Alternate Wetting and Drying of Rice Reduced CH4 Emissions but Triggered N2O Peaks in a Clayey Soil of Central Italy

Reducing CH4 and N20 emissions from rice cropping systems while sustaining production levels with less water requires a better understanding of the key processes involved. Alternate wetting and drying （AWD） irrigation is one promising practice that has been shown to reduce CH4 emissions. However, little is known about the impact of this practice on N20 emissions, in particular under Mediterranean climate. To close this knowledge gap, we assessed how AWD influenced grain yield, fluxes and annual budgets of CH4 and N20 emissions, and global warming potential （GWP） in Italian rice systems over a 2-year period. Overall, a larger GWP was observed under AWD, as a result of high N20 emissions which offset reductions in CH4 emissions. In the first year, with 70% water reduction, the yields were reduced by 33%, CH4 emissions decreased by 97%, while N20 emissions increased by more than 5-fold under AWD as compared to PF; in the second year, with a 40% water saving, the reductions of rice yields and CH4 emissions （13% and 11%, respectively） were not significant, but N20 fluxes more than doubled. The transition from anaerobic to aerobic soil conditions resulted in the highest N20 fluxes under AWD. The duration of flooding, transition to aerobic conditions, water level above the soil surface, and the relative timing between fertilization and flooding were the main drivers affecting greenhouse gas mitigation potential under AWD and should be carefully planned through site-specific management options.

Which are the most favourable conditions for reducing soil CO_(2)emissions with no-tillage?Results from a meta-analysis

No-tillage practices have a recognised beneficial impact on soil and water conservation while reducing erosion processes and enhancing soil organic matter content.However,scientists continue to debate the effectiveness of no-tillage in reducing soil carbon dioxide(CO_(2))emissions from farming.Following the same line of inquiry pursued by the authors who reviewed the impact of conservative practices on direct CO_(2)emissions,we applied meta-analytic and machine learning techniques to unravel the effect of no-tillage under contrasting pedo-environmental conditions and agricultural management.We analysed fifty-six experimental studies investigating direct soil CO_(2)emissions from no-tillage and conventional tillage practices(102 paired observations),considering pedological(soil texture,soil organic carbon content),environmental(climate type),and management(crop rotation,experiment duration)factors.We estimated the effect of different practices on the daily amount of soil CO_(2)emissions,and the impact of tillage in the period immediately following the event.The main insights of this study are:(i)the conditions leading to the highest reduction of CO_(2)emissions due to no-tillage were long-term experiments(standardised mean differenceβ=0.64)conducted in arid environments(β=0.76)and clay soils(β=0.81),with low organic carbon content(β=0.79)where crop rotations(β=0.65)were performed;(ii)the same conditions were associated with the lowest absolute CO_(2)emissions,irrespective of soil management;(iii)the highest contribution to the variability of absolute soil CO_(2)emissions was associated with soil texture(mean decrease in accuracy of Random Forest models,MDA=4.57),rotation(MDA=3.07),experiment duration(MDA=2.93)and soil organic carbon content(MDA=2.24),rather than to tillage practices;(iv)soil CO_(2)emissions almost doubled in the first day after a tillage event,consistently across studies(p=0.001).This meta-analysis offers quantitative figures on the impact of tillage practices on soil CO_(2)emissions and releases data for informing policies aimed at promoting climate change mitigation.