Soil N2O and NOx emissions are directly linked with N-cycling enzymatic activities

Manure application and crop residue returning have been performed to enhance crop yield and reduce soil reactive nitrogen (N-r) gas (N2O and NOx) emissions. However, the dynamic characteristics of soil nitrogen-cycling (N-cycling) enzyme activities, and the relationship among N-cycling enzyme activities, N-r contents and N-r gas emissions are not clearly understood. Six fertilization regimes, including control (CK-no fertilizer), synthetic N fertilizer alone (N), synthetic nitrogen, phosphorus and potassium fertilizer (NPK), pig manure (OM), pig manure (30% of N rate) with synthetic NPK (70% of N rate) (OMNPK), and crop residues (20% of N rate) with synthetic NPK (80% of N rate) (CRNPK), were used for 11 years to form stable soil conditions. The total N application rate in each fertilization regime, except CK, is the same (130 kg N ha(-1)) in the winter wheat season. The results showed that the cumulative emissions of N2O plus NOx in OM increased significantly by 43.7%, while those in OMNPK and CRNPK decreased by 8.2 and 15.3% compared to conventional fertilization regimes of NPK. The long-term application of manure or returning crop residues to partly replace synthetic N fertilizer may significantly enhance soil hydrolase activities compared with NPK at most stages of wheat growth. In contrast to OM, OMNPK and CRNPK primarily inhibited the activities of nitrate reductase (NR) and nitrite reductase (NiR) but increased hydroxylamine reductase (HyR) activity. Redundancy analysis (RDA) and stepwise multiple linear regression showed positive NR and NiR activities, while PRO activity is negative for N2O emission. NOx emissions were mainly controlled by HyR activities. Thus, soil reactive nitrogen (N-r) gas emissions may be mitigated by different fertilization regimes through the regulation of soil enzyme activities. Fertilization regimes, such as manure application and returning crop residues with synthetic NPK, could be recommended as optimal N fertilization strategies in Regosols to mitigate N-r gas emissions.