Abstract: The nitrogen (N) cycle in an ideal agroecosystem is efficient: it requires the smallest possible N inputs which it derives from the least energy-intensive sources, effectively directs N into biomass production rather than atmospheric or leaching losses, and retains any remaining N in pools that are relatively stable yet also accessible to plants. To achieve such a system, agronomic practices will need to harness biologically-driven soil biogeochemical processes to tighten the agricultural N cycle, reduce N input requirements, supplant synthetic or costly fertilizers, and build long-term soil fertility.
In this presentation we explore the effects of agronomically-feasible management practices on the soil N cycle, particularly as it is mediated by plant-microbe-mineral interactions, focusing on three key aspects of the N budget: inputs, retention/recycling, and accumulation of organic N. 1) Inputs: we synthesize a survey of published values to estimate the potential for free-living diazotrophs to fix N in the rhizospheres of non-leguminous crops and in bulk soil. To this we add estimates of how cover-cropping and intercropping with legumes and/or grasses can fix additional N and/or stabilize dissolved N that would otherwise be vulnerable to loss. 2) Retention and recycling: We consider how agronomic practices can shape plant-microbe interactions to increase storage of organic N in mineral-associated organic matter (MAOM)—in particular in the “active” fraction of MAOM which has short-to-medium residence times—and mine N from this pool during periods of crop demand. 3) Accumulation: we extrapolate long-term increases in soil fertility resulting from the incorporation of cover crop residues and stabilization of plant- and microbe-derived organic N, and predict how building such reserves can help agroecosystems weather global change stressors like drought.
This presentation integrates data with emerging concepts with the intent to stimulate discussion at the intersection of soil ecology, biogeochemistry, and agronomy, and offers a model for a soil N cycle in which organic forms of N predominate and that leverages biological processes to optimize agroecosystem efficiency and sustainability.
