Nitrous oxide emissions from fruit orchards: A review

Agricultural soils are a dominant source of atmospheric nitrous oxide (N2O). A clear understanding of N2O emission from fruit cropping systems is urgently needed to improve the global budget and establish mitigation options. The primary aims of this study were to (i) quantify the variations in N2O emissions from fruit orchards, (ii) evaluate the major controls of N2O emissions, and (iii) discuss potential mitigation strategies across climates, soil types and field managements. We summarized 123 determinations of N2O emissions under perennial fruit trees from peer-reviewed publications from 1997 to 2019, with measurement periods covering at least one whole growing season. Cumulative N2O emissions ranged widely from -0.116-26 kg N ha(-1) per year or growing season and increased linearly with nitrogen (N) fertilizer input rates on a global basis (r(2) = 0.39, p < 0.001). Climate and form of N fertilizers also contributed to the large variability. In particular, tropical orchards should be a priority for N2O management because they display a large emission factor (approximately 2% of applied fertilizer N). A meta-analysis revealed that the application of nitrification inhibitors significantly reduced N2O emissions by 73% on average (95% confidence intervals: -87% to -51%). The effects of mulching and cover cropping were not significant, mostly depending on the mulch material and species of cover crop along with the management of cropping, which modified the availability of soil water and nutrients. More measurements are highly needed to evaluate the effectiveness of mitigation options across diverse experimental environments.