Responses of soil nematodes to water and nitrogen additions in an old-field grassland

Given that soil water and nitrogen (N) are two major limiting factors for plant growth, changing precipitation regime and atmospheric N deposition may have profound impacts on ecosystem structure and function. However, the effects of changing precipitation regime and atmospheric N deposition on soil fauna abundance and trophic complexity are still poorly understood. A field manipulation experiment was established to investigate the responses of soil nematode communities to water and N additions in an old-field grassland ecosystem. Results showed that water addition significantly increased soil nematode abundance and generic richness, but had no effects on the relative abundance of different trophic groups. Nitrogen addition did not affect nematode abundance, but reduced their generic richness, and significantly altered community structure by promoting the abundance of bacterivores and suppressing that of fungivores and omnivores-predators. Compared to water addition, N addition resulted in a community with higher enrichment index (EI), and lower maturity index (MI) and channel index (CI), consistent with a soil food web with domination of bacterial decomposition channel. The resource availability might be responsible for the observed changes in nematode population size, whereas the alterations in nematode community trophic structure could be attributed to the changes in soil nitrate concentration. Our findings reveal different but independent effects of water and N additions on soil nematode communities in an old-field grassland of North China Plain.