Non Technical Summary
California's agricultural communities face the dual challenges of increasingly variable and limited irrigation water supplies, alongside persistent groundwater quality degradation from nitrate leaching that has imperiled drinking water for nearly one million residents of rural communities in the state. These challenges come together most acutely in California's Central Coast region, the "Salad Bowl" of the U.S., where intensive, high-value vegetable and berry production dominates. The ecosystem services provided by adding winter cover crops to these annual cropping systems are considered essential to improving groundwater quality, but perceptions among farmers and policymakers that cover crops consume too much water make a formidable barrier to adoption in this Mediterranean climate. Here, we propose to address the question of how farmers in Mediterranean, semi-arid climatic regions can implement cover cropping in a water scarce environment while meeting water quality regulatory goals. Through both controlled trials and on-farm, participatory research, we aim to determine how different cover crop management practices affect the balance between water regulation and provisioning and water quality protection, as well as the supply of nitrogen (N) for cash crops. We will form a community of practice, including farmers, technical assistant providers, researchers, and communities lacking access to clean drinking water to test these questions and facilitate co-learning that is embedded in the research process.

Classification

Knowledge Area (KA)	Subject of Investigation (SOI)	Field of Science (FOS)	Percent
102	0210	1070	50%
111	0110	1000	50%

Knowledge Area
102 - Soil, Plant, Water, Nutrient Relationships; 111 - Conservation and Efficient Use of Water;

Subject Of Investigation
0110 - Soil; 0210 - Water resources;

Field Of Science
1070 - Ecology; 1000 - Biochemistry and biophysics;

Goals / Objectives
The overall goals of this proposal are to 1) Assess the impact of varying cover crop management practices on water cycle regulation, water and nitrogen provisioning to the cash crop, and water quality protection and potential tradeoffs among these ecosystem services in a water scarce region 2) Develop a long-lasting community of practice amongst farmers, technical assistance providers, and researchers, which will provide a framework for co-learning the challenges and successes of implementing cover cropping, with the goal of increasing adoption and the resulting ecosystem services. To achieve our overarching research goal of increasing cover cropping in semi-arid regions to better regulate the water cycle and adapt to climate uncertainty while improving the quality of water resources, we will investigate the following three objectives:Objective 1 Evaluate the impacts of cover cropping practices on soil water provisioning and water use, including possible mechanisms, and assess possible tradeoffs with groundwater quality protection.Objective 1.1 Establish replicated, 3-year long research station trial -- "parent trial" -- to compare cover crop management (monoculture vs polyculture, planting timing, cover crop biomass removal). (All personnel)Objective 1.2 Compare changes in soil moisture content during cover crop growth and post termination to determine soil moisture availability for cash crop establishment, and assess how components of the soil water balance drive changes. (Waterhouse, Bowles & Ochoa, with Loik)Objective 1.3 Quantify plant-soil N dynamics across cover crop treatments, including soil nitrate dynamics and potential for leaching during cover crop growth as well as cover crop biomass N uptake and sources (residual soil N vs. newly fixed atmospheric N). (Waterhouse, Bowles & Ochoa)Objective 2 Determine best cover crop management practices to balance regulation of water quality with supporting adequate cash crop N provisioning.Objective 2.1 Within the parent trials, assess how cover crop treatments affect post-termination soil N dynamics, especially potential for mitigating nitrate losses to groundwater while reducing the need for external N inputs for cash crops. (lead Waterhouse, with Bowles & Ochoa)Objective 2.2 Determine the potential for harvested cover crop biomass to be transformed into a liquid fertilizer that better synchronizes cover crop N availability and cash crop N demand. (Waterhouse & Bowles)Objective 2.3 Trace the fate of cover crop N into soil organic and inorganic pools, as well as cash crop biomass, to understand ways to manage cover crop termination and species selection for improved cash crop performance and reduced water quality risks. (lead Waterhouse)Objective 3 Support adoption of cover crops across small to large scale farms while determining key factors that modulate tradeoffs between water use and water quality protection across the agricultural landscape.Objective 3.1 Co-create and facilitate a community of practice on Central Coast farms, organized as a cohort of farmers implementing on-farm comparisons -- "baby trials" -- of farmer-selected cover crop management practices vs. winter fallow. (leads Lozano, Peterson)Objective 3.2 Assess how variation in soil types and management contexts affect indicators of water use and water quality in the baby trials. (lead Waterhouse, with Bowles & Ochoa)Objective 3.3 Facilitate co-learning across farmers, technical assistance providers, and researchers based on farmers' experiences with on-farm trials. (All personnel)We address program goals by focusing on how to increase the provisioning of multiple ecosystem services from agroecosystems that are currently under stress due to climate change. We will conduct outreach to policymakers, regulators, and environmental justice organizations to help incentivize, or at least reduce barriers to, cover cropping to meet water quantity and quality goals.

Project Methods
We will conduct three years of field research at the UCSC Center for Agroecology research farm to compare cover crop planting timing and species vs. bare fallow in intensive vegetable rotations (Obj. 1.1). To assess potential tradeoffs between water use and protection of water quality, we will intensively sample soil water content across and beyond the cover crop growing season until the establishment of the cash crop (Obj 1.2). Using a combination of soil water δ2H/δ18O stable isotope analysis, monitoring of runoff and drainage, and measurements of plot-level evapotranspiration, we will quantify water budgets of cover crop treatments vs. winter fallow to assess mechanisms underlying differences in water use and provisioning (Obj 1.2), paired with estimates of soil nitrate flows and plant δ15N stable isotope analysis to compare sources of cover crop biomass N (Obj. 1.3). For Obj. 2, we will focus on N dynamics in the period between cover crop termination and cash crop establishment with a focus on minimizing N losses while provisioning for bioavailable N to cash crops (Obj. 2.1). With the harvested biomass, we will conduct pilot research on a novel fertilizer product: "juicing" the cover crop biomass to separate liquid--rich with soluble nutrients--from compostable solids, and use greenhouse experiments to understand its N provisioning potential to a vegetable crop (Obj. 2.2). In turn, we will use greenhouse 15N tracer studies to understand how cover crop species (rye vs rye-vetch mixtures) and soil type affect N in different soil pools, as well as the cash crop (Obj 2.3). Simultaneously with this parent trial research, we will establish a Central Coast Cover Cropping Community of Practice (5CoP) to conduct on-farm baby trials on up to 30 farms for two years (Obj. 3.1). On-farm sampling of cover crop treatments will allow us to assess how differences in management histories and soil properties affect tradeoffs between water quality protection and water use (Obj. 3.2). We will facilitate farmer-to-farmer sharing of experiences as well as co-learning among farmers, researchers and technical assistance providers in an iterative process to inform subsequent research (Obj. 3.3).?