Contrasting Drivers of Soil Bacterial, Fungal, and Nematode Diversity Along a Litter Stoichiometry Gradient

Human activities are modifying the chemical elements of plant litter, reshaping soil communities in terrestrial ecosystems. This shift most likely impacts ecosystem functions and services, yet, little is known about how litter stoichiometry affects community diversity across various soil trophic groups and their assembly mechanisms. To better understand these linkages, we investigated the diversity of soil microbial and nematode communities in a field experiment where we added six types of plant litter, creating a broad gradient of litter C:N stoichiometry. We particularly focused on community β-diversity, which is important in capturing the variability in species composition among different locations. The results revealed contrasting responses in community β-diversity among soil bacteria, fungi, and nematodes in relation to litter C:N stoichiometry. Specifically, with increasing litter C:N ratio, β-diversity of soil bacteria initially decreased and then increased, while β-diversity of fungi showed a unimodal response, and β-diversity of nematodes showed a declining trend. Stochastic processes were dominant in driving the community assembly of soil bacteria, and this remained relatively stable along the litter C:N stoichiometry gradient. In contrast, deterministic and stochastic processes were co-dominant in driving soil fungal community assembly, with soil nutrients serving as important predictors. Conversely, the relative contribution of stochastic processes in shaping nematode communities decreased with increasing litter C:N ratio, with soil nutrients and habitat (i.e., soil moisture) emerging as key environmental drivers. Taken together, we found that as litter C:N increases, the importance of stochastic processes relative to deterministic processes decreases with trophic levels. These findings provide valuable insights into the mechanisms underlying the assembly and spatial turnover of soil communities under predicted global changes in plant litter stoichiometry.