembola or under elevated temperature. The same applied for soil ergosterol or K content for example. Soil nitrate content was exclusively affected by the presence/absence of Collembola and not by the temperature. Finally, significantly interactive effects between temperature and Collembola upon several plant or soil variables (root biomass, number of flowers, soil K and Mg content) were shown. Our study suggests that biotic factors (here soil fauna) may strongly interact with climatic variables in regulating plant growth through changes in soil nutrient availability. We provide experimental evidence that climate warming modifies the effect of soil Collembola on the reproduction strategy of the plant.

Introduction

A biotic response to climate warming remains one of the greatest challenges in ecology in terms of predicting how ecosystems will respond to the currently high rates of warming [START_REF] Blois | Climate change and the past, present, and future of biotic interactions[END_REF]. How plant growth responds to temperature has been well documented over the last decades (see metanalysis of [START_REF] Lin | Climate warming and biomass accumulation of terrestrial plants: a meta analysis[END_REF] with a general increase of biomass (+12.3%) associated with warming, without differences of increased patterns between shoots or root biomasses. Furthermore, plants experiencing elevated temperatures have a different phenology and physiology, as they may, for example, flower significantly earlier [START_REF] Fitter | Rapid changes in flowering time in British plants[END_REF][START_REF] Menzel | European phenological response to climate change matches the warming pattern[END_REF][START_REF] Anderson | Phenotypic plasticity and adaptive evolution contribute to advancing flowering phenology in response to climate change[END_REF] or increase their root exudation rate [START_REF] Yin | Enhanced root exudation stimulates soil nitrogen transformations in a subalpine coniferous forest under experimental warming[END_REF], although this last point has mainly been documented for trees.

Furthermore, according to the metabolic scaling theory, metabolism increasing with temperature drives ecological processes at different levels of biological organisation [START_REF] West | A general model for the origin of allometric scaling laws in biology[END_REF][START_REF] Brown | Toward a metabolic theory of ecology[END_REF]. Thus, in a context of global warming that affects plant performance, it seems necessary not only to focus on the direct effect of the temperature factor upon plants but also on its indirect effect on other biotic factors to deepen our understanding of the effect of climate change on biotic responses. Within this framework, while above-ground responses to climate changes have elicited the most attention, the responses of biota below-ground and their consequences on aboveground organisms are notoriously understudied [START_REF] West | Effects of plant species diversity, atmospheric CO2, and N addition on gross rates of inorganic N release from soil organic matter[END_REF][START_REF] Bardgett | Aboveground-belowground linkages: biotic interactions, ecosystem processes, and global change[END_REF], despite the acknowledged importance of above-ground/below-ground links in ecosystem functioning [START_REF] Bardgett | Aboveground-belowground linkages: biotic interactions, ecosystem processes, and global change[END_REF] . Soil biota encompasses a wide array of organisms both directly (e.g. pathogens, symbionts, rhizophages) and indirectly (e.g. detritivores, predators) by interacting physically with plant roots. To date, most of studies have investigated the interactive effects between temperature and soil biota upon plant growth, with a focus on soil microorganisms [START_REF] Van Grunsven | Plant-soil feedback of native and range-expanding plant species is insensitive to temperature[END_REF][START_REF] Thakur | Effects of soil warming history on the performances of congeneric temperate and boreal herbaceous plant species and their associations with soil biota[END_REF], and very few have considered soil fauna [START_REF] Walther | Plants in a warmer world[END_REF][START_REF] Cornelissen | Global negative vegetation feedback to climate warming responses of leaf litter decomposition rates in cold biomes[END_REF]. These few papers, rather, have suggested a change in interactions between plants and the soil faunal community as a result of global climate change, but without focusing on mechanisms.

Soil faunal activity exerts a strong influence on the composition, structure and functioning of above-ground communities [START_REF] De Deyn | Soil invertebrate fauna enhances grassland succession and diversity[END_REF]. For example, Collembola, a dominant soil microarthropod group, is known to regulate flows of nutrients to plants by shaping the biomass and activity of the microbial community (both bacteria and fungi), either directly, through feeding, or indirectly, through the comminution of organic matter and dissemination of microbial propagules [START_REF] Moore | Arthropod regulation of micro-and mesobiota in below-ground detrital food webs[END_REF][START_REF] Coulibaly | Functional Assemblages of Collembola Determine Soil Microbial Communities and Associated Functions[END_REF]. Furthermore, various Collembolan species have been shown to act upon plant root biomass without any clear pattern, either directly, by exerting rhizophagous pressure [START_REF] Endlweber | Collembola switch diet in presence of plant roots thereby functioning as herbivores[END_REF], or indirectly, by interacting with microbes as detailed above. In previous studies, we were able to show that the presence of Collembola stimulates plant growth and accelerate flowering, potentially through increasing nutrient availability and uptake by plants [START_REF] Forey | Flowering phenology of a herbaceous species (Poa annua) is regulated by soil Collembola[END_REF][START_REF] Winck | Functional collembolan assemblages induce different plant responses in Lolium perenne[END_REF].

