Tightly-Coupled Plant-Soil Nitrogen Cycling and Crop Productivity on Contrasting Organic Farms Across an Intensively-Managed Agricultural Landscape.

Organic production relies on soil microbially-derived ecosystem functions for plant available nutrients, nutrient retention, and ultimately, crop productivity, and thus may be a model system for investigating the ability of soil in managed systems to mediate both provisioning and supporting ecosystem services. The overall objective of this research is to improve understanding of how variability, in terms of management practices and soil characteristics, affect plant-soil-microbial interactions on working organic farms and to consider the implications for ecosystem functions related to crop production and environmental quality. We intensively surveyed 13 organically-managed Roma-type tomato fields in California’s Sacramento Valley over the course of a growing season and measured variables from multiple scales (molecular ecophysiology to landscape). The 13 fields spanned a three-fold gradient in soil C (6.7 to 20.0 g C kg^-1 soil) ranging in soil C:N ratio from 8.05 to 9.71 on soil with similar parent material and little variation in soil texture or pH. Nutrient inputs varied across farms with two general groups based on primary organic matter amendment applied (manure or composted green material). Nine of 13 fields had similar yields (104 ± 3.6 tons ha^-1) that were above average (86 tons ha^-1, both organic and conventional) for the area, but there was substantial variability in both soil chemical and biological variables, including soil inorganic N pools, bioassays for N availability, and microbial activity. Based on multivariate analyses of plant, soil and microbial variables, we propose a typology of N cycling scenarios that characterize these fields and may be applicable to other systems: “N deficit”, “N saturated”, and “tightly-coupled N cycling”. The latter scenario occurred on fields with the highest soil C, above average yields, and extremely low inorganic N pools over the course of the season. These fields also showed higher potential activities of soil N cycling enzymes and high expression levels of glutamine synthetase in tomato roots, a key gene in N metabolism, indicative of N cycling even though pools did not build up. These results demonstrate that select combinations of soil and management simultaneously achieve crop productivity and environmental quality in this landscape. The use of multiple indicators can help differentiate these scenarios and may help farmers navigate a transition from either N deficit or N saturated to tightly-coupled N cycling.