Divergent responses of nematodes in plant litter versus in top soil layer to nitrogen addition in a semi-arid grassland

Nematodes are a crucial component of the soil food web, and play a key role in maintaining biodiversity and regulating soil nutrient cycling. Many studies have been done on the effects of N addition on the soil biota, but the effects of N addition on nematodes in plant litter versus in soil are not well understood, especially in semiarid steppe ecosystems. Here we investigated the composition of nematode communities in plant litter versus in surface soil in two climatically contrasting years in a simulated N enrichment experiment with four N levels (0, 25, 50 and 100 kg N ha(-1) yr(-1)) in a temperate semi-arid grassland. We found that nematodes in plant litter were more sensitive to N addition than that in surface soil, that is, the abundance and generic richness of nematodes in plant litter, but not in surface soil, manifest a significant difference among N addition rates at both community and guild levels. The abundance and generic richness of nematodes were significantly higher in 2015 (a year with relatively wet weather) than 2016 (a year with relatively dry weather) in soil, but the opposite was true on the abundance of nematodes in litter. The plant parasites and omnivores-predators were more abundant, whereas the bacterivores and fungivores were less abundant, in 2015 than 2016. Our results also indicate that litter C/N ratio and soil moisture have greater impact than litter biomass and plant species richness on the abundance of nematodes in litter layer, while soil moisture and resource availability have greater impact than soil N availability and pH on the abundance of nematodes in the top soil. These results provide new insights into the dynamics of nematode communities, and suggest that the litter layer is very important to maintain a stable nematode community in the steppe ecosystem under the fluctuating climate.