Short-term effects of straw return on N₂O emissions from wheat and maize rotation fields in Guanzhong area

Objectives The responses of environmental factors and soil N₂O emissions to short-term straw return were analyzed for more accurate evaluation of the benefits of short-term straw return in terms of N₂O emissions under fertilization.

Methods A field trial was conducted in Guanzhong area, Shaanxi Province, from 2020 to 2021. The cropping system was winter wheat and summer maize rotation. A two-factor split-zone design was adopted, with the main factor of straw return (W1) and no return (W0), and the sub-factor of fertilization, composing 6 treatments, as: no fertilizer (W1, W0), nitrogen fertilizer (W1N, W0N) and nitrogen and phosphorus fertilizer (W1NP, W0NP). Soil water content, temperature, NO₃⁻-N, NH₄⁺-N, and available P content, N₂O emission fluxes were measured, crop yields were investigated, and the relationships between soil N₂O emissions and environmental factors were explored.

Results The soil water content in W1 treatment was 1.1%–16.3% higher than in W0, and the peak soil NO₃^--N content in W1N treatment was 17.6%–30.5% higher than in W0N. The soil NO₃⁻-N and NH₄⁺-N contents of two N and two NP treatments first increased rapidly and then decreased slowly as the growth period progressed, and the peak soil NO₃⁻-N of N treatments was 17.0%−20.8% higher than that of NP treatment. The soil available P content in NP treatments increased rapidly and then decreased slowly as the growth period progressed, and significantly higher than those in no fertilizer and N treatments on average. The soil N₂O emission fluxes arrived peaks at 6 days and 12 days after fertilization in winter wheat and summer maize seasons, respectively. Compared with W0N and W0NP, W1N and W1NP increased the peak soil N₂O emissions by 5.0% and 38.5%, and the annually cumulative N₂O emissions by 291.13 g/hm² and 379.99 g/hm², respectively (P<0.05). The cumulative annual soil N₂O emission in treatment W1N was 298.14 g/hm² higher than in W1NP (P<0.05). W1NP decreased 2.43% and 298.14 g/hm² of the peak and cumulative annual N₂O emission, compare to W1N, while W0NP decreased 54.67% and 386.99 g/hm² of peak and cumulative annual N₂O emission, compared to W0N . Correlation analysis revealed that N₂O emission fluxes were positively correlated with soil temperature and water content in maze season, and NO₃⁻-N, NH₄⁺-N, and available P contents in both maize and wheat seasons (P<0.01). Under the same straw return condition, the N₂O emission intensity showed descent order of N fertilization, NP application, and no fertilization. Under the same fertilization, the N₂O emission intensity with straw return was higher than without straw return

Conclusions Whether straw was returned to the field or not, the combined N and P application was more effective in reducing cumulative N₂O emissions than N fertilizer alone, as the increased soil available P content and lower soil NO₃⁻-N peak content. Although straw returning to the field also increased the soil available phosphorus content in the short term, its higher soil moisture content and NO₃⁻-N content led to higher N₂O emission flux and cumulative N2O emissions from farmland. This short term adverse ecological impact should be considered in the evaluation of straw return.