Primarily associated with soil fertility management practices, nitrous oxide (N2O) is a potent greenhouse gas (GHG) whose emission from farmland is a concern for environmental quality and agricultural productivity. In California, agriculture and forestry account for 8% of the total GHG emissions, of which 50% is accounted for by N2O (CEC, 2005). Furrow irrigation and high temperatures in the Central Valley, together with conventional fertilization, are ideal for the production of food, but also N2O production. These conditions can promote N2O emissions, but also suggest great potential to reduce N2O emissions by optimizing fertilizer and irrigation management. Smaller, more frequent fertilizer applications increase the synchrony between available soil nitrogen (N) and crop N uptake and may result in less N loss to the atmosphere. Given that the ecosystem processes regulating the production of N2O respond to and interact with multiple factors influenced by environmental and managerial conditions, it is not always feasible to approach the study of integrated agricultural systems and their affect on GHG emissions by use of a factorial experiment alone. On-farm studies are therefore an important precursor to research station trials to determine which management practices and components of a complete management system should be targeted and isolated for future study. Farm-based trials also provide a realistic evaluation of current management practices subject to practical and economic constraints. The following study took place on existing farms in order to assess the effect of active, operational farm field conditions and current managements on GHG emissions and to thoroughly characterize two typical management systems. In this study, I determined how management practices, such as fertilization, irrigation, tillage, and harvest, affect direct N2O emissions in tomato cropping systems under two contrasting irrigation managements and their associated fertilizer application method, i.e. furrow irrigation and knife injection (conventional system) versus drip irrigation, reduced tillage, and fertigation (integrated system). Field sites were located on two farms in close proximity, on the same soil type, and were planted with the same crop cultivar. This project demonstrated that shifts in fertilizer and irrigation water management directly affect GHG emissions. More fertilizer was applied in the conventional system (237 kg N ha−1 growing season−1) than the integrated system (205 kg N ha−1 growing season−1). The amount of irrigated water was comparable between the two systems; 64 to 70 cm was applied in the conventional system and 64 cm in the integrated system. Total weighted growing season emissions were 3.4 times greater in the conventional system (2.39 ± 0.17 kg N2O-N ha−1) than the integrated system (0.58 ± 0.06 kg N2O-N ha−1), with a higher tomato yield in