Effect of fertilizer N rates and straw management on yield-scaled nitrous oxide emissions in a maize-wheat double cropping system

Mitigating greenhouse gas emissions and ensuring crop productivity simultaneously is a major challenge that demands the balance of the amounts of nitrogen (N) and straw (S) applied to agricultural soil. This study seeks to determine whether higher grain yields and lower nitrous oxide (N2O) emissions could be realized concomitantly by optimizing synthetic N rates and straw returning. The effect of fertilizer N rates and straw application on the inter-annual yield-scaled N2O emission variations was measured over a three-year period (2011-2014) in a maize-wheat cropping system on the North China Plain (NCP). Yield-scaled N2O emissions were expressed as g N2O-N per Mg grain. Six treatments with three synthetic N levels (zero N [N0], optimized N [Nopt] and conventional N [Ncon]) and two straw management practices (straw removal [i.e., N0, Nopt and Ncon] and straw return [i.e., N0 + S, Nopt + S and Ncon + S]) were used. Optimized N (Nopt, Nopt + S) refers to the use of approximately 50% of the fertilizer N that is used in conventional farming practices (Ncon, Ncon + S), with no significant decrease in grain yields (P > 0.05). Optimized N reduced cumulative N2O emissions by 18-37%, which in turn significantly decreased yield-scaled N2O emissions by 38-42% (P < 0.05). The effects of fertilizer N rates and grain yields on cumulative N2O emissions are described by linear and exponential models, respectively. Straw return had a positive effect on mean yield-scaled N2O emissions both in the maize season and annually. Yield-scaled N2O emissions are constructive considering the trade-off between grain yield and N2O emission mitigation from intensive crop production. Optimized fertilizer N rate combined with straw return reduced yield-scaled N2O emissions significantly in maize-wheat rotations.