Functional traits of soil nematodes define their response to nitrogen fertilization

1. 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. 2. 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 eight-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, i.e., body size, maximum body length, maximum body width,
stylet length, esophagus 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). 3. 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 esophageal 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. 4. 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).