Regulatory Effects of Nitrogen Fertilization on Soil Extracellular Enzyme Activity and Greenhouse Gas Emissions in Paddy Fields with Straw Return

Straw return improves paddy soil quality and nutrient cycling, but its combined effects with nitrogen application on extracellular enzyme activities and greenhouse gas emissions in cold-region paddies remain unclear. A field experiment was conducted in Northeast China under full straw return (8.8 t ha−1) with six nitrogen rates (0, 110, 120, 130, 140, and 150 kg ha−1); conventional nitrogen application without straw return (130 kg ha−1) was the control (CK), while N0 distinguished straw input from nitrogen effects. Soil properties, extracellular enzyme activities, and CO2, CH4, and N2O emissions were measured 20, 50, 80, 110, and 140 days after straw return. At 140 days, compared with CK, straw return increased the NH4+-N and organic matter in the 0–15 cm soil layer by 41.75% and 28.69%, respectively, and reduced pH by 4.34%. Under N110–N150, straw return enhanced the carbon- and nitrogen-acquiring enzymes and oxidative enzymes by 15.88–162.23%. In particular, β-glucosidase, phenol oxidase, and peroxidase activities were significantly higher under N130–N140 than under CK. Compared with N150, N130–N140 maintained organic matter turnover without further increasing greenhouse gas emissions. Overall, under full straw incorporation in the Mollisol paddies of cool Northeast China, N130–N140 sustained high yield while balancing nutrient cycling, enzyme activity, and greenhouse gas mitigation.