The mixed effects of conservation agriculture principles including fertilizer management on greenhouse gas emissions: A scoping review and multi-study synthesis

Context Conservation agriculture (CA) has gained attention over the last decade for its potential to mitigate climate change, among other benefits. However, no clear pattern of CA's effect on greenhouse gas emissions exist, due to complex and interactive processes from CA's three main principles including appropriate fertilizer use as the fourth proposed principle of CA. Objective A comprehensive literature search was conducted to review evidence on the effect of CA's three main principles (reduced tillage, mulching, crop diversification) including appropriate fertilizer management on greenhouse gas (nitrous oxide–N 2 O, carbon dioxide–CO 2 , methane–CH 4 ) emissions. Methods We reviewed 422 out of 3452 publications to assess the relative strength of evidence on effects of reduced tillage, mulching and fertilizer management on greenhouse gas emissions, with an in-depth multi-study synthesis on crop rotation and experimental characteristics. Control treatments included conventional tillage, zero mulch, monocropping and standard mineral nitrogen (N) fertilizer application. Results and conclusions There was good (73%) and some (46%) evidence that reduced tillage practices reduce CO 2 and N 2 O emissions, respectively. There was some evidence that mulching increases N 2 O (46%) and CO 2 (55%) emissions while the effects of different fertilizer management strategies on greenhouse gas emissions were mixed. The multi-study analysis showed an association of nitrogen fertilizer category, N fertilizer rate and overall rotation type with CH 4 emission. However, the CH 4 distribution was highly inconsistent. Overall, continuous legume-legume rotation was associated with decreases in CO 2 compared to continuous cereal-cereal rotations. The cereal-cereal rotation up to and including the current crop was associated with an increase in CO 2 emission relative to a monocrop. Nitrous oxide was associated with the N fertilizer application rate with the medium N fertilizer rate (102–175 kg N ha −1 ) associated with increases in N 2 O compared to the zero N control. Significance Our study highlights the effect of CA's three main principles and appropriate fertilizer use on greenhouse gas emissions under different cropping systems and agroecological regions, and the need for standardised reporting to ensure continued importance of field experiments, particularly long-term experiments, in providing empirical evidence on impacts of agronomic practices on global environmental change.