Collembola, ectothermic organisms, may exhibit greater activity and higher growth and reproductive rates with moderate warming, resulting in elevated metabolic demands [START_REF] Gillooly | Effects of size and temperature on metabolic rate[END_REF][START_REF] Mallard | From individuals to populations: How intraspecific competition shapes thermal reaction norms[END_REF]. A few studies focusing on the fate of the Collembola community have tried to disentangle the effects of temperature from those of soil moisture [START_REF] Wolters | Long-term dynamics of a collembolan community[END_REF][START_REF] Xu | Seasonal exposure to drought and air warming affects soil Collembola and mites[END_REF][START_REF] Santonja | Increasing temperature and decreasing specific leaf area amplify centipede predation impact on Collembola[END_REF][START_REF] Mallard | From individuals to populations: How intraspecific competition shapes thermal reaction norms[END_REF]. These studies generally reported an increase in the growth rate, abundance and diversity of Collembola in warmer soil conditions if soil humidity remained favourable. Indeed, as described above, many factors may explain the changes in plant performance following an increase in temperature (Fig. 1), ranging from the direct effects of temperature upon plants to its indirect effects through changes in the soil-available nutrients regulated by the interactions between microbes and their consumers (e.g. Collembola).

Thus, in a microcosm experiment, we explored the interaction of temperature and Collembola on the growth of one of the most common grass species in the world, Poa annua L. As Figure 1 shows, we hypothesised that the positive effect of Collembola on P. annua growth and flowering would be stronger under a higher air temperature (+3°C), mimicking climate warming compared to the actual temperature. Indeed, we expected that a higher temperature would stimulate Collembola metabolism and growth, leading to an increase in their number and a shift in their community composition and structure. This increase in Collembola activities would result in a higher amount of nutrients (N, P and K) available in the soil for plants (see Fig. 1), ultimately promoting plant growth (biomasses) and eventually flowering. In other words, we assumed that there was a strong interaction of Collembola and temperature that affected soil nutrient status and plant performance (both growth and reproductive investment). Each treatment was replicated 15 times in microcosms (10x9x9cm PVC pots), where individuals of P. annua were grown. The soil was a rendosol (organic matter content = 6.17%; pHH2O = 7.79) collected in September 2017 from a chalk grassland (1°703000E, 49°2202200N) in Normandy, France. Part of the soil was defaunated by deep-freezing and thawing over the course of one week and then repeated the following week, as described in [START_REF] Forey | Flowering phenology of a herbaceous species (Poa annua) is regulated by soil Collembola[END_REF]. The 60 microcosms were meshed (0.250mm) to prevent any escape of Collembola and filled with approximately 245g FW of previously sieved (5mm) and defaunated soil. The soil density obtained at the start of the experiment was of 0.47g.cm -3 , which is particularly light but may result from using a 2mm-sieved soil and not compacting the soil when filling in the microcosms in order to leave available pore space for Collembola (especially large species).

Material and Methods

In

From the remaining soil, microorganisms (fungi and bacteria) were extracted and reinoculated in the 60 microcosms prior to setting the different treatments (see [START_REF] Forey | Flowering phenology of a herbaceous species (Poa annua) is regulated by soil Collembola[END_REF], for further details). Re-inoculation of microorganisms in all microcosms was followed two weeks later by the addition of Collembola only in the 15 'High T°-Coll' and 15 'Low T°-Coll' microcosms. This time lap of two weeks was applied in order to let the microbial components colonise the soil and not suffer from, for example, overgrazing. Each microcosm of the Low T°-Coll and High T°-Coll was reinoculated with natural Collembola assemblages extracted from 150g of fresh soil. To do this, we first extracted, using the Berlese-Tullgren device [START_REF] Tullgren | Ein sehr einfacher Ausleseapparat fur terricole Tierformen[END_REF], the Collembola of each 150g of fresh soil into pots filled with moist plaster. Under a stereomicroscope, we captured and transferred Collembola with pooters into the different microcosms. These specific and manual transfers ensured that only introduce Collembola and no other mesofauna groups were transferred. As Collembola species could only be determined with certainty under a microscope, we selected under the stereomicroscope different morphospecies, i.e. species distinguished from others only by their morphology, that were easily recognisable. This allowed us to introduce the same set of species in each microcosm. To obviate for too much discrepancy between number of individuals introduced per species or morphospecies under the stereomicroscope, we selected 3 to 5 individuals per morphospecies except for the dominant morphospecies (later identified as Parisotoma notabilis), of which 10 to 15 individuals were introduced. As mortality may have occurred during the re-inoculation process, making it difficult to estimate the number of individuals that establish themselves, Collembola were extracted from 5 microcosms of each treatment at the end of the experiment using the Berlese-Tullgren device [START_REF] Tullgren | Ein sehr einfacher Ausleseapparat fur terricole Tierformen[END_REF], allowing us to confirm that no contamination was observed in the treatment without Collembola, as none was found in either the treatment 'Low T°-None' nor in 'High T°-None'. It also allowed us to compare Collembolan communities between treatments at the end of the experiment.

