Nicolas Legay

Relative contributions of plant traits and soil microbial properties to mountain grassland ecosystem services.

Summary 1. Plant functional diversity and soil microbial community composition are tightly coupled. Changes in these interactions may influence ecosystem functioning. Links between plant functional diversity, soil microbial communities and ecosystem functioning have been demonstrated in experiments using plant monocultures and mixtures, using broad plant and microbial functional groups, but have not been examined in diverse natural plant communities. 2. We quantified the relative effects of plant and microbial functional properties on key ecosystem functions. We measured plant functional diversity, soil microbial community composition and parameters associated with nitrogen (N) cycling and key nutrient cycling processes at three grassland sites in different parts of Europe. Because plant structure and function strongly influence soil microbial communities, we determined relationships between ecosystem properties, plant traits and soil community characteristics following a sequential approach in which plant traits were fitted first, followed by the additional effects of soil micro-organisms. 3. We identified a continuum from standing green biomass and standing litter, linked mostly with plant traits, to potential N mineralization and potential leaching of soil inorganic N, linked mostly with microbial properties. Plant and microbial functional parameters were equally important in explaining % organic matter content in soil. A parallel continuum ran from plant height, linked with above-ground biomass, to plant quality effects captured by the leaf economics spectrum, which were linked with the recycling of carbon (C) and N. 4. More exploitative species (higher specific leaf area, leaf N concentrations and lower leaf dry matter content) and taller swards, along with soil microbial communities dominated by bacteria, with rapid microbial activities, were linked with greater fodder production, but poor C and N retention. Conversely, dominance by conservative species (with opposite traits) and soil microbial communities dominated by fungi, and bacteria with slow activities, were usually linked with low production, but greater soil C storage and N retention. 5. Synthesis – Grassland production, C sequestration and soil N retention are jointly related to plant and microbial functional traits. Managing grasslands for selected, or multiple, ecosystem services will thus require a consideration of the joint effects of plant and soil communities. Further understanding of the mechanisms that link plant and microbial functional traits is essential to achieve this.

Contribution of above- and below-ground plant traits to the structure and function of grassland soil microbial communities

BACKGROUND AND AIMS Abiotic properties of soil are known to be major drivers of the microbial community within it. Our understanding of how soil microbial properties are related to the functional structure and diversity of plant communities, however, is limited and largely restricted to above-ground plant traits, with the role of below-ground traits being poorly understood. This study investigated the relative contributions of soil abiotic properties and plant traits, both above-ground and below-ground, to variations in microbial processes involved in grassland nitrogen turnover. METHODS In mountain grasslands distributed across three European sites, a correlative approach was used to examine the role of a large range of plant functional traits and soil abiotic factors on microbial variables, including gene abundance of nitrifiers and denitrifiers and their potential activities. KEY RESULTS Direct effects of soil abiotic parameters were found to have the most significant influence on the microbial groups investigated. Indirect pathways via plant functional traits contributed substantially to explaining the relative abundance of fungi and bacteria and gene abundances of the investigated microbial communities, while they explained little of the variance in microbial activities. Gene abundances of nitrifiers and denitrifiers were most strongly related to below-ground plant traits, suggesting that they were the most relevant traits for explaining variation in community structure and abundances of soil microbes involved in nitrification and denitrification. CONCLUSIONS The results suggest that consideration of plant traits, and especially below-ground traits, increases our ability to describe variation in the abundances and the functional characteristics of microbial communities in grassland soils.

