Search

A new method, based on the latent variable measurement model, was recently developed to quantify generalised soil fertility (FG) in grassland ecosystems; i.e. the productive capacity of a soil at a multispecific scale when non-soil variables are held constant. To predict FG values, this method uses the relative growth rates (RGR) of Festuca rubra, Trifolium pratense, Triticum aestivum and Arabidopsis thaliana grown in intact soil. We investigated three simplifications of this method; namely, (i) the use of sieved soil instead of intact cores, (ii) the use of a single measure of biomass production instead of RGR and (iii), a combination of these two modifications. Using 26 grassland soils from southern Quebec (Canada), FG values were predicted for each method and compared using Pearson correlation coefficients. We also evaluated the performances of these different FG values, as well as measures of NO3−, P and K in predicting aerial net primary production (NPP) of the vegetation in these sites in a common non-soil environment. The four methods had comparable and very consistent FG values and, although they were not numerically equivalent, they were equivalent in predicting natural NPP of plant communities, and these predictions were better than the direct measures of NO3−, K and P flux rates as measured by Plant Root Simulator probes. Therefore, any of these methods could be used to assess the generalized fertility.

International audience; Is it possible to generalize relationships between certain plant traits and soil fertility? In particular, are there quantitative relationships between community-weighted mean (CWM) trait values of leaf dry-matter content (LDMC), specific leaf area (SLA), plant height, and Grime’s competitor-stress tolerator-ruderal (CSR) strategy scores and the generalized soil fertility, FG (i.e., the capacity of a soil to produce biomass when all nonsoil variables are held constant) that are generalizable across different species assemblages and geographical areas? We assessed FG in 21 sites in southern Quebec and 7 sites in southern France using a previously published method based on structural equation modeling. We then determined the CWM values of LDMC, SLA, plant height, and CSR scores in the 21 Quebec sites to obtain quantitative relationships between FG and these CWM traits. Using these relationships, we independently tested the generality of the trait–fertility relationships using data from French sites. The relationships between FG and the CWM traits were nonlinear, but displayed the expected qualitative trends as reported in the literature. In particular, the S score and CWM LDMC decreased with increasing soil fertility, and the R score and CWM SLA increased. CWM traits were more strongly correlated to measures of FG (r2 up to 0.48) than to measures of other soil characteristics (r2 up to 0.17 for nitrogen flux). Importantly, the independently tested French FG–trait relationships showed no significant deviations from these quantitative relationships. Further investigation is necessary to confirm if the same trend applies to other regions and or ecosystems.

International audience; In the global change context, the basal respiration (BR), the estimated number of bacterial spores (SP) and the community level physiological profiles (CLPPs) were investigated in three different Mediterranean soils following different hydric and thermic stress scenarios. The treatments consisted in an increasing number (1, 2, 4, and 7) of drying-rewetting (DRWc) or freezing-thawing cycles (FR) at 20, 40 or -20 degrees C. The results highlighted that the different soils responded differently to the same treatment and that the three variables considered were weakly related one to each another. In almost all soils and modalities, the BR increased significantly during the first cycles before decreasing during the last. With regards to SP, it appeared that, for a given soil, the capacity of microbial communities to sporulate and/or germinate can be considerably more influenced by the temperature rather than by the hydric stress. Finally, the CLPPs literally collapsed with the treatment at 40 degrees C, irrespective of the soil considered. This suggested a progressive replacement of the catabolically diversified original bacterial communities by another showing lower functional diversities.

We propose an operational definition of soil "fertility" that is applicable to plant community ecology and develop a method of measuring and quantifying it, using structural equations modeling, that is generalizable to soils in different regions whose fertility has different causes. To do this, we used structural equation modeling (SEM). The measurement submodel predicts the latent "generalized fertility," FG , of a soil using four indicator variables: the relative growth rates of Festuca rubra, Trifolium pratense, Triticum aestivum, and Arabidopsis thaliana. The direct causes of FG in this study were the supply rates of NO3 - , P, and K as well as three indirect causes consisting of three physical soil properties, but these can change between studies. The model was calibrated using 76 grassland soils from southern Quebec, Canada and independently tested using aboveground net primary productivity (NPP) of the natural vegetation over two growing seasons. Both the measurement submodel and the full SEM fit the data well. The FG values predicted 51% of the variance in NPP and were a better predictor than any other single variable, including the actual nutrient flux rates. Furthermore, this model can be applied to grassland soils anywhere because of its modular nature in which the causes and effects of soil fertility are clearly separated.

International audience; The aim of this study was to determine the impact of climatic stress scenarios on soil microbial functions. To this end, three Mediterranean soils (south exposed soil, S; north exposed soil, N; soil from riparian forest, R) were sampled and subjected to climatic stress events consisting of an increasing number of drying-rewetting (DRWc) or freezing thawing (FFc) cycles (1, 2, 4, and 7 cycles) at 20, 20, and 40 degrees C. The microbial functions considered were: fluorescein diacetate hydrolase (FDAse), lipase, acid phosphomonoesterase (PhA), and beta-glucosidase (beta-glu). Results showed that the soils had different enzymatic patterns, the S soil being less active than the two others. Considering the climatic stresses, the repeated effects of hydric and thermic stresses resulted, in most cases, in enzymatic activity decreases. The soils with colder and moister conditions (i.e. R and N soils) had globally more vulnerable enzyme activities to DRWc, a phenomenon which was amplified by increased temperatures, 40 degrees C being the more deleterious temperature. On the contrary, the S soil appeared more resistant to climatic stresses and, in some cases, certain enzyme activities, namely beta-glu and PhA, were unaffected by the various treatments. gamma-irradiation was also used to assess the biotic and abiotic parts in soil FDAse response to stresses. Results showed that in soils devoid of active microbial biomass, the FDAse activity was very sensitive to DRWc at high temperature (40 degrees C), activity losses ranging from 90 to 95%. Moreover, comparison with unsterilized soils indicated that microbial communities were able to reload efficiently this activity between stresses. (C) 2015 Elsevier Ltd. All rights reserved.