Isotopic constraints on soil carbon and nitrogen storage among long-term agricultural management practices in California

Carbon (C) and Nitrogen (N) cycling are inextricably linked chemically and biologically, with substantial implications for soil health, climate change and food security. While studies have examined nitrogen and carbon budgets across an array of agricultural management practices, longer-term (decadal) impacts of conventional, cover-cropping and organic inputs on soil carbon and nitrogen patterns remain unclear. Nitrogen in particular displays strong temporal and spatial variations in the soil, making it challenging to extrapolate point measurements to long-term patterns of agricultural nutrient use. Moreover, different management practices could enhance carbon storage, allowing for healthier soil conditions to sustain food production and offset green-house gas emissions. Here, we examine the impacts of organic (manure and cover crop), conventional (synthetic fertilizer), and legume (synthetic fertilizer and cover crop) over nearly 20 years to determine impacts on nutrient depth distribution and concentration. We focus on tomatoes, corn and wheat in long-term acre-scale plots at the Russell Ranch Experiment at the University of California, Davis. We compare measurements of natural abundance of carbon (13C/12C) and nitrogen (15N/14N) from surface to 2 meters of soil depth in 1993 and 2012 across agricultural cropping systems and management practices. We find varying degrees of carbon and nitrogen storage across treatments, particularly in the surface soils (less than 15 cm depth), with especially enhanced nitrogen and carbon concentrations in the organic treatment. Carbon and nitrogen isotopic data reveal evidence for incorporation of organic amendments into longer-term soil carbon pools, with less carbon and nitrogen storage in conventional treatments. Differences in isotopic expression and source-mixing in conventional suggest mobilization of nitrogen to the air and water at a rate that exceeds cover cropping and organic treatments. These results suggest opportunities to alter management practices in a way that reduces losses of nitrogen to the environment and helps to restore and regenerate carbon and nitrogen over the long-term.