• Energy Research

AbstractHeterotrophic microorganisms are commonly thought to be stoichiometrically homeostatic but their stoichiometric plasticity has rarely been examined, particularly in terrestrial ecosystems. Using a fertilization experiment in a tropical rainforest, we evaluated how variable substrate stoichiometry may influence the stoichiometry of microbial communities in the leaf litter layer and in the underlying soil. C:N:P ratios of the microbial biomass were higher in the organic litter layer than in the underlying mineral soil. Regardless of higher ratios for litter microbial communities, C, N, and P fertilization effects on microbial stoichiometry were strong in both litter and soil, without any fundamental difference in plasticity between these two communities. Overall, N:P ratios were more constrained than C:nutrient ratios for both litter and soil microbial communities, suggesting that stoichiometric plasticity arises because of a decoupling between C and nutrients. Contrary to the simplifying premise of strict homeostasis in microbial decomposers, we conclude that both litter and soil communities can adapt their C:N:P stoichiometry in response to the stoichiometric imbalance of available resources.

SignificanceEcosystems are responding to climate change and increasing atmospheric CO2concentrations. Interactions between these factors have rarely been assessed experimentally during and after extreme climate events despite their predicted increase in intensity and frequency and their negative impact on primary productivity and soil carbon stocks. Here, we document how a grassland exposed to a forecasted 2050s climate shows a remarkable recovery of ecosystem carbon uptake after a severe drought and heat wave, this recovery being amplified under elevated CO2. Over the growing season, elevated CO2entirely compensated for the negative impact of extreme heat and drought on net carbon uptake. This study highlights the importance of incorporating all interacting factors in the predictions of climate change impacts.

The relationship between biodiversity and biogeochemical processes gained much interest in light of the rapidly decreasing biodiversity worldwide. In this article, we discuss the current status, challenges and prospects of functional concepts to plant litter diversity and microbial decomposer diversity. We also evaluate whether these concepts permit a better understanding of how biodiversity is linked to litter decomposition as a key ecosystem process influencing carbon and nutrient cycles. Based on a literature survey, we show that plant litter and microbial diversity matters for decomposition, but that considering numbers of taxonomic units appears overall as little relevant and less useful than functional diversity. However, despite easily available functional litter traits and the well-established theoretical framework for functional litter diversity, the impact of functional litter diversity on decomposition is not yet well enough explored. Defining functional diversity of microorganisms remains one of the biggest challenges for functional approaches to microbial diversity. Recent developments in microarray and metagenomics technology offer promising possibilities in the assessment of the functional structure of microbial communities. This might allow significant progress in measuring functional microbial diversity and ultimately in our ability to predict consequences of biodiversity loss in the decomposer system for biogeochemical processes.

Abstract Microbial denitrification plays a central role in nitrous oxide (N 2 O)-emitting processes, which are involved in ecosystem services such as crop production and climate regulation. Field characterization of N 2 O-emitting processes being time-consuming due to great variability, laboratory determination of potential denitrification (upon incubation) is often used as a valuable test. Near infrared reflectance spectroscopy (NIRS) is a time- and cost-effective approach that has been reported to allow accurate determination of several soil properties. The objective of the present study was to assess the interest of NIRS for determining potential denitrification over a set of 460 topsoils sampled under crops, tree plantations, savanna or rainforest, originating from Madagascar, Congo-Brazzaville, Brazil, and French Guiana. Prediction of potential denitrification using NIRS was satisfying over the total set, especially with LOCAL calibration, which builds a model for each sample separately using its spectral neighbours in the calibration subset ( R 2 = 0.79 for validation). For the other sets, either textural or geographical, global calibration only was performed, involving for each set a unique prediction model built with all calibration samples. The accuracy of NIRS determination depended on the sample set, decreasing in the following order: Malagasy clayey set > total set ≈ Brazilian sandy loam set > coarse-textured set (Congo-Brazil) ≈ Guianese sandy clay loam set ≈ Congolese sandy set > non-clayey set (Congo-Brazil-Guiana), with cross-validation R 2 ranging from 0.88 to 0.44 (external validation was not carried out for small-sized sets). Thus NIRS prediction was more accurate over the clayey homogeneous set than over the non-clayey heterogeneous set. As a result of global calibration, potential denitrification was expressed as a linear combination of absorbance at every wavelength. Wavelengths that contributed most to NIRS prediction of soil potential denitrification corresponded to wavelengths that literature has assigned to organic nitrogenous compounds, amide-containing ones especially, and to carbonaceous compounds such as cellulose or including CH 3 or CH 2 groups. This related to the importance of amides in soil organic nitrogen and microbial biomass, and to the dependence of denitrification on soil organic matter. In short, NIRS is a time- and cost-effective approach that proved relevant for determining soil potential denitrification with acceptable accuracy, especially for clayey samples or when LOCAL calibration was performed.

The consequences of predicted climate change on ecosystem processes is difficult to evaluate, because biodiversity is also susceptible to change resulting in complex interactions on ecosystem functioning. With an experimental approach, we aimed to understand how plant community diversity (through different plant litter mixtures) and climate change (through decreased precipitation) may impact microbial abundance and diversity and affect C and N cycling in a Mediterranean shrubland. Along a natural plant diversity gradient, we manipulated the amount of precipitation and followed leaf litter decomposition during one year. We found that multi-species litter mixtures had higher microbial abundance, lower bacterial diversity and higher fungal diversity than predicted from single-species litter. In addition, C and N release increased with increasing litter species richness. Microbial abundance and diversity were positively, but weakly, correlated to the litter mixture effects on C and N release. Drier conditions increased microbial diversity but had no effect on microbial abundance. The net release of N from decomposing litter was lower with reduced precipitation irrespective of litter species richness and composition, while that of C was higher or lower depending on litter species composition. The relationships between microbial communities and litter mixture effects on C and N release were altered under drier conditions. Our data provide clear evidence that microbial decomposers and the processes they drive, respond to changing plant community diversity and composition in a Mediterranean shrubland. We highlighted the importance of Quercus coccifera that appears to be a key species in shaping microbial communities and driving synergistic effects on C and N release more than the three other shrub species. Our study also suggests that shifts in the plant community composition may have stronger impacts on litter decomposition and nutrient cycling than relatively subtle changes in precipitation as simulated in our study.