Within each microcosm, a single seedling of P. annua was transplanted into each microcosm according to the methods used by [START_REF] Forey | Flowering phenology of a herbaceous species (Poa annua) is regulated by soil Collembola[END_REF]. All the seedlings were obtained from seeds collected at a single spot in a calcareous grassland in Moult, Normandy.

The microcosms dedicated to the low-temperature treatments (Low T°-None and Low T°-Coll) were incubated in climate chambers (temperature: 16°C; daily light/dark 16h/8h), while the microcosms dedicated to the high temperature treatments (High T°-none and High T°-Coll) were incubated in similar climatic chambers with the same photoperiod but at a 19°C air temperature. For all microcosms, the soil moisture was kept to 60% of the soil's waterholding capacity during the experiment by weighing the microcosms every 3 days and adding water accordingly. This process was stopped after 13 weeks, when more than 80% of P. annua were flowering.

At the end of the experiment, several plant traits were measured. On ten replicates, root traits (root length and number of root tips) were assessed using WinRHIZO TM image-analysis software (V. reg. 2009c, Regent Instruments, Canada), as described by [START_REF] Forey | Flowering phenology of a herbaceous species (Poa annua) is regulated by soil Collembola[END_REF].

On ten replicates, plant height, the number of leaves, the number of spikelets (thereafter 'flowers') and the number of ramets were measured. To estimate plant biomass allocation, both root (R) and shoot (S) biomasses were measured after drying them at 65°C for 48 h.

The S/R ratio was then calculated. Finally, the carbon and nitrogen content of leaves was measured by chromatography with a CHN pyrolysis micro-analyser (Flash 2000 Series, CHNS/O Analysers Thermo Scientific, France). Mineral elements (Mg, Mn and P, n = 8) in the leaves were analysed by atomic absorption spectrophotometry (AAS, ICE 3000 SERIES, Thermo Scientific, China; see Forey et al., 2015, for methods). We also determined for each microcosm the first flowering date, calculated as the number of days between the seedling transplantation into the microcosms and the flowering.

Correlation tests between plant traits were performed in order to avoid autocorrelative parameters in further analyses. According to the results, we only kept the shoot and root biomasses and their ratio as markers of vegetative growth and thus excluded plant height, number of leaves and number of ramets. The number of flowers and the first flowering date were kept as markers of reproductive status. Finally, the root length was used as a marker of root architecture, and we therefore excluded the number of root tips for further analyses.

Soil properties were also investigated at the end of the experiment; these included fungal biomass (i.e. ergosterol concentration), determined according to the method of [START_REF] Gong | A rapid method to extract ergosterol from soil by physical disruption[END_REF], microbial carbon biomass (Cmic), determined using the fumigation-extraction method [START_REF] Jenkinson | The effects of biocidal treatments on metabolism in soil-V: A method for measuring soil biomass[END_REF] and mineral N (N-NO3 -and N-NH4 + ), determined colourimetrically (Sequential Analyser Gallery, Thermoscientific). All these variables were measured on only ten replicates per treatment, and were not investigated in the five microcosms per treatment dedicated to the extraction of soil Collembola. Furthermore, nutrient content (Mg, Mn, P and K), pHH2O and soil water content (see [START_REF] Forey | Flowering phenology of a herbaceous species (Poa annua) is regulated by soil Collembola[END_REF] full methods) were also investigated in all microcosms (n = 15) in each treatment. To answer our main hypothesis on the joint effects of temperature and Collembola in determining the response of P. annua and soil variables, we used a Generalised Linear Model (GLM) approach with the presence or absence of Collembola and the two levels of temperature as fixed factors. We fitted the model according to the distribution of the data (details in Tables 1 and Fig. 1 and2). A Bonferroni correction for multiple tests was applied for both plant and soil variables. Differences in Collembola communities between treatments were tested using a Permanova test with 9999 permutations, Bray-Curtis distance, and Bonferroni's correction applied to the p-values. Wann-Whitney U tests were used to depict differences in the abundance of each Collembolan species between both temperature treatments. Finally, to test whether the response of reproduction simply results from the effect on plant biomass or whether the temperature also affects reproductive investments, we tested the residual of the number of flowers~plant biomass by means of a GLM. All tests were computed using Statistica software package (version 10.0, Statsoft, Tulsa, 2010).

Results

Effect of temperature and springtails on plant variables

Three plant variables-the number of flowers, the first flowering date (number of days to flower emergence) and the root biomass-were significantly affected by the interactive term 'Temperature * Collembola' (Fig. 2 a-c).

