| Nitrogen loss from agriculture to the environment has negative ecological consequences. While much research has been conducted to determine the sources of and minimize the loss from conventional production systems, little work has been done for systems that rely on organic fertility amendments. In this dissertation I present work from several studies with the overarching goal of providing information to organic vegetable growers to help optimize fertility management, using farms from California's Central Coast as example agroecosystems. Following the introduction, Chapter 2 presents data on soil and plant N dynamics in fields with and without winter legume-cereal cover crops, as well as overall nutrient budgets to compare N inputs and outputs on two farms. Our results suggest there is both fall and spring NO3-N leaching from the top 90 cm of the soil profile, and that cover crops reduce leaching losses. We also saw positive average annual N balances of 51 and 153 kg N/ha for the two farms studied. Chapter 3 examines one of the N inputs, biological N fixation (BNF) from legume cover crops, in greater detail. Through greenhouse and field studies using the natural abundance 15N isotopic method, we found high rates of BNF for bell bean (Vicia faba) and woollypod vetch (Vicia dasycarpa), even in soils with high inorganic N concentrations. We also found evidence of early BNF activity in seedlings, including in soils with high available inorganic N. The BNF estimates we saw were 57 to 98% for bell beans and 61 to 98% for vetch, higher than the common rule of thumb of 50%. This difference in BNF estimates has a significant effect on N budget balances. The δ15N of the reference plants that were used to calculate BNF varied greatly and was not related to cover crop management history and only weakly related to time since the most recent compost application. In Chapter 4, puts the work in a larger context, examining how organic vegetable growers are co-judged with their products on agronomic and environmental quality. Finally, Chapter 5 draws conclusions about the work and offers suggestions for future lines of inquiry. |
