Impact of Cover Crop and Irrigation Method on Soil Organic Matter Composition and Distribution.

Agriculture faces enormous challenges due to global climate change and associated changes in precipitation patterns. Especially in water-stressed regions, the capacity for crops to respond to periods of drought will impact food security as well as local and regional economies. Climate change impacts on agriculture are a global issue highlighted by the recent drought in California, which cost the state economy $2.7 billion and over 17,000 lost jobs in 2015 alone. Soil, and particularly soil carbon and its microbiome, plays a critical role in agricultural water use efficiency and drought resiliency for crops. The type and amount of organic matter contained in soil is a key determinant of soil structure, but how management (cover cropping, organic amendments), tillage practices, and irrigation methods (furrow, microirrigation) affect soil aggregate formation and stability are not well understood at a mechanistic level. We collected soil samples from the Russell Ranch Sustainable Agricultural Facility and analyzed carbon content in soil size and density fractions of a matrix of management covering irrigation techniques and cover cropping. By utilizing scanning transmission X-ray microscopy (STXM) and C1s near-edge x-ray absorption fine structure (NEXAFS) spectroscopy we determined differences in C chemical composition in both size- and density-fractions of soil representing each management type. In the smallest fractions (<53 microns) cover cropping leads to increased aromatic and aliphatic C at the expense of phenolic and carboxyl C. Improved mechanistic understanding of the role soil carbon plays on aggregate formation and water retention in soil will be generalizable across a variety of soil types and ultimately used to design precision agriculture management strategies that integrate mechanistic understanding of soil carbon and water dynamics in soil.