Trophic Group Specific Responses of Alpine Nematode Communities to 18 Years of N Addition and Codominant Plant Removal

Background and Aims Human-driven nitrogen (N) deposition can alter soil biogeochemistry and plant communities, both critical to soil biota. However, understanding the relative impact of the relationship between nutrient resources and plants on soil communities has been hindered by a lack of experimental manipulations of both factors. We hypothesized that soil nematode communities would be structured predominantly by N addition via overall increased abundance, decreased diversity, and compositional shifts to dominance of r-selected bacterial-feeding nematodes. In contrast, we expected plant effects to be less evident and restricted to nematodes directly associated with plants. Methods We used a long-term (18-yrs) experiment in moist meadow alpine tundra involving N addition and codominant plant (nitrophilic Deschampsia cespitosa and nitrogen-sensitive Geum rossii ) removal. We characterized nematode communities via 18S rRNA metabarcoding and used soil biogeochemistry, plant, and microbial variables to determine factors shaping their communities. Results The N addition treatment increased overall nematode abundance, decreased diversity, and affected the composition of all nematode trophic groups. Overall, nematode communities shifted to dominance of bacterial-feeding nematode taxa adapted to N-enriched environments. The likely drivers of this shift were increased soil nitrate and lower pH. The direct effects of codominant plants were more limited, with only changes in Geum rossii appearing to affect nematode responses. Conclusion Overall, nematode communities in N-limited alpine ecosystems are highly sensitive to increases in N availability, irrespective of the nature of N preferences of codominant plants. The resulting nematode community restructuring could signify future shifts in soil functioning throughout alpine landscapes.