Which are the most favourable conditions for reducing soil CO2 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 (CO2) emissions from farming. Following the same line of inquiry pursued by the authors who reviewed the impact of conservative practices on direct CO2 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 CO2 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 CO2 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 CO2 emissions due to no-tillage were long-term experiments (standardised mean difference βˆ = 0.64) conducted in arid environments (βˆ = 0.76) and clay soils (βˆ = 0.81), with low organic carbon content (βˆ = 0.79) where crop rotations (βˆ = 0.65) were performed; (ii) the same conditions were associated with the lowest absolute CO2 emissions, irrespective of soil management; (iii) the highest contribution to the variability of absolute soil CO2 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 CO2 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 CO2 emissions and releases data for informing policies aimed at promoting climate change mitigation.