Spatial scale and intraspecific trait variability mediate assembly rules in alpine grasslands

Summary 1.Assembly of grassland communities has long been scrutinized through the lens of functional diversity. Studies generally point to an overwhelming influence of climate on observed patterns of functional diversity, despite experimental evidence demonstrating the importance of biotic interactions. We postulate that this is because most observational studies neglect both scale-dependencies of assembly processes and phenotypic variation between individuals. Here, we test for changes in the importance of abiotic filtering and biotic interactions along a stress gradient by explicitly accounting for different scales. In addition to quantifying intraspecific trait variability, we also vary the two components of spatial scale, including grain (i.e. community size) and extent (i.e. the geographic area that defines the species pool). 2.We sampled twenty grassland communities in ten sites distributed along a 975 m elevation gradient. At each site, we measured seven functional traits for a total of 2,020 individuals at different spatial grains. We related community functional diversity metrics to the main environmental gradient of our study area, growing season length, and assessed the dependence of these relationships on spatial grain, spatial extent and intraspecific trait variability. 3.At large spatial grain and extent, the imprint of environmental filtering on functional diversity became more important with increasing stress (i.e. functional diversity decreased with shorter growing season length). At small spatial grain and extent we found a convex relationship between functional diversity and growing season length congruent with the hypothesis that competition is dominant at low stress levels while facilitative interactions are dominant at high stress levels (i.e. high functional diversity at both extremes of the stress gradient). Importantly, the effect of intraspecific variability on assembly rules was noticeable only at small spatial grain and extent. 4.Synthesis. Our study reveals how the combination of abiotic stress and biotic interactions shape the functional diversity of alpine grasslands at different spatial scales, and highlights the importance of phenotype variation between individuals for community assembly processes at fine spatial scale. Our results suggest that studies analysing trait-based assembly rules but ignoring intraspecific trait variability and focusing on a single spatial scale are likely to miss essential features of community diversity patterns. This article is protected by copyright. All rights reserved.

Rhizodeposition of organic carbon by plants with contrasting traits for resource acquisition: responses to different fertility regimes

Background and aimsRhizodeposition plays an important role in mediating soil nutrient availability in ecosystems. However, owing to methodological difficulties (i.e., narrow zone of soil around roots, rapid assimilation by soil microbes) fertility-induced changes in rhizodeposition remain mostly unknown.MethodsWe developed a novel long-term continuous 13C labelling method to address the effects of two levels of nitrogen (N) fertilization on rhizodeposited carbon (C) by species with different nutrient acquisition strategies.ResultsFertility-induced changes in rhizodeposition were modulated by root responses to N availability rather than by changes in soil microbial biomass. Differences among species were mostly related to plant biomass: species with higher total leaf and root biomass also had higher total rhizodeposited C, whereas species with lower root biomass had higher specific rhizodeposited C (per gram root mass). Experimental controls demonstrated that most of the biases commonly associated with this type of experiment (i.e., long-term steady-state labelling) were avoided using our methodological approach.ConclusionsThese results suggest that the amount of rhizodeposited C from plants grown under different levels of N were driven mainly by plant biomass and root morphology rather than microbial biomass. They also underline the importance of plant characteristics (i.e., biomass allocation) as opposed to traits associated with plant resource acquisition strategies in predicting total C rhizodeposition.

Grassland species show similar strategies for sulphur and nitrogen acquisition

Backgrounds and aimsPlant nutrition strategies play a crucial role in community structure and ecosystem functioning. However, these strategies have been established only for nitrogen (N) acquisition, and it is not known whether similar strategies hold for other macronutrients such as sulphur (S). The aim of our study was to determine whether strategies for S acquisition of some grassland species were similar to those observed for N acquisition, and to analyse the relationships between these plant strategies and the soil microbial activity involved in soil organic S mineralisation.MethodsWe used three exploitative and three conservative grass species grown with and without S fertilisation. We measured a set of plant traits, namely root and shoot biomass, leaf area, root length, N and S content, leaf nutrient use efficiency, and sulphate uptake rates in plants, and one microbial trait linked to S mineralisation, namely soil arylsulphatase activity.ResultsThe set of plant traits differentiated exploitative from conservative species. Close relationships were found between traits associated with strategies for N acquisition, namely total N content and Leaf N Use Efficiency (LNUE), and traits associated with strategies for S acquisition, namely total S content and Leaf S Use Efficiency (LSUE). Exploitative species exhibited similar or lower sulphate uptake capacities per unit of biomass than conservative species, but acquired more S through their larger root systems. Greater arylsulphatase activity was observed in the rhizosphere of the most exploitative species.ConclusionOverall, our results show that nutrient strategies defined in grassland species for N acquisition can be extended to S.

