Precision Control of Soil Nitrogen Cycling through Soil Functional Zone Management.

Soil nitrogen (N) cycling is a critical, microbially-mediated ecosystem service underpinning soil fertility. Agronomic management that precisely controls the soil environment to elicit microbial N turnover processes in desired places and times may reduce N loss and increase crop N uptake. Soil functional zone management (SFZM) may offer such control. SFZM separates management of crop row and inter-row positions, creating zones of ‘active-turnover’ and ‘soil building’. The crop row is managed to enhance microbial activity and N turnover; the inter-row is managed to retain N in labile, organic forms. SFZM contrasts with conventional tillage that is applied uniformly across a field. We investigated soil N cycling in corn fields across the northern US Corn Belt, managed under ridge till (RT) and chisel plow (CP), as model SFZM and conventional tillage systems, respectively, with and without cover cropping. Via a series of structural equation models, we determined that SFZM and cover cropping altered N cycling processes at two key points in the growing season. Prior to planting, cover cropping reduced available N. Potentially mineralizable N (PMN) was greatest in RT, and RT also increased available N in the crop row. During the period of peak corn N demand, RT increased available N and PMN in the crop row above that in CP, while reducing N availability in the inter-row. The results demonstrate that SFZM can concentrate available and mineralizable N close to crop roots, while conventional tillage maximizes N away from roots in the inter-row. By creating functionally distinct zones, SFZM regulates soil N cycling more precisely than conventional tillage, and may improve synchrony between soil N availability and crop requirements. Such husbandry of microbial processes may increase agricultural N-use efficiency.