Ammonia emission from staple crops in China as response to mitigation strategies and agronomic conditions: Meta-analytic study

Numerous strategies for mitigating ammonia volatilization (AV) from agricultural land have been investigated with limited evaluation of their efficacy and response to differing field conditions across China. Therefore, this meta-analysis seeks to draw conclusions about AV in China, and assess the effect of crop type (wheat, maize, and rice as staple crops), climatic conditions, and soil properties on AV and variations in responses that can guide selection of appropriate strategies under different conditions. Novel analysis indicates nitrogen (NH₃–N) losses of 7.62 Tg due to AV from these three crops in China. AV is 12.8 and 18.0% (95% CI) higher from maize than from wheat and rice, respectively, without N application under warm and sub-humid conditions (MM), and 13.8 and 2.8% higher under hot and humid conditions (HH). AV increases notably under HH as compared with cold and sub-arid conditions (LL) by 11.5% and 28.4% under the control treatment and application of 150 kg N ha⁻¹, respectively in rice fields while it is most substantially under maize (52.5%) with application of 150 kg N ha⁻¹. The ratio of change in AV between soil grades 3 and 1 (potential AV >1.0 and < 0 cmol kg⁻¹) is significantly greater for wheat and rice (2.5%) than for maize (1.9%). The strategy of reducing N rate is more effective for maize and rice than wheat, where applying N at a rate of 150–220 kg N ha⁻¹ to them has similar effects to applying 300 kg N ha⁻¹ to wheat. For wheat and maize, urea should be partially substituted with organic amendments, while improved urea is the preferable N source for rice. Subsurface application has competitive efficacy, especially for maize, and higher N rates. Applying inhibitors achieves the highest AV reduction, especially with rice under various conditions, creating more actively options for future research to increase applicability.