Functional traits of soil nematodes define their response to nitrogen fertilization

Nitrogen (N) fertilization and warming are two crucial global change factors affecting the soil nematode communities. The effects of N fertilization and warming, however, on nematode communities in soils are inconsistent across ecosystems and maybe be even opposite. One key reason is that the commonly used taxonomic diversity is less sensitive to environmental changes than the seldom‐used trait‐based indicators. To verify this, we performed an 8‐year field experiment with four N fertilization levels with and without soil warming and collected an extensive dataset consisting of (i) six traits related to the nematode performance, that is, body size, maximum body length, maximum body width, stylet length, oesophagus length and intestinal length; (ii) the taxonomic alpha (richness and abundance) and beta diversity (Bray–Curtis dissimilarities) of the whole nematode community and each nematode functional group, (iii) soil food web resources (the total taxonomic richness and abundance of plant, bacterial and fungal communities) and (iv) soil properties (pH, total, ammonium and nitrate N, microbial N and C, total and available P and soil water content). We found that N fertilization altered plant diversity and soil nitrate levels, which in turn decreased taxonomic alpha diversity of two nematode functional groups (phytophagous nematodes and predators), but taxonomic diversity for the whole nematode community remained stable. The decreased taxonomic alpha diversity of phytophagous nematodes resulted in increased maximum body width, but decreased stylet length and oesophageal length. Mild warming (~ 0.7°C) had no effects on soil properties and soil food web resources, and the taxonomic diversity or nematode traits remained unchanged. Our results reveal that nematode functional traits show strong responses to N fertilization as individual nematode groups adapted quickly to changed soil properties and food web resources. Taxonomic diversity indices, however, were more stable under these changes showing that the functional composition of nematode communities may respond in the short term despite little effects on species diversity. Thus, the trait‐based indicators not only reveal how nematodes respond to N fertilization, but also how nematodes mediate the effects of N fertilization on ecosystem functioning (i.e. soil nutrient cycling). Read the free Plain Language Summary for this article on the Journal blog.