Reconciling opposing soil processes in row-crop agroecosystems via soil functional zone management

Sustaining soil productivity in agricultural systems presents a fundamental agroecological challenge: nutrient provisioning depends upon aggregate turnover and microbial decomposition of organic matter (SOM); yet to prevent soil depletion these processes must be balanced by those that restore nutrients and SOM (soil building processes). These nutrient provisioning and soil building processes are inherently in conflict; management practices that create spatial separation between them may enable each to occur effectively within a single growing season, thereby supporting high crop yield while avoiding soil depletion. Soil functional zone management (SFZM), an understudied but increasingly adopted strategy for annual row-crop production, may help meet this agroecological challenge by creating spatial heterogeneity in biophysical conditions between crop rows and inter-rows. However, the process-level effects of this spatial heterogeneity on nutrient provisioning and soil building processes have not been characterised. We assessed the magnitude and spatial distribution of nutrient provisioning and soil building processes in model SFZM (ridge tillage) and conventional tillage (chisel plough) systems in four US states encompassing a major global agricultural production region. For soil building we measured bulk density, aggregation and permanganate oxidisable carbon (POXC); for nutrient provisioning we measured microbial decomposition activity, nutrient mineralisation and plant-available nitrogen. After two years, POXC increased under ridge tillage (0–20 cm depth) compared with chisel plough. Ridge tillage also enhanced nutrient provisioning processes in crop rows, increasing plant-available nitrogen in synchrony with maize peak nitrogen demand. Structural equation modelling revealed that improvement in soil building processes under ridge tillage caused rapid enhancement of nutrient provisioning processes in SOM-poor soils. Increases in crop row POXC stimulated microbial decomposition activity, which was associated with increased plant-available nitrogen during the phase of maize peak nitrogen demand. The decimetre-scale spatial heterogeneity created by ridge tillage enables reconciliation of nutrient provisioning and soil building processes in row-crop agroecosystems. In doing so, ridge tillage promotes critical soil processes necessary for increasing the range of ecosystem services provided by intensive production systems. SFZM approaches may have particular value in regions with SOM-poor soils, which would benefit from rapid increases in surface organic carbon. Also, by concentrating and promoting nutrient provisioning processes around crop roots during crop peak nitrogen demand, ridge tillage may enhance nitrogen-use efficiency and reduce current fertiliser requirements