Co-occurrence patterns in the N-loading system
A co-occurrence network was built for the identified nematodes at the genus level. Compared with N0, there was a slight rising of positively correlated edges at N1 treatment, then decreased at N2 and N3 treatments (Fig. 4a). N1 treatment had the most positive correlations (74.2%) and network connections (31 edges), while N2 and N3 levels had more negative correlations. The structure values in the N-loading networks were greater than or equal to those in N0 (Fig. 4b).
Pathways determining the abundances of soil nematode trophic groups
SEM analysis was performed to reveal the relationships between the abundance of nematode trophic groups and environmental attributes under a good model fit using χ2 test (χ2=7.55, Fig. 5). As for the four nematode trophic groups, the SEM explained 0.8-42.5% of total variance with N loading. The abundance of the PF group (r2 =0.42) was directly affected by N loading (P <0.001), and was indirectly affected by soil pH under N-loading regulation (P <0.001, Fig. 5a). By altering soil available nutrients and soil water content, N loading significantly changed the abundance of FF (P <0.001,r2 =0.31) and BF groups (P <0.01, r2 =0.43). Moreover, N loading was as a whole beneficial to the fungal- (Fig. 5b; Table S2) and bacterial-feeding (Fig. 5c; Table S2) nematodes, while was detrimental for the plant-feeding nematodes (Fig. 5d; Table S2).