Soil legacy effects of climatic stress, management and plant functional composition on microbial communities influence the response of Lolium perenne to a new drought event

Background and aimsDrought events, agricultural practices and plant communities influence microbial and soil abiotic parameters which can feedback to fodder production. This study aimed to determine which soil legacies influence plant biomass production and nutritional quality, and its resistance and recovery to extreme weather events.MethodsIn a greenhouse experiment, soil legacy effects on Lolium perenne were examined, first under optimal conditions, and subsequently during and after drought. We used subalpine grassland soils previously cultivated for two years with grass communities of distinct functional composition, and subjected to combinations of climatic stress and simulated management.ResultsThe soil legacy of climatic stress increased biomass production of Lolium perenne and its resistance and recovery to a new drought. This beneficial effect resulted from higher nutrient availability in soils previously exposed to climatic stresses due to lower competitive abilities and resistance of microbial communities to a new drought. This negative effect on microbial communities was strongest in soils from previously cut and fertilized grasslands or dominated by conservative grasses.ConclusionIn subalpine grasslands more frequent climatic stresses could benefit fodder production in the short term, but threaten ecosystem functioning and the maintenance of traditional agricultural practices in the long term.

The added value of including key microbial traits to determine nitrogen-related ecosystem services in managed grasslands

Despite playing central roles in nutrient cycles and plant growth, soil microbes are generally neglected in the study of ecosystem services (ES), due to difficulties to assess their diversity and functioning. However, to overcome these hurdles, new conceptual approaches and modern tools now provide a means to assess the role of micro-organisms in the evaluation of ES. In managed grasslands, soil microbes are central in providing nitrogen (N)-related ES such as maintenance of soil fertility and retention of mineral forms of N. Here, we applied state-of-the-art techniques in microbial ecology and plant functional ecology to uncover the intrinsic link between N-related bacterial functional groups, important plant functional traits, environmental factors and three proxies of maintenance of soil fertility and potential for N-leaching across managed grasslands in three regions of Europe. By constructing well-defined structural equation modelling, we showed that including key microbial traits improve on average more than >50% of the total variances of ES proxies, that is, ammonium (math formula) or nitrate (math formula) leaching, and soil organic matter content. Geographic differences arose when considering the direct relationships of these ES proxies with specific microbial traits: nitrate leaching was positively correlated to the maximum rate of nitrification, except in the Austrian site and potentially leached math formula–N was negatively correlated to the fungi/bacteria ratio, with the exception of the French site. Synthesis and applications. The integration of soil microbial functional traits in the assessment of nitrogen-related grassland ecosystem services has direct contributions for understanding sustainable management of grassland ecosystems. The fundamental aspects of this study suggest that integrating a soil microbial component in grassland management may enhance sustainability of such grass-based agroecosystems.

