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Decomposition of dead fine roots contributes significantly to nutrient cycling and soil organic matter stabilization. Most knowledge of tree fine-root decomposition stems from studies in monospecific stands or single-species litter, although most forests are mixed. Therefore, we assessed how tree species mixing affects fine-root litter mass loss and which role initial litter quality and environmental factors play. For this purpose, we determined fine-root decomposition of 13 common tree species in four European forest types ranging from boreal to Mediterranean climates. Litter incubations in 315 tree neighborhoods allowed for separating the effects of litter species from environmental influences and litter mixing (direct) from tree diversity (indirect). On average, mass loss of mixed-species litter was higher than those of single-species litter in monospecific neighborhoods. This was mainly attributable to indirect diversity effects, that is, alterations in microenvironmental conditions as a result of tree species mixing, rather than direct diversity effects, that is, litter mixing itself. Tree species mixing effects were relatively weak, and initial litter quality and environmental conditions were more important predictors of fine-root litter mass loss than tree diversity. We showed that tree species mixing can alter fine-root litter mass loss across large environmental gradients, but these effects are context-dependent and of moderate importance compared to environmental influences. Interactions between species identity and site conditions need to be considered to explain diversity effects on fine-root decomposition.

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International audience; 1. Plant community composition influences soil microbial communities through plant trait variations that lead to changes in nutrient and organic carbon inputs into the soil by root exudates and plant litter. Although plant litter and living roots are known to influence microbial functioning independently, their relative effects are rarely measured simultaneously in naturally occurring plant communities. 2. Here, we sought to evaluate how forest floor litter and absorptive roots affect broad functions of soil microbial communities, and how this may be influenced by tree species mixing. To do so, forest floor litter, absorptive roots, and soil were sampled from mono-specific and 3-species mixed stands in four mature, natural forest ecosystems across Europe. The direct effects of tree species mixing, its indirect effects via litter and root traits, and the effects of soil parameters on microbial biomass, catabolic activity and diversity, and denitrification were analyzed.3. Results did not show direct tree mixture effects on the soil microbial parameters we measured but did suggest indirect influences via tree mixture effects on traits of aboveground litter and absorptive roots and soil parameters. Mixed forests composed of any three tree species modified soil microbial functioning by influencing nutrient availability in forest floor litter and root resource acquisition. Tree mixing also modified soil microbial functioning and catabolic diversity by influencing soil fertility and physicochemical properties. 4. Our findings suggest an indirect but present influence of tree species mixing on the activity of heterotrophic soil microbial communities across four different forest ecosystems ranging from Mediterranean to boreal forests. Our study contributes to a better mechanistic understanding of mixed tree species effects on soil microbial functioning via the modification of forest floor litter properties and traits of absorptive roots represented by the tree community beyond simple species numbers consideration, and potentially via soil properties.

In forest ecosystems, deadwood can improve carbon storage, nutrient availability, and water holding capacity in soils. Yet the effect of organic matter from deadwood such as lignin on these soil functions and their regulators are unknown. We hypothesized that carbon storage, exchangeable cations, and pore space increase with the quantity of lignin-derived phenolic acids from beech deadwood. We also hypothesize that the most pronounced differences occur in more advanced decay classes, in the forest floor at sites with moder forest floors, and in the Ah horizon at sites with mull forest floors. Cupric oxide-oxidation products were used to determine lignin concentration, composition, and oxidation from paired reference and test samples next to 42 downed European beech (Fagus sylvatica L.) deadwood logs in ten stands in Southwest Germany. Compared to reference points, the sum of vanillyl, syringyl and cinnamyl lignin-derived phenols increased next to beech deadwood (within 10–20 cm). The composition and oxidation of lignin-derived phenols also changed near beech deadwood: syringyl/vanillyl ratios increased while cinnamyl/vanillyl and aldehyde/acid ratios for vanillyl decreased. Water-extractable organic carbon (OC) and its aromaticity also increased next to beech deadwood as did total OC and particulate OC separated by density fractionation relative to total and mineral-bound OC. These changes occurred namely in the organic horizons of moder forest floors, and in the Ah horizon underneath mull forest floors. These observations indicated that phenols predominantly entered soil in fluxes of fragmented and dissolved organic matter from beech deadwood. Changes to soil nutrient availability and porosity were linked to increasing lignin-derived phenols from beech deadwood especially in nutrient-poor soils and near heavily decayed deadwood. This is evidence that soils close to beech deadwood, a substrate, are spatially limited pedogenic hot-spots that have increased soil carbon, available nutrients, and pore space depending on the forest floor and parent material.