The number of flowers per plant was significantly higher in the 'High T°-Coll' treatment than in the other three treatments, with an increase ranging from +120% compared to the 'Low T°-None' treatment to +150% compared to the 'Low T°-Coll' treatment (Fig. 2a). The number of flowers was also significantly affected by the factor 'Temperature' (+67% in the High T° treatments compared to the Low T° treatments) and by the factor 'Collembola' (+75% in the treatments with Collembola compared to the treatments without Collembola (Fig. 2a).

Furthermore, the reproductive investment of P. annua influenced by the interactive term 'Collembola * Temperature' was independent of the plant biomass (GLM, F = 0.08; p = 0.78).

The number of days to emergence was significantly reduced, by approximately one week for a growth period of around 7-8 weeks in 'High T°-Coll' compared to both 'Low T°-Coll' and 'High T°-None', 'Low T°-None' had an intermediate value (Fig. 2b and Fig. S1). This variable was influenced neither by the temperature nor by the presence of Collembola alone (Fig. 2b).

The shoot biomass and the P of the leaves showed higher values in the 'High T°' treatments compared to the 'Low T°' treatments (by 64% and by 15%, respectively; Fig. 2d). In parallel, the P content in the leaves was also higher (+23%) in the presence of Collembola. On the other hand, the shoot/root ratio (-49%), the C/N (-6%) and the Mg content (-16%) of the leaves were all significantly lower in the treatments with Collembola compared to those without (Table 1). The root biomass was significantly higher in the 'High T°-Coll' treatment compared to all other treatments, with an increase ranging from +62% compared to the 'Low T°-Coll' to +153% compared to the 'Low T°-None'. These last two treatments also differed significantly, with +56% of root biomass in 'Low T°-Coll' compared to 'Low T°-None'. Both factors 'Temperature' and 'Collembola' affected the root biomass, with mean values 41% higher in the 'High-T°' treatments and 87% higher in the presence of Collembola (Fig. 2c).

The root length was not influenced by both factor or their combination (Fig. 2f).

Effect of temperature and Collembola on soil properties

The concentrations of two soil nutrients, Mg and K, were affected by the interaction between 'Temperature' and 'Collembola' (Fig. 3a-b), with approximately 10% less Mg in the 'High T°-Coll' soil compared that of both 'Low T°-Coll' and 'Low T°-None'. In parallel, soil Mg content was also significantly but slightly lower (-5%) in the presence of Collembola (Fig. 3a). Soil K content was higher in 'High T°-Coll' compared to all other treatments: +22% compared to both 'High T°-None' and 'Low T°-None' and +17% compared to 'Low T°-Coll'. This variable was also affected by both the 'Temperature' and 'Collembola' factors, with +9% and +13% of soil K content under high temperature and in the presence of Collembola, respectively (Fig. 3b).

Besides soil Mg and K content, several other soil variables were affected by either the 'Temperature' or 'Collembola' factor. The microbial biomass (+29%; Table 2), the ergosterol content (+20%; Table 2) and the soil NH4 + content (+79%; Fig. 3c) had higher mean values under high temperature. The presence of Collembola, regardless of temperature, significantly increased the ergosterol (+16%; Table 2), NO3 -(+95%; Fig. 3d), and P (+23%; Fig. 3e) and content (Table 2). The concentration of Mn into the soil was not affected by both factors of their interactions (Fig. 3f).

Effect of Temperature on Collembola communities

The total amount of Collembola was significantly higher: 79% higher in the 'High T°' compared to the 'Low T°' soils (Table 2). The significant PERMANOVA showed that both communities were different from each other (Table 2). Of the eleven taxa found at the end of the experiment, three species occurred only in the 'Low T°' treatments, while two were found only in the 'High T°'. The abundance of only one species, Isotomurus palustris, however, was significantly affected by the factor 'Temperature', with 8.7 times more individuals in the 'High-T°' compared to the 'Low-T°' groups (Table 2). Finally, Parisotoma notabilis was the dominant species in both treatments, constituting a 60.5% and 46.6% share of the 'Low-T°' and 'High-T°' groups, respectively.

