Global patterns and drivers of soil nematodes in response to nitrogen enrichment

Nitrogen (N) deposition significantly lessens ecosystem N limitation, and can even cause N saturation and soil acidification. The effects of N enrichment have been globally assessed for plants, microbial diversity and biomass. However, at higher trophic levels, the global patterns and controlling factors of soil nematodes fed by plants or microbes remain unclear in context of N enrichment. Here, we presented a meta-analysis of 66 N addition experiments on nematode diversity and abundance. Our results revealed a negative dependence of nematode diversity on the N addition rate; however, not for nematode abundance. Across climate gradients, N addition reduced nematode diversity in dry regions but enhanced it in moist regions. Furthermore, N inputs suppressed nematode abundance in cold regions but promoted it in warm regions. Between different trophic levels, the abundance of high trophic-level nematodes (omnivorous-carnivorous) was reduced more than low-level groups (plant-, bacteria- and fungi-feeding nematodes). This occurred through negative N-induced effects, including increased ammonium (NH4+), soil acidification, and reduced microbial biomass. This study indicates that the considerable reduction in nematode communities due to higher N loads in the future will diminish their top-down impacts on plant and microbial activities and even simplify soil food web, particularly in cold and dry areas.