International audience; Soil fauna communities are major drivers of many forest ecosystem processes. Tree species diversity and composition shape soil fauna communities, but their relationships are poorly understood, notably whether or not soil fauna diversity depends on tree species diversity. Here, we characterized soil macrofauna communities from forests composed of either one or three tree species, located in four different climate zones and growing on different soil types. Using multivariate analysis and model averaging we investigated the relative importance of tree species richness, tree functional type (deciduous vs. evergreen), litter quality, microhabitat and microclimatic characteristics as drivers of soil macrofauna community composition and structure. We found that macrofauna communities in mixed forest stands were represented by a higher number of broad taxonomic groups that were more diverse and more evenly represented. We also observed a switch from earthworm-dominated to predator-dominated communities with increasing evergreen proportion in forest stands, which we interpreted as a result of a lower litter quality and a higher forest floor mass. Finally, canopy openness was positively related to detritivore abundance and biomass, leading to higher predator species richness and diversity probably through trophic cascade effects. Interestingly, considering different levels of taxonomic resolution in the analyses highlighted different facets of macrofauna response to tree species richness, likely a result of both different ecological niche range and methodological constraints. Overall, our study supports the positive effects of tree species richness on macrofauna diversity and abundance through multiple changes in resource quality and availability, microhabitat, and microclimate modifications.

International audience; Mixed-species forests often enhance the provision of ecosystem functions, both above- and below-ground. Several of these effects are mediated by the amount and spatial distribution of tree tissues. However, previous studies on tree diversity effects on fine-root biomass (FRB) have returned inconsistent results and did not distinguish between absorptive and transport fine roots. Furthermore, owing to the lack of species-specific data, it is not well understood whether complementarity or selection effects contribute more to these mixing effects.Here, we analysed tree species mixing effects on fine-root traits while considering the respective tree species contributions and root functional types. Specifically, we tested whether tree species mixing increases FRB and root length density (RLD) and results in vertical root stratification. We quantified FRB and RLD in 30-cm deep soil profiles for 13 tree species in mixed and pure stands across four widespread European forest types. The differentiation of different fine-root species in mixtures allowed us to disentangle complementarity and selection effects.Across all sites, mixtures supported on average less FRB than pure stands, which was reflected in negative complementarity and selection effects. RLD of absorptive fine roots did not change across the soil profile and even increased in the topsoil, which was associated with positive complementarity effects. There was no evidence for vertical root stratification. Conifer proportion, which was mainly associated with selection effects, dampened net diversity effects. Root functional type further influenced tree species mixing effects.Synthesis. Despite the underyielding of FRB in mixtures, overall soil occupation by absorptive fine roots (RLD) did not decrease in mixtures, pointing to morphological root trait adaptations associated with higher resource-use efficiency. Increased RLD in the most nutrient-rich layer in mixtures further indicates complementary interactions among species and a greater resource uptake capacity. This work illustrates that considering only one aspect of trait-functioning relationships, for example, root biomass, may not capture the full effect of plant diversity on ecosystem functioning. The integration of a larger range of relevant traits is required. Moreover, traditional classification of fine roots based on the 2-mm diameter cut-off may obscure responses of roots to environmental changes.