Discussion

As expected, we observed joint effects of both the 'Temperature' and 'Collembola' factors on P. annua reproductive (flowering) or vegetative (root biomass) variables, as well on as soil Mg and K content. This clearly demonstrates the interactive effect of temperature and Collembola on plant growth and development. To our knowledge, our study is the first to examine and demonstrate experimentally such interactive effects among soil fauna, Collembola and temperature upon plant growth and reproduction. We also expected that the interactions between 'Collembola' and 'Temperature' would result in a shift of the Collembolan assemblage (species and abundances) at high temperature compared to the low-temperature treatment. We effectively found different assemblages of Collembola between the two temperatures that partly explained the interactive effects observed. Indeed, increasing temperature can have strong direct effects on Collembola demographic parameters (e.g. fecundity, reproduction, development; [START_REF] Hopkin | Biology of the springtails:(Insecta: Collembola)[END_REF], thereby affecting their densities. In fact, our results lend insights into the Collembola response to warming that have often been found to be more species-specific than general (e.g. [START_REF] Wolters | Long-term dynamics of a collembolan community[END_REF][START_REF] Makkonen | Traits explain the responses of a sub-arctic Collembola community to climate manipulation[END_REF]. Drought has often been reported as the main negative factor explaining a decrease in Collembola experiencing global warming [START_REF] Kennedy | Simulated climate change: a field manipulation study of polar microarthropod community response to global warming[END_REF][START_REF] Jucevica | Global warming affect Collembola community: A long-term study[END_REF][START_REF] Makkonen | Traits explain the responses of a sub-arctic Collembola community to climate manipulation[END_REF][START_REF] Petersen | Collembolan communities in shrublands along climatic gradients in Europe and the effect of experimental warming and drought on population density, biomass and diversity[END_REF]. In contrast to these studies, we only manipulated soil temperature, with soil moisture maintained as constant. Although we do not have the exact number of individuals introduced at the beginning of the experiment, the higher total abundance of Collembola found in our case within the 'High T°-Coll' group supports the results of other studies focusing on temperature increase [START_REF] Day | Response of plants and the dominant microarthropod, Cryptopygus antarcticus, to warming and contrasting precipitation regimes in Antarctic tundra[END_REF][START_REF] Kardol | Soil ecosystem functioning under climate change: plant species and community effects[END_REF][START_REF] Xu | Seasonal exposure to drought and air warming affects soil Collembola and mites[END_REF]Santonja et al., 2019). Only one Collembola species, however, had a significantly different abundance between the two temperatures of incubation. It is unlikely that an increase in temperature will increase the survival of arthropods, so the overall higher density is probably due to increased fecundity and reduced generation time at the higher temperature. Recently, [START_REF] Mallard | From individuals to populations: How intraspecific competition shapes thermal reaction norms[END_REF] found that the growth rate of a Collembolan species increased with temperature but that this thermal reaction was modulated by intraspecific competition. This, when taken into account along with interspecific competition, may explain why in our case only one species benefited from an increased temperature. Three species with a low number of individuals were found only at low temperature, but as we did not introduce the same number of individuals to each microcosm, it is difficult to draw a conclusion from this finding. Nevertheless, as found in previous research [START_REF] Hodkinson | Global change and Arctic ecosystems: conclusions and predictions from experiments with terrestrial invertebrates on Spitsbergen[END_REF][START_REF] Makkonen | Traits explain the responses of a sub-arctic Collembola community to climate manipulation[END_REF][START_REF] Petersen | Collembolan communities in shrublands along climatic gradients in Europe and the effect of experimental warming and drought on population density, biomass and diversity[END_REF], our results support the role of temperature as a determinant of Collembolan communities. In the literature, deep-living small euedaphic species were found to have difficulties coping with an increase in temperature, unlike larger epedaphic species living on top of the soil [START_REF] Jucevica | Global warming affect Collembola community: A long-term study[END_REF][START_REF] Makkonen | Traits explain the responses of a sub-arctic Collembola community to climate manipulation[END_REF]. Our results showed an increase of only one epedaphic species, partly supporting the previous findings.

Changes in Collembolan communities under increasing temperature might also be mediated by plant-soil feedbacks (PSFs). Indeed, increasing temperature is known to affect almost every biological processes in a plant, including morphogenesis, respiration and photosynthesis [START_REF] Quint | Molecular and genetic control of plant thermomorphogenesis[END_REF][START_REF] Dusenge | Plant carbon metabolism and climate change: elevated CO2 and temperature impacts on photosynthesis, photorespiration and respiration[END_REF]. Generally, these metabolic processes, such as photosynthesis, occur between 20°C and 30°C depending on species (Berry and Björkman, 1980). As a consequence, elevated temperatures may increase the amount and nature of compounds transferred to the rhizospheric soil by plants through increasing root respiration and root exudation [START_REF] Atkin | Response of root respiration to changes in temperature and its relevance to global warming[END_REF][START_REF] Yin | Enhanced root exudation stimulates soil nitrogen transformations in a subalpine coniferous forest under experimental warming[END_REF]. These changes in soil carbon dynamic and inputs can stimulate soil microbial communities from the rhizosphere [START_REF] Lange | Biotic and abiotic properties mediating plant diversity effects on soil microbial communities in an experimental grassland[END_REF], thereby potentially modifying through bottom-up control Collembola communities. Our experiment was not designed to test or quantify the importance of these PSFs. Many questions, however, need to be addressed to understand how climate change will alter PSFs and their potential consequences for ecosystem functioning. In a recent meta-analysis on this topic, [START_REF] Pugnaire | Climate change effects on plant-soil feedbacks and consequences for biodiversity and functioning of terrestrial ecosystems[END_REF] stressed the need to understand how warming affects these PSFs through the importance of rhizodeposits, which are often overlooked in the literature.

