Mechanisms underlying the negative effects of nitrogen addition on soil nematode communities in global grassland ecosystems

Abstract

Reactive nitrogen (N) can significantly influence the richness and abundance of a myriad of organisms; however, it remains unclear how the addition of N affects soil nematodes and their associated ecosystem functions in natural grasslands on a global scale. For this study, we synthesized data on the responses of soil nematode taxon richness and abundance to N addition based on 72 published papers that investigated different grassland ecosystems. Subsequently, we explored the potential mechanisms that underlie the impacts of N addition on soil nematode communities. We found that N addition generally reduced the total nematode taxon richness and abundance of fungal-feeding and omnivorous-carnivorous nematodes in grasslands worldwide. These negative effects of N enrichment on nematodes primarily existed in temperate grassland and meadows, which were influenced through the addition of ammonium nitrate, and became stronger with higher rates of N addition. Further, our structural equation model revealed that the rate of N addition reduced the total nematode taxon richness and abundances of multiple trophic groups (e.g., plant-feeding, bacterial-feeding, fungal-feeding, and omnivorous-carnivorous nematodes) by increasing soil ammonium concentrations and by decreasing microbial biomass carbon, despite the influence of N on plant biomass promoting nematode richness and abundances. In addition, higher soil acidification following the increased N addition rate, directly and indirectly reduced nematode taxon richness and the abundance of different nematode groups, by decreasing plant biomass and microbial biomass carbon. In terms of ecosystem functionality, the negative impacts of N addition on soil nematodes were found to be closely linked with the reduced soil carbon mineralization rate. Taken together, our results suggested that N enrichment negatively impacted the taxon richness and abundance of nematode communities and their associated soil carbon mineralization across global grassland ecosystems. Earth system models that predict belowground communities and linkages to carbon cycling should consider the increases in soil ammonium concentrations and soil acidification under future scenarios of N deposition.