Effect of straw returning on ammonia emissions from soil in a wheat-maize multiple cropping system in the Guanzhong region, China

Reducing agricultural ammonia emissions is considered the most economical and effective method to mitigate haze pollution. Notably, ammonia emissions caused by fertilizer application in agricultural practices are a significant contributor to atmospheric ammonia. In this context, this study aimed to explore the effect of straw returning on ammonia emissions from soil using a winter wheat-summer maize multiple cropping system to provide a theoretical basis for reducing agricultural ammonia emissions to control haze pollution. The study was conducted from June 2018 to June 2019, in the Shaanxi Guanzhong region, China. The experiment used a random block design encompassing different straw returning treatments—no straw returning (S0), half straw returning (S0.5), and full straw returning (S1), as main treatments; and different fertilizer applications—no fertilization (F0), 20% fertilizer reduction (F0.8), and conventional fertilization (F1), as sub-treatments. Soil ammonia emissions, inorganic N (in 0-40 cm of soil), and crop yield, were measured for all treatments. The results showed that straw returning and fertilization, and the interaction between the two, had a significant effect on cumulative ammonia emissions (C) in the summer maize season. Conversely, straw returning had no significant effect on C in the winter wheat season. The C of the entire wheat-maize multiple cropping system was 1.31-19.26 kg·hm^-2, accounting for 2.17%-4.69% of the fertilizer application. Performance among the treatments was as follows: S0F1 > S0.5F1 > S1F1 > S0F0.8 > S0.5F0.8 > S1F0.8 > S1F0 > S0.5F0 > S0F0. Notably, straw returning showed an increase in C without fertilization. However, when compared with no straw returning, straw returning combined with fertilization significantly reduced the C and the amount of ammonia loss that occurred. There was no significant difference in ammonia emissions between full and half straw return treatments. Notably, the ammonia emission reduction effect of S1F0.8 and S0.5F0.8 treatments were the most significant for the study, reducing 38.64% and 37.35% from S0F0.8 treatment, respectively. Straw returning combined with N fertilizer also demonstrated a significant reduction in NO₃^--N and NH₄₊-N in soil, while increasing the yield of summer maize by 6.23%-20.20%, and winter wheat by 16.60%-28.17%. Further to this, PCA analysis indicated that S0.5F0.8 and S1F0.8 treatments were the optimal treatment combinations of those tested for the study, providing a balance between ammonia emission reduction and increased crop production. Therefore, our findings indicate that long-term straw returning combined with fertilizer reduction could improve crop yield as well as reduce soil ammonia emissions, suggesting an agricultural practice that can assist in the reduction of agricultural ammonia emissions, and thus help mitigate haze pollution in the Guanzhong region.