We further hypothesised that an increase of Collembola with increasing temperature would result in higher amount of nutrients available in the soil for plants and ultimately higher plant growth and faster flowering. While we effectively found an effect of the interaction between temperature and Collembola on several soil nutrients (Mg and K), this was not the case for all Collembola. Furthermore, even if the interactions between 'Collembola' and 'temperature' affected the concentration of several soil nutrients, plant nutrients were not affected. The temperature-associated potential changes in the specific needs for nutrients (stoichiometry) of microbial components, Collembola and plants may partly explain the discrepancies found in the nutrient responses either in the soil (with only K and Mg affected) or within the plants to the interaction of 'Collembola' and 'Temperature'. The factor 'Collembola' alone, however, has been found to affect soil and plant nutrients strongly, as found by [START_REF] Forey | Flowering phenology of a herbaceous species (Poa annua) is regulated by soil Collembola[END_REF] and [START_REF] Winck | Functional collembolan assemblages induce different plant responses in Lolium perenne[END_REF]. This suggests that Collembola might first stimulate mineralisation performed by microorganisms, leading to an increase in soil nutrients, but also that they might limit the ability of the microbial biomass to grow and compete with plants for N and P, even if they do not reduce the total microbial biomass. This could probably be achieved through selective grazing by promoting (in relative proportion) different microbial components that may be less competitive for N and P acquisition and also for other nutrients [START_REF] Ngosong | Collembola grazing on arbuscular mycorrhiza fungi modulates nutrient allocation in plants[END_REF][START_REF] Kutáková | Soil microarthropods alter the outcome of plant-soil feedback experiments[END_REF][START_REF] Coulibaly | Functional Assemblages of Collembola Determine Soil Microbial Communities and Associated Functions[END_REF]. We did not find, however, the interactive effect upon microbial variables that we might have expected regarding the conceptual scheme (Fig. 1). It could be that the coarse parameters used to describe the microbial component were not sensitive enough to detect changes that may have occurred within the assemblages according to the interaction between 'Collembola' and 'temperature'.

Alternatively, PSFs might also have masked these interactive effects on the microbial variables. Our design did not allow us to clearly separate a Collembola effect due to an increase of either their abundance or their metabolism or through a change in their community composition and structure with temperature, which may have prevented us from detecting such interactive effect on nutrients. An intriguing but very challenging project would be to disentangle how these variables may participate in causing the joint effects observed of temperature on plant growth.

In general, the functional consequences for soil functioning and ultimately the plant growth of trophic interactions between soil invertebrates and microbial components remain poorly understood [START_REF] Kutáková | Soil microarthropods alter the outcome of plant-soil feedback experiments[END_REF]. Nevertheless, in microcosms, selective grazing of microarthropods upon micro-organisms has been proven to regulate plant development (see [START_REF] Friberg | Influence of soil fauna on fungal plant pathogens in agricultural and horticultural systems[END_REF], for a review). Furthermore, the recent results obtained by a field study by [START_REF] Schuldt | Belowground top-down and aboveground bottom-up effects structure multitrophic community relationships in a biodiverse forest[END_REF] support the view that top-down regulation below ground might be more important than previously thought, operating through feedbacks among the different microbial components, detritivores and plant communities, adding complexity to the PSFs.

Interestingly, even if the nutrient status of the plant was not affected by the interaction between the two treatments, both plant reproduction and a marker of plant growth were. This suggests that, besides regulating nutrient availability through microbial grazing in a temperature-dependent manner, Collembola may also interact through other mechanisms with plants and that those synergistic interactions or mechanisms are partly temperaturedependent. In a review paper, Puga-Freitas and Blouin (2015) highlighted that very different soil organisms, such as bacteria or Collembola, may activate similar hormone signalling pathways, leading to mutual consequences upon plant growth or defence. This should be further investigated, but may likely play a role in the interaction pathways between plants and soil. For example, in a different context, [START_REF] Rodríguez-Echeverría | A role for below-ground biota in plant-plant facilitation[END_REF] showed that belowground biota (specifically microbial communities) shaped by nurse (or benefactor) plant species play an important role in the facilitation of other plant (or beneficiary) species. They hypothesised that such PSF effects could be related to the increased nutrient availability promoted by soil microbes and/or to production of phytohormones by plant growth, promoting bacteria.

