Global patterns and controls of soil nematode responses to nitrogen enrichment: A meta-analysis

Nematodes are important components of soil food webs and have important ecosystem functions. However, the responses of soil nematode communities to nitrogen (N) deposition have not been well assessed over large spatial scales. Here, we conducted a global-scale meta-analysis of the effects of N-addition regimes (N form, N addition approach, and N addition rate) and environmental factors (ecosystem type, soil depth, climate, and soil property) on the responses of soil nematodes to N addition. The results showed that N addition significantly increased the relative abundance of bacterial-feeding nematodes (23.54%), but decreased the relative abundance of plantfeeding nematodes (-20.24%) and omnivorous-predatory nematodes (-22.28%). Nitrogen addition, however, did not significantly affect total nematode abundance or the relative abundance of fungal-feeding nematodes. Understory N addition usually had larger effects on nematode abundance than canopy N addition. The responses to N addition increased with N addition rate in the case of bacterial-feeding nematode abundance, but decreased with N addition rate in the case of plant-feeding nematode and fungal-feeding nematode abundances. Nematode abundance in grasslands were more responsive to N addition than in forests or croplands, and the responses were affected by soil depth. The response ratios of bacterial-feeding nematodes to N addition decreased with increases in mean annual temperature and mean annual precipitation, while the response ratios of plant-feeding nematodes showed the opposite trend. Soil pH and organic carbon did not affect the response ratios of nematode parameters to N addition except plant parasite index. In summary, the responses of soil nematodes to N addition on a global scale were found to be affected by both N-addition regimes and environmental factors. We suggest that our understanding of how N deposition affects soil nematodes will be increased by additional studies of canopy N addition and of tropical ecosystems.