Agricultural ecosystems could help mitigate climate change by increasing carbon (C) stored in soils via legume cover crops (LCCs) and organic fertilizer application. However, both of these mechanisms require additional water use, and many crops, especially in California, already require active irrigation. Understanding the tradeoffs between yields, C storage, and water use is critical for determining management practices as climate solutions, especially given projections of increased droughts. We test these tradeoffs using the ecosys model to parse out the effects of LCCs and organic fertilizer (composted poultry manure). This model has been rigorously tested in agricultural ecosystems. For this study we applied the model with data from a Mediterranean ecosystem, the long-term Russell Ranch Experimental Farm in the California Central Valley, where the effects of different management practices on yields and ecosystem properties have been studied since 1993. Specifically, we ran the model with site-specific information on soil properties, crop types, climate, and management for the three tomato-corn systems at Russell Ranch (conventional; conventional + LCC; and organic fertilizer + LCC) for ten years, from 1994-2003, and included an additional treatment of organic fertilizer without LCC. We tested the model at this site using measured yields and soil C data. We compare the yields, soil C storage over ten years, and yield water use efficiency (WUEY), which we define as crop yield / transpiration.

The field and modeling data show that the tomato yields were not statistically different over these ten years (p < 0.05), but we found significant differences in WUEY and soil C storage between management treatments. Fields with a LCC had significantly lower yield WUEY ( p < 0.05) and lower soil C storage than those without (p<0.05), regardless of fertilizer type. Increases in soil C were greatest when using organic fertilizer without a LCC, followed by organic with a LCC, conventional without a LCC, and conventional with a LCC. Our conclusions support the use of organic fertilizers but not LCCs for water savings and soil C storage in this particular Mediterranean agro-ecosystem, though LCCs may have other benefits including erosion mitigation, nitrogen fixation, and biodiversity.