High Sewage Sludge Loading Drives N2O-Dominated Warming Potential and Destabilizes Soil Microbial Networks during Ryegrass Cultivation

Abstract The application of sludge as a fertilizer contributes to greenhouse gas (GHG) emissions, but the associated microbial and geochemical mechanisms across sludge application rates remain unclear. A field experiment evaluated three sludge application rates (0, 3, and 30 kg/m2) under seedling transplanting (T0, T3, and T30), with an additional direct-seeding treatment (S30) at 30 kg/m2 (wet-weight basis). The results indicated that higher sludge rates elevated cumulative CO2 and N2O emissions, with N2O contributing 69.26–97.74% to the total global warming potential. In contrast, sludge application shifted cumulative CH4 fluxes from net emission in T0 to net uptake in all sludge-amended treatments. S30 produced 1.14 and 2.94 times more CO2 and N2O, respectively, than did T30. The K-edge XANES of calcium revealed dynamic shifts in soil calcium speciation associated with GHG variations, with CaC2O4 (14–23%) peaking at maximum GHG emissions and Cax(C6H8O6)y increasing to 32.8% at later stages, consistent with a calcium pump effect. High sludge application altered microbial diversity, co-occurrence patterns, and network stability. Functional gene analysis indicated an enhanced incomplete denitrification potential through increased norC abundance and reduced N2O reduction capacity. Overall, the sludge application rate was the primary driver of N2O-dominated warming potential, while the planting method exerted additional effects under high sludge loading.