Effects of long-term conservation tillage on N and NO emission rates and NO emission microbial pathways in Mollisols.

Conservation tillage is widely used in farmland management for soil carbon sequestration, but it can also lead to potential emissions of nitrous oxide (NO). Therefore, our study is aimed to investigate the effects of 15 years of no-tillage combined with four straw mulching levels of 0 % (NT0), 33 % (NT33), 67 % (NT67), and 100 % (NT100) compared to ridge tillage (RT) on the rates of NO and N emissions and the respective contributions of four microbial pathways to NO emissions. The incubation experiments were conducted at two different moisture levels (55 % and 100 % WFPS) by using dicyandiamide inhibition and N-labeling techniques. Soil samples were collected from the 0-20 cm and 20-40 cm soil depths across three maize growth stages: seedling, jointing, and maturity. Our results showed that conservation tillage significantly decreased the NO + N emission at 55 % WFPS, but it has a reverse influence in NO + N emission at 100 % WFPS. The proportion of NO in gaseous N loss were higher at 100 % WFPS than at 55 % WFPS. Among the four microbial pathways for NO emissions, autotrophic nitrification was the dominant pathway 55 %WFPS. The contribution of autotrophic nitrification remarkably decreased, co-denitrification and denitrification increased at 100 %WFPS. Overall, at 100 % WFPS, NO emissions from all major microbial pathways were positively correlated with GWC, temperature, TC, TN, NH-N, and NO-N, but negatively correlated with soil pH and C/N ratios. Our results suggest that long-term conservation tillage increases NO and N emissions from the soil under water-saturated conditions by regulating soil nutrient levels, soil moisture, and microbial pathways. Therefore, we should consider the impact of conservation tillage on NO emission risk when we attach importance to the role of conservation tillage in improving soil quality and increasing crop yields.