variation in nematode richness. However, the unchanged ammonium concentration under the N addition observed in our experiment suggests that there should be some other factors regulating nematode generic richness. The positive dependences of nematode generic richness on plant density and plant species richness indicate that the alteration in plant community induced by N addition may be more important than the direct influence of ammonium toxicity. In our study, N addition significantly decreased plant density and species richness, which subsequently leads to a decline in nematode generic richness.
In terms of nematode ecological indices, N addition marginally increased EI, but had no influence on SI. The values of EI could reflect the increased availability of resources to the soil food web and the response of primary decomposers to the resources, which are found directly related to the cumulative amounts of N mineralized in the soil (Ferris and Matute, 2003). In this study, the CI values were significantly decreased by N addition, which is consistent with previous findings that the values of CI in N addition plots were significantly lower than those in control (Azpilicueta et al., 2014; Pan et al., 2015). The changes in CI could be largely explained by the observed variations in the abundance of bacterivores and fungivores. CI has been used as an indicator of decomposition pathways (Ferris et al., 2001). The lower CI implies that N addition drives the soil food web to bacterial decomposition channels. The MI is a sensitive indicator for assessing fertilization disturbance of soil (Bongers et al., 1997). The lower MI under N addition treatment is associated with bacterivore dominance and a reduction in omnivores-predators. The decreasing values of MI suggest that the structure of the nematode community is deteriorating and the complexity of soil food web declines with increasing N deposition.