Aboveground fungal pathogens can substantially reduce biomass production in grasslands. However, we lack a mechanistic understanding of the drivers of fungal pathogen infection and impact. Using a grassland global change and biodiversity experiment we show that the trade-off between plant growth and defense is the main determinant of infection incidence. In contrast, nitrogen addition only indirectly increased incidence via shifting plant communities towards faster growing species. Plant diversity did not decrease incidence, likely because spillover of generalist pathogens or dominance of susceptible plants counteracted negative diversity effects. A fungicide treatment increased plant biomass production and high levels of infection incidence were associated with reduced biomass. However, pathogen impact was context dependent and infection incidence reduced biomass more strongly in diverse communities. Our results show that a growth-defense trade-off is the key driver of pathogen incidence, but pathogen impact is determined by several mechanisms and may depend on pathogen community composition.

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1. Nitrogen (N) enrichment has direct effects on ecosystem functioning by altering soil abiotic conditions and indirect effects by reducing plant diversity and shifting plant functional composition from dominance by slow to fast growing species. Litter decomposition is a key ecosystem function and is affected by N enrichment either by a change in litter quality (the recalcitrance of the plant material) or through a change in soil quality (the abiotic and biotic components of the soil that affect decomposition). The relative importance of soil and litter quality and how the direct and effects of N alter them remains poorly known. 2. We designed a large grassland field experiment manipulating N enrichment, plant species richness and functional composition in a full factorial design. We used three complementary litter bag experiments and a novel structural equation modelling approach to quantify the relative effects of the treatments on litter and soil quality and their importance for total decomposition. 3. Our results indicate that total decomposition was mostly driven by changes in litter quality rather than soil quality. Litter quality was affected by the nutrient contents (N and calcium) and structural components of the litter (leaf dry matter content, fibres). N enrichment increased litter decomposition mostly indirectly through a shift in functional composition toward faster growing plant species producing higher quality litter. N enrichment also had effects on soil, by directly and indirectly affected vegetation cover, but this had relatively few consequences for the total decomposition rate. 4. Synthesis. Our approach provides a mechanistic tool to test the drivers of litter decomposition across different ecosystems. Our results show that functional composition is more important than richness or soil quality in determining litter decomposition and that N enrichment effects mainly occur via above-rather than belowground processes. This highlights the importance of considering shifts in plant species composition when assessing the effects of N enrichment 52 on decomposition. 53. CC-BY-NC-ND 4.0 International license It is made available under a (which was not peer-reviewed) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.

Plant functional traits can provide a mechanistic understanding of community responses to global change and of community effects on ecosystem functions. Nitrogen enrichment typically shifts trait composition by promoting the dominance of acquisitive plants (high specific leaf area [SLA] and low leaf dry matter content [LDMC]), translating into high biomass production. Changes in mean trait values can be due to shifts in species identity, relative abundances and/or intraspecific trait values. However, we do not know the relative importance of these shifts in determining trait responses to environmental changes, or trait effects on ecosystem functioning, such as biomass production. We quantified the relative importance of species composition, abundance and intraspecific shifts in driving variation in SLA and LDMC, and how these shifts affected above‐ and below‐ground biomass. We measured traits in a grassland experiment manipulating nitrogen fertilisation, plant species richness, foliar fungal pathogen removal and sown functional composition (slow vs. fast species). We fitted structural equation models to test the importance of abundance and intraspecific shifts in determining (a) responses of functional composition to treatments and (b) effects on above‐ and below‐ground biomass. We found that species intraspecific shifts were as important as abundance shifts in determining the overall change in functional composition (community weighted mean trait values), and even had large effects compared to substantial initial variation in sown trait composition. Intraspecific trait shifts resulted in convergence towards intermediate SLA in diverse communities; although convergence was reduced by nitrogen addition and enhanced by pathogen removal. In contrast, large intraspecific shifts in LDMC were not influenced by the treatments. However, despite large responses, intraspecific trait shifts had no effect on above‐ or below‐ground biomass. Only interspecific trait variation affected functioning: below‐ground biomass was reduced by SLA and increased by LDMC, while above‐ground biomass was increased by SLA. Synthesis. Our results add to a growing body of literature showing large intraspecific trait variation and emphasise the importance of using field collected data to determine community functional composition. However, they also show that intraspecific variation does not necessarily affect ecosystem functioning and therefore response–effect trait relationships may differ between versus within species.