Crop traits drive soil carbon sequestration under organic farming

Organic farming (OF) enhances top soil organic carbon (SOC) stocks in croplands compared with conventional farming (CF), which can contribute to sequester C. As farming system differences in the amount of C inputs to soil (e.g. fertilization and crop residues) are not enough to explain such increase, shifts in crop residue traits important for soil C losses such as litter decomposition may also play a role. To assess whether crop residue (leaf and root) traits determined SOC sequestration responses to OF, we coupled a global meta-analysis with field measurements across a European-wide network of sites. In the meta-analysis, we related crop species averages of leaf N, leaf-dry matter content, fine-root C and N, with SOC stocks and sequestration responses in OF vs. CF. Across six European sites, we measured the management-induced changes in SOC stocks and leaf litter traits after long-term ecological intensive (e.g. OF) vs. CF comparisons. Our global meta-analysis showed that the positive OF-effects on soil respiration, SOC stocks, and SOC sequestration rates were significant even in organic farms with low manure application rates. Although fertilization intensity was the main driver of OF-effects on SOC, leaf and root N concentrations also played a significant role. Across the six European sites, changes towards higher leaf litter N in CF also promoted lower SOC stocks. Our results highlight that crop species displaying traits indicative of resource-acquisitive strategies (e.g. high leaf and root N) increase the difference in SOC between OF and CF. Indeed, changes towards higher crop residue decomposability was related with decreased SOC stocks under CF across European sites. Synthesis and applications. Our study emphasizes that, with management, changes in crop residue traits contribute to the positive effects of organic farming (OF) on soil carbon sequestration. These results provide a clear message to land managers: the choice of crop species, and more importantly their functional traits (e.g. leave and root nitrogen), should be considered in addition to management practices and climate, when evaluating the potential of OF for climate change mitigation.

Assessing the dynamic changes of rhizosphere functionality of Zea mays plants grown in organochlorine contaminated soils

The persistent organochlorine pesticides (OCPs) in soils are suspected to disturb soil biogeochemical cycles. This study addressed the dynamic changes in soil functionality under lindane and chlordecone exposures with or without maize plant. Decreases in soil ammonium concentration, potential nitrogen mineralization and microbial biomass were only OCP-influenced in bulk soils. OCPs appeared to inhibit the ammonification step. With plants, soil functionality under OCP stress was similar to controls demonstrating the plant influence to ensure the efficiency of C- and N-turnover in soils. Moreover, OCPs did not impact the microbial community physiological profile in all tested conditions. However, microbial community structure was OCP-modified only in the presence of plants. Abundances of gram-negative and saprophytic fungi increased (up to +93% and +55%, respectively) suggesting a plant stimulation of nutrient turnover and rhizodegradation processes. Nevertheless, intimate microbial/plant interactions appeared to be OCP-impacted with depletions in mycorrhizae and micro/meso-fauna abundances (up to -53% and -56%, respectively) which might have adverse effects on more long-term plant growth (3-4 months). In short-term experiment (28days), maize growth was similar to the control ones, indicating an enhanced plasticity of the soil functioning in the presence of plants, which could efficiently participate to the remediation of OCP-contaminated soils.

Management versus site effects on the abundance of nitrifiers and denitrifiers in European mountain grasslands

It is well established that the abundances of nitrogen (N) transforming microbes are strongly influenced by land-use intensity in lowland grasslands. However, their responses to management change in less productive and less fertilized mountain grasslands are largely unknown. We studied eight mountain grasslands, positioned along gradients of management intensity in Austria, the UK, and France, which differed in their historical management trajectories. We measured the abundance of ammonia-oxidizing bacteria (AOB) and archaea (AOA) as well as nitrite-reducing bacteria using specific marker genes. We found that management affected the abundance of these microbial groups along each transect, though the specific responses differed between sites, due to different management histories and resulting variations in environmental parameters. In Austria, cessation of management caused an increase in nirK and nirS gene abundances. In the UK, intensification of grassland management led to 10-fold increases in the abundances of AOA and AOB and doubling of nirK gene abundance. In France, ploughing of previously mown grassland caused a 20-fold increase in AOA abundance. Across sites the abundance of AOB was most strongly related to soil NO3--N availability, and AOA were favored by higher soil pH. Among the nitrite reducers, nirS abundance correlated most strongly with N parameters, such as soil NO3--N, microbial N, leachate NH4+-N, while the abundance of nirK-denitrifiers was affected by soil total N, organic matter (SOM) and water content. We conclude that alteration of soil environmental conditions is the dominant mechanism by which land management practices influence the abundance of each group of ammonia oxidizers and nitrite reducers.