The shoot biomass, as opposed to root biomass, did not respond to the interaction between 'Temperature' and 'Collembola'. This difference in the response of shoot and root biomass was also found in our case regarding the simple effect of the factor 'Collembola' and may result from the close physical relationship between Collembola and roots. For example, several Collembola are known to feed on roots occasionally [START_REF] Endlweber | Collembola switch diet in presence of plant roots thereby functioning as herbivores[END_REF], which may lead to a compensatory growth of roots. [START_REF] Lin | Climate warming and biomass accumulation of terrestrial plants: a meta analysis[END_REF], however, in a meta-analysis found that shoot and root biomass of grasses exposed to climate warming increased by 5% and 33%, respectively, showing a different response of above-or below-ground plant parts independently of soil fauna. In our case, while root biomass increased to the same magnitude (+41%) with temperature, the shoot biomass increased much more (+64%) than the mean values reported in the meta-analysis (+5%). Such a discrepancy may be explained by the absence of plant competition in our experiment and by our choice to maintain constant soil moisture rather than limiting moisture throughout the whole experiment. Several authors also pointed to the fact that root traits are species-specific and highly plastic to abiotic or biotic (i.e. microorganisms) environmental factors [START_REF] Eissenstat | Building roots in a changing environment: implications for root longevity[END_REF][START_REF] De Kroon | Root responses to nutrients and soil biota: drivers of species coexistence and ecosystem productivity[END_REF][START_REF] Hendriks | Spatial heterogeneity of plant-soil feedback affects root interactions and interspecific competition[END_REF]. We also cannot exclude the possibility that the size of our microcosms may have restricted the growth of the roots.

The reproductive success of P. annua (i.e. its number of flowers) was doubled in presence of both elevated temperature and Collembola. Most importantly, only the joint effect of high temperature coupled with the presence of Collembola led to influence the flowering phenology of P. annua, with the number of days to flower emergence being reduced by 14% compared to both the 'Low T°-Coll' and 'High T°-None' groups. Other studies have demonstrated a positive influence of arthropods on plant reproduction or on phenology [START_REF] Ando | Aphid as a network creator for the plant-associated arthropod community and its consequence for plant reproductive success[END_REF][START_REF] Forey | Flowering phenology of a herbaceous species (Poa annua) is regulated by soil Collembola[END_REF], but have focused on temperature as an interactive factor. As noticed by [START_REF] Forey | Flowering phenology of a herbaceous species (Poa annua) is regulated by soil Collembola[END_REF], changes induced both in soil chemistry and within the soil microbial community may lead shorten flowering emergence. Without being able to disentangle these mechanisms, our findings of higher soil K and ergosterol content in parallel with shorter flowering emergence and higher flowering numbers in 'High T°-Coll' support this assertion.

Therefore, any suggestions about the mechanisms involved would be rather speculative.

Although these points were beyond the scope of our study, they would certainly be worth addressing. The results of temperature on plant growth via soil biota are supported by a recent study that found soil warming to affect the relationship between plants and soil biota, e.g. nematodes feeding on microbes [START_REF] Thakur | Effects of soil warming history on the performances of congeneric temperate and boreal herbaceous plant species and their associations with soil biota[END_REF].

Conclusion

The effects of soil invertebrates upon plants are of high importance when considering their specific responses to global warming. For example, P. annua is a Eurasian species from a temperate climate that is invasive on sub-Antarctic islands [START_REF] Molina-Montenegro | Occurrence of the non-native annual bluegrass on the Antarctic mainland and its negative effects on the native plants[END_REF].

Understanding the dynamic and adaptation of this alien species in northern regions experiencing climate warming requires the integration of both climate change and changes in below-ground compartments, as suggested by our study and demonstrated by the significant interactive terms 'Collembola' * 'Temperature' regarding several plant and soil variables.

Few Species Distribution Models include biotic interactions, and even fewer include the role of soil fauna in plant growth. We thus strongly recommend testing the effect of climate change with more complex plant communities and soil fauna groups, as our results for temperature effects on plant growth via soil biota confirm previous results [START_REF] Thakur | Effects of soil warming history on the performances of congeneric temperate and boreal herbaceous plant species and their associations with soil biota[END_REF][START_REF] Heinze | Soil temperature modifies effects of soil biota on plant growth[END_REF]. It remains unclear, however, whether the effects found in our case in a simplified model system are relevant within natural complex soil food webs.

Furthermore, other factors, such as changes in precipitation patterns or elevated atmospheric CO2, must also be taken into account, as they may regulate the activities of below-ground organisms and plants. Table 1. Means (± SD) of plant leaves chemistry and soil variables according to the factor "Temperature -T°" (High or Low) and the combined presence of "Collembola" (Coll.) or not (None). Means and SD were calculated per treatment. Results of Glms with the two factors are given. F or W and P-values are given. n.s: p> 0.05; *: p < 0.05; **: p < 0.01; ***: p < 0.001. For clarity, significant results are in bold. In case of a significant interactions, different letters denote significantly different values. Cmic: microbial biomass; DW: dry weight. [START_REF] Gange | Arbuscular mycorrhizal fungi, Collembola and plant growth[END_REF][START_REF] Partsch | Decomposers (Lumbricidae, Collembola) affect plant performance in model grasslands of different diversity[END_REF], Endlweber and Scheu, 2006, Endlweber and Scheu, 2007), and indirectly through regulation of microflora [5] [START_REF] Ngosong | Collembola grazing on arbuscular mycorrhiza fungi modulates nutrient allocation in plants[END_REF][START_REF] Coulibaly | Functional Assemblages of Collembola Determine Soil Microbial Communities and Associated Functions[END_REF][START_REF] Winck | Functional collembolan assemblages induce different plant responses in Lolium perenne[END_REF], enhancing mineralization and therefore increasing the pool of soil nutrients [6] [START_REF] Forey | Flowering phenology of a herbaceous species (Poa annua) is regulated by soil Collembola[END_REF] available for plants or by reducing plant pathogens [7] [START_REF] Endlweber | Collembola switch diet in presence of plant roots thereby functioning as herbivores[END_REF].

