Climate and soil pH modulate global negative effects of nitrogen enrichment on soil nematodes

Abstract

Soil nematodes account for ∼80 % of all Metazoans and play key roles in supporting terrestrial ecosystem functions, yet their responses to pervasive nitrogen (N) deposition, a significant global change factor, and their potential context-dependencies remain unclear. In this study, we conducted a global meta-analysis of 629 paired observations from 91 studies to examine how climatic and pedological factors modulate the effects of N enrichment on soil nematodes, and to assess the spatial patterns of these N-induced effects. We found that N enrichment significantly decreased soil nematode diversity (−3.2 %), maturity index (−6.1 %), structure index (−12.3 %), and the abundance of omnivorous-predatory nematodes (−28.3 %), but had minimal effect on the nematode enrichment index and the abundance of total and other trophic groups of nematodes. The negative effects of N on soil nematodes were greater in grasslands than in forests and croplands, and the effects of NH4NO3 were greater than those of urea. Pedological factors (soil pH and ammonium), climate (mean annual temperature and aridity index), and N enrichment regimes (rates and types of N enrichment) were dominant modulators of N effects. With increasing rate of N enrichment, the effects of N on most soil nematode parameters shifted from negative in dry and cold climates to positive in wet and warm climates, and from positive to negative with increasing background soil pH. Our spatial prediction identified East Asia, South Asia, and Europe as critical regions where soil nematodes are most negatively affected by current N deposition. These findings suggest that N enrichment has global negative effects on soil nematode communities and food web complexity by soil acidification and NH4+ accumulation, which may trigger cascading effects on the entire ecosystem.