Contrasting Responses of Soil Nematode Trophic Groups to Long-Term Nitrogen Addition

Increases in nitrogen (N) deposition greatly affect soil processes and functions, but there is limited information about the effects of long-term N deposition on soil nematodes. A field experiment was initiated in 2004 with four levels of N addition (0, 60, 120, and 240 kg N ha(-1) y(-1)) in a subtropical Cunninghamia lanceolata forest. After 10 years of treatment, soil samples were collected at three depths (0-20, 20-40, and 40-60 cm), and the community structure, trophic groups, and diversity of soil nematodes was determined. In addition, a meta-analysis with 220 observations from 52 field manipulation experiments was performed to determine whether the results of the current study were consistent with previous results obtained after a range of treatment durations. Long-term N addition significantly increased the abundance of bacterial-feeding nematodes but decreased the abundance of plant-feeding nematodes in the C. lanceolata forest. The plant parasite index and enrichment index decreased but the basal index and channel index increased, which reduced the importance of the bacterial-based energy channel but increased the importance of the fungal-based energy channel as soil depth increased. The meta-analysis confirmed that for treatment durations that did not exceed 10 years, N addition increased the abundance of bacterial-feeding nematodes but reduced the abundance of plant-feeding nematodes. Further analysis of our experimental data indicated that N addition directly affected the abundance of plant-feeding nematodes and indirectly affected the abundances of bacterial-, fungal-, and plant-feeding nematodes via changes in soil nutrients, soil water content, and pH. These findings suggest that N addition can change the abundance and community composition of soil nematodes, which would affect the soil food web and biogeochemical cycling under future global-change scenarios.

Automated summary

Long-term nitrogen deposition can change the abundance and community composition of soil nematodes, affecting the soil food web and biogeochemical cycling. In Cunninghamia lanceolata forest, nitrogen addition increased the abundance of bacterial-feeding nematodes but reduced the abundance of plant-feeding nematodes. Furthermore, nitrogen addition indirectly affected the abundances of bacterial, fungal, and plant-feeding nematodes via changes in soil nutrients, soil water content, and pH.