Trophic reorganization may contribute to shifts in nitrogen transformations following alleviation of saline alkaline stress

Soil salinization is a widespread form of land degradation, yet how saline-alkaline stress is linked to belowground micro-food webs and shapes nitrogen transformations remains poorly understood. Here we compare natural saline–sodic and reclaimed farmland soils in northeastern China using microscopy and genetic sequencing. Farmland has higher nematode and prokaryote abundances and greater nematode, protist and prokaryote richness, whereas fungal richness is higher in solonetz soils. Nitrogen mineralization rates and nitrification potential are 118.2% and 72.3% higher in farmland, respectively. Estimated farmland networks contain more conditional associations, with more consumer–prokaryote but fewer consumer–fungal associations. Inferred consumer associations with nitrogen-cycling genes are more numerous in farmland, involving mainly protists. Nitrogen-cycling gene profiles show greater relative representation of ammonia assimilation, ammonification and nitrification in farmland, whereas denitrification is more represented in solonetz soils. Structural equation models link saline–alkaline stress indirectly to nitrogen processes through soil resources and micro-food web attributes. These findings inform nitrogen-cycling management in salt-affected soils worldwide.