The impact of grazing and fertilization intensity on functional trait diversity and assembly processes of soil mesofauna in grasslands

Assessing the impact of land-use intensification on soil communities is important for developing conservation strategies to maintain grassland ecosystem functioning. We aimed to establish a trait-based mechanistic understanding of how soil community composition responds to varying grazing and fertilization intensity, and whether this alters assembly processes of soil microarthropod metacommunities (Collembola and Oribatida). Microarthropods were sampled at 150 grassland plots of the German DFG Biodiversity Exploratories, covering gradients in three regions (north, centre, south). First, functional community responses (functional richness, functional dispersion) to grazing and fertilization gradients were analysed using a trait-based approach. Hierarchical assembly models then estimated contributions of stochastic (dispersal-driven) vs. niche-based (environmental filtering, limiting similarity) processes. We compared low- vs. high-intensity plots to test whether trait-based responses shifted assembly processes. Trait–environment relationships were assessed using RLQ analyses. Increased grazing reduced functional diversity and increased similarity in collembolan communities. Environmental filtering dominated Collembola assembly in intensively grazed grasslands in north and central Germany, but not in the south. Grazing had no effect on oribatid assembly processes. The strength of environmental filtering was region-specific. Fertilizer input was positively correlated with functional richness in Collembola but not Oribatida. Environmental filtering contributed less to Collembola assembly in intensively fertilized than unfertilized sites, suggesting low nutrient availability limits community structure. Oribatida appeared more resistant to land-use change than Collembola. We conclude that grazing intensification reduces soil microarthropod functional diversity. Under high-intensity use, this promotes environmental filtering (especially for Collembola), altering assembly processes. Medium-intensity grazing reduced some functional groups but communities remained more resilient than at high-intensity sites, where filtering excluded entire groups. We recommend reducing high-intensity grazing to conserve soil biodiversity and ecosystem functioning.