At the global scale, most forest research on biodiversity focuses on aboveground organisms. However, understanding the structural associations between aboveground and belowground communities provides relevant information about important functions linked to biogeochemical cycles. Microorganisms such as soil fungi are known to be closely coupled to the dominant tree vegetation, and we hypothesize that tree traits affect fungal guilds and soil functionality in multiple ways. By analysing fungal diversity of 64 plots from four European forest types using Illumina DNA sequencing, we show that soil fungal communities respond to tree community traits rather than to tree species diversity. To explain changes in fungal community structure and measured soil enzymatic activities, we used a trait-based ecological approach and community-weighted means of tree traits to define 'fast' (acquisitive) versus 'slow' (conservative) tree communities. We found specific tree trait effects on different soil fungal guilds and soil enzymatic activities: tree traits associated with litter and absorptive roots correlated with fungal, especially pathogen diversity, and influenced community composition of soil fungi. Relative abundance of the symbiotrophic and saprotrophic guilds mirrored the litter quality, while the root traits of fast tree communities enhanced symbiotrophic abundance. We found that forest types of higher latitudes, which are dominated by fast tree communities, correlated with high carbon-cycling enzymatic activities. In contrast, Mediterranean forests with slow tree communities showed high enzymatic activities related to nitrogen and phosphorous. Our findings highlight that tree trait effects of either 'fast' or 'slow' tree communities drive different fungal guilds and influence biogeochemical cycles.

Deadwood plays a crucial role in forest ecosystems, yet its impact on soil properties and specifically soil organic matter (SOM) stabilization is hitherto not fully understood or studied. We hypothesized that downed deadwood would enhance the light, labile SOM fraction in forest topsoils, and that those changes would be enhanced by advanced decay and higher rates of soil bioturbation that would move deadwood fragments into mineral soil. To test our hypotheses, we took topsoil samples directly next to European beech ( Fagus sylvatica L.) downed deadwood and samples from paired reference points at eight stands in Southwest Germany. From those samples we separated SOM into three density fractions linked to physical and chemical SOM stabilization processes: the free light fraction, the aggregate-occluded light fraction and the mineral-adsorbed heavy fraction. On silicate bedrock, deadwood increased the free light fraction by 57% ( 6.0 ± 4.2 mg g −1 ) compared to reference points. In contrast on calcareous bedrock, deadwood decreased the free light fraction by 23% ( 9.0 ± 3.5 mg g −1 ) compared to reference points. Deadwood with advanced decay from all sites increased the aggregate-occluded light fraction by 40% ( 3.7 ± 1.1 mg g −1 ) as well as total soil organic carbon (SOC) stocks by 24% ( 12.8 ± 4.5 mg cm −3 ) as compared to reference points. In summary, the light fraction of SOM was affected by deadwood depending on site conditions and the more stable, aggregate-occluded fraction eventually increased near decayed deadwood through interactions between stimulated biological activity and both particulate and dissolved organic matter. Altogether these results indicate that deadwood increases SOC stocks at sites where SOM decomposition is slow enough to enable occlusion of particulate organic matter within aggregates.

Deadwood is a key factor in forest ecosystems, yet how it influences forest soil properties is uncertain. We hypothesized that changes in soil properties induced by deadwood mainly depend on the amount of released phenolic matter. Consequently we expected softwood- and hardwood-related deadwood effects on soil to be explained by unequal enrichment of phenolic substances. We measured differences in the quantity and composition of soil organic matter (SOM), pH, nutrient concentrations, and enzymatic activity between paired control and treatment points influenced by deadwood of silver fir ( Abies alba Mill.) and European beech ( Fagus sylvatica L.), and checked for correlations with total C and phenolic matter; the latter was quantified as aromaticity of water-extractable organic C through specific UV absorbance at 280 nm. Near fir deadwood, aromaticity and effective cation exchange capacity (CEC) increased while pH decreased. In comparison, concentrations of water-extractable organic C, effective CEC, exchangeable Ca 2+ and Mg 2+ , base saturation, and available molybdenum-reactive P increased near beech deadwood while exchangeable Al 3+ decreased. For fir deadwood, soil properties strongly correlated almost exclusively with total C. For beech deadwood, numerous strong correlations with aromaticity indicated that extractable phenolics influenced soil properties. These differences in correlations imply that deadwood affects soil through the composition of added phenolic matter, which would stem from differing decay processes and organisms. Decayed, particulate lignin from brown-rot in fir deadwood as opposed to oxidized, dissolved lignin from white-rot in beech deadwood would account for our observations.