Biodiversity effects on ecosystem functioning can be partitioned into complementarity effects, driven by many species, and selection effects, driven by few. Selection effects occur through interspecific abundance shifts (dominance) and intraspecific shifts in functioning. Complementarity and selection effects are often calculated for biomass, but very rarely for secondary productivity, that is, energy transfer to higher trophic levels. We calculated diversity effects for three functions: aboveground biomass, insect herbivory and pathogen infection, the latter two as proxies for energy transfer to higher trophic levels, in a grassland experiment (PaNDiv) manipulating species richness, functional composition, nitrogen enrichment, and fungicide treatment. Complementarity effects were, on average, positive and selection effects negative for biomass production and pathogen infection and multiple species contributed to diversity effects in mixtures. Diversity effects were, on average, less pronounced for herbivory. Diversity effects for the three functions were not correlated, because different species drove the different effects. Benefits (and costs) from growing in diverse communities, be it reduced herbivore or pathogen damage or increased productivity either due to abundance increases or increased productivity per area were distributed across different plant species, leading to highly variable contributions of single species to effects of diversity on different functions. These results show that different underlying ecological mechanisms can result in similar overall diversity effects across functions.

281. Nitrogen (N) enrichment has direct effects on ecosystem functioning by altering soil abiotic 29 conditions and indirect effects by reducing plant diversity and shifting plant functional 30 composition from dominance by slow to fast growing species. Litter decomposition is a key 31 ecosystem function and is affected by N enrichment either by a change in litter quality (the 32 recalcitrance of the plant material) or through a change in soil quality (the abiotic and biotic 33 components of the soil that affect decomposition). The relative importance of soil and litter 34 quality and how the direct and effects of N alter them remains poorly known. 35 2. We designed a large grassland field experiment manipulating N enrichment, plant species 36 richness and functional composition in a full factorial design. We used three complementary 37litter bag experiments and a novel structural equation modelling approach to quantify the 38 relative effects of the treatments on litter and soil quality and their importance for total 39 decomposition. 40 3. Our results indicate that total decomposition was mostly driven by changes in litter quality 41 rather than soil quality. Litter quality was affected by the nutrient contents (N and calcium) 42 and structural components of the litter (leaf dry matter content, fibres). N enrichment 43 increased litter decomposition mostly indirectly through a shift in functional composition 44 toward faster growing plant species producing higher quality litter. N enrichment also had 45 effects on soil, by directly and indirectly affected vegetation cover, but this had relatively few 46 consequences for the total decomposition rate. 47 4. Synthesis. Our approach provides a mechanistic tool to test the drivers of litter decomposition 48 across different ecosystems. Our results show that functional composition is more important 49 than richness or soil quality in determining litter decomposition and that N enrichment effects 50 mainly occur via above-rather than belowground processes. This highlights the importance 51 3 of considering shifts in plant species composition when assessing the effects of N enrichment 52 on decomposition. 53 54 55

Plant functional traits can provide a more mechanistic understanding of community responses to global change and effects on ecosystem functions. In particular, nitrogen enrichment shifts trait composition by promoting dominance of fast growing, acquisitive plants (with high specific leaf area [SLA] and low leaf dry matter content [LDMC]), and such fast species have higher aboveground biomass production. Changes in mean trait values can be due to a shift in species identity, a shift in species relative abundance and/or a shift in intraspecific trait values. However, we do not know the relative importance of these three shifts in determining responses to global change and effects on function. We quantified the relative importance of composition, abundance and intraspecific shifts in driving variation in SLA and LDMC. We collected leaf samples in a large grassland experiment, which factorially manipulates functional composition (slow vs. fast species), plant species richness, nitrogen enrichment and foliar fungal pathogen removal. We fitted structural equation models to test the relative importance of abundance shifts, intraspecific shifts and sown trait composition in contributing to overall variation in community weighted mean traits and aboveground and belowground biomass production. We found that intraspecific shifts were as important as abundance shifts in determining community weighted mean traits, and even had large effects relative to a wide initial gradient in trait composition. Intraspecific trait shifts resulted in convergence towards intermediate SLA, in diverse communities, although convergence was reduced by nitrogen addition and enhanced by pathogen removal. In contrast, large intraspecific shifts in LDMC were not influenced by the treatments. Belowground biomass was reduced by SLA and increased by LDMC, while aboveground biomass increased in communities dominated by high SLA species. However, despite large intraspecific trait shifts, intraspecific variation in these traits had no effect on above or belowground biomass production. Our results add to a growing body of literature showing large intraspecific trait variation and emphasise the importance of using field sampled data to determine community composition. However, they also show that intraspecific variation does not affect ecosystem functioning and therefore trait response-effect relationships may differ between vs. within species.

Aboveground fungal pathogens can substantially reduce biomass production in grasslands. However, we lack a mechanistic understanding of the drivers of fungal infection and impact. Using a global change biodiversity experiment we show that the trade-off between plant growth and defense is the main determinant of fungal infection in grasslands. Nitrogen addition only indirectly increased infection via shifting plant communities towards more fast growing species. Plant diversity did not decrease infection, likely because the spillover of generalist pathogens or dominance of susceptible species counteracted dilution effects. There was also evidence that fungal pathogens reduced biomass more strongly in diverse communities. Further, fungicide altered plant-pathogen interactions beyond just removing pathogens, probably by removing certain fungi more efficiently than others. Our results show that fungal pathogens have large effects on plant functional composition and biomass production and highlight the importance of considering changes in pathogen community composition to understand their effects.