Effects of aridity and plant invasion on soil nematode food webs.

Climatic drying and plant invasions are intensifying globally, yet their combined effects on soil fauna remain poorly understood. Soil nematodes are key components of belowground food webs, contributing to nutrient cycling, organic matter decomposition, and trophic energy transfer. Here, we used paired invaded and non-invaded plots across a natural dry-to-humid climatic gradient to examine how Xanthium strumarium L. invasion was associated with changes in soil nematode communities. Soil moisture and soil organic carbon were greater at more humid sites but were lower in invaded soils. Extractable organic carbon (EOC) increased toward more humid conditions in non-invaded soils, whereas this relationship was weakened in invaded soils. Invaded soils, however, exhibited higher microbial biomass carbon and nitrogen and elevated inorganic nitrogen concentrations. Both drier conditions and invasion were associated with lower total nematode abundance. Nematode diversity and species richness were higher at more humid sites but lower in invaded than in non-invaded plots, with greater invasion-associated declines in both metrics observed at humid sites. The maturity index and functional metabolic footprint of nematode communities were reduced under drier conditions and in invaded soils. Redundancy analysis suggested that EOC was related to variation in nematode community structure. Structural equation modeling indicated negative statistical relationships between X. strumarium invasion and soil nematode communities, including direct relationships and additional relationships involving lower EOC. Overall, these findings highlight the importance of evaluating variation in belowground faunal communities in relation to plant invasion across environmental gradients, especially under ongoing climatic drying.