Tables

Collembola may also directly increase the pool of soil nutrients through their faeces [8] [START_REF] Hopkin | Biology of the springtails:(Insecta: Collembola)[END_REF]). An increase pool of soil nutrients may favour shoot biomass over root biomass [9] [START_REF] Ericsson | Growth and shoot: root ratio of seedlings in relation to nutrient availability[END_REF].

  a microcosm experiment performed in October 2017, four treatments were established corresponding to the presence ('Coll.') or absence ('None') of Collembola, with two air temperature levels: Low T° (16°C) and High T° (19°C). The low temperature represents the mean air temperature of the growing season of P. annua in late spring and summer (from May through August) in Normandy (the mean data from 1981 to 2010 registered by a local weather station at Boos-Rouen; https://www.infoclimat.fr). The high temperature level (19°C) mimics what could be expected in the near future in Normandy during spring and summer according to the different climate change scenarios (RCP 6.0 and RCP 8.5) presented in the fifth report of the IPCC (2014).

Fig. 2a -

 2a Fig. 2a-f. Mean (and SD) of morphological variables of Poa annua : a) Number of flowers, b)First flowering date, c) Root biomass d) Shoot biomass, e) Shoot/root ratio, f) Root length, grown with Collembola ("Coll") or without ("None") under two temperatures conditions (Low T° = 16°C; High T° = 19°C). For a single variable, results of two-ways GLMs are given with the factor "Temperature", "Collembola" and their interactive terms. In case of a significant interactions different letters denote significant differences between the treatments (Tukey HSD test). For all variables, except the number of flowers with a poisson distribution, a gaussian distribution was used in the GLMs. Temp: Temperature; Coll: Collembola.

  Figure 1:

Table 2 .

 2 Means and standard deviations (SD) of collembolan species abundance (number of individuals) in microcosms (650ml of soil) under two different temperatures with a seedling of Poa annua at the end of the 13 weeks of experiment. Species are ordered according to their

	abundance in the Low T° treatment. Results of two-sided Mann-Whitney U test Z and P-level
	of significance are given. n.s: p > 0.05; *: p < 0.05. Low T°: 16°C; High T°: 19°C; n=5. Hemi:
	Hemiedaphic species; Ep: Epedaphic species; Eu: Euedaphic species.
	Life-forms	Species	Mean Abundance (± SD)
			Low T°	High T°	Z	P-level
	Hemi	Parisotoma notabilis (SCHÄFFER 1896)	24.2 (12.9)	33.4 (6.0)	1.04	n.s
	Hemi	Folsomia fimetaria (LINNE 1758)	8.8 (5.5)	24.0 (18.9)	1.78	n.s
	Hemi	Pseudosinella octopunctata (PACKARD 1873)	2.2 (1.9)	3.6 (3.6)	0.42	n.s
	Hemi	Folsomia decemoculata (STACH 1946)	1.2 (1.8)	0.0 (0.0)	1.04	n.s
	Ep	Isotomurus palustris (LUBBOCK 1862)	0.8 (1.1)	7.0 (6.2)	2.24	*
	Eu	Protaphorura armata (TULLBERG 1869)	0.8 (1.1)	0.0 (0.0)	1.04	n.s
	Eu	Mesaphorura sp.	0.8 (1.1)	2.0 (2.0)	0.90	n.s
	Eu	Megalothorax minimus (WILLEM 1900)	0.8 (0.8)	0.0 (0.0)	1.57	n.s
	Ep	Lepidocyrtus lanuginosus (TULLBERG 1871)	0.4 (0.9)	0.4 (0.9)	0.01	n.s
	Hemi	Folsomia quadrioculata (FABRICIUS 1783)	0.0 (0.0)	0.8 (1.8)	0.52	n.s
	Ep	Sminthurinus elegans (FITCH 1863)	0.0 (0.0)	0.4 (0.9)	0.52	n.s
		Total	40.0 (15.8)	71.6 (17.8)	-2.4	*

Acknowledgments

The authors would like to thank the Botanical Garden of Caen for providing seeds. The authors are grateful to Philippe Delporte and Marthe Akpa-Vinceslas (Ecodiv lab, University of Rouen Normandy) and the FR SCALE CNRS 3730 for technical assistance. This study was financially supported by GRR TERA and SFR SCALE (Région Haute-Normandie, France). The authors would like to thanks two anonymous reviewers for their comments

improving an earlier version of this manuscript.