Effects of nitrogen addition and warming on nematode ecological indices: A meta-analysis

Nematodes occupy vital positions in soil food webs and are sensitive to environmental disturbances. Although soil nematode ecological indices are useful indicators of soil environmental conditions and processes, the overall effects of nitrogen addition and warming on nematode ecological indices are unclear. We conducted a meta-analysis to understand how nematode ecological indices are affected by experimental treatments: nitrogen addition and warming. The results showed that nitrogen addition significantly reduced the maturity index (MI) by 7% and the structure index (SI) by 11% but significantly increased the plant-parasitic index (PPI) by 1%. Warming reduced the channel index (CI) by 35% but did not significantly affect the other ecological indices. The effects of nitrogen addition and warming on ecological indices depended on the experimental system (ecosystem, duration, and methods) and environmental factors (mean annual temperature, precipitation, and latitude). The ecological indices were affected more by long-term than by short-term experiments. The MI consistently decreased when NH4+ or urea was added. The addition of NH4NO3 significantly increased the PPI by 1%. The response of ecological indices to nitrogen addition and warming varied among ecosystems. For example, the decrease in SI was significant only in cropland under N addition. The PPI was significantly decreased in forests under warming conditions. In addition, a high level of warming (≥2 °C) increased the MI by 10%. This synthesis increases our understanding of the effects of climate change on the soil environment and ecosystem functions.