Conservation agriculture practices drive maize yield by regulating soil nutrient availability, arbuscular mycorrhizas, and plant nutrient uptake

Conservation agriculture (CA) can sustainably increase crop productivity through improved soil chemical, physical, and biological properties, among others. However, the implementation of all its three main components (i.e., no-tillage, organic soil cover/mulch, and crop diversification) in southern Africa is often challenging, resulting in variable yield responses. Disentangling the contributions of CA practices is necessary to understand the drivers of maize grain yield within the region. Here we analysed two 6-year long component omission experiments, one at a sandy soil location and the other at a clay soil location. In these two experiments, soil chemical parameters, total plant nutrient uptake, rate of crop residue decomposition, and arbuscular mycorrhizal fungi (AMF) colonization of maize roots were assessed. Soil chemical properties only differed across systems at the sandy soil location with the mulched systems under no-tillage (NT) resulting in increased soil organic carbon levels, total nitrogen, and soil available phosphorus as compared to conventional tillage with no mulch or rotation (CT). Conventional tillage-based systems resulted in fastest decomposition of maize residues, while systems with NT and rotation resulted in highest AM fungal root colonization rate of maize at the clay soil location. Total plant N uptake was almost 2-fold higher in tilled and no-tilled systems with both mulch (M) and rotations (R) (i.e., NT+M+R and CT+M+R) as compared to CT. Structural equation modeling was used to disentangle the links between cropping systems, soil chemical and biological properties, plant nutrient uptake, and maize grain yield. Cropping systems had direct and indirect influences on yield at both locations. At both locations, cropping systems influenced yield via plant N uptake, with the NT+M+R and CT+M+R systems having more beneficial effects compared to other systems, as shown by their higher path coefficients. In conclusion, we recommend a more holistic approach to cropping system assessment that includes a higher number of abiotic and biotic determinants. This would allow for a more rigorous evaluation of the drivers of yield and increase our understanding of the effects and performance of practices under the prevailing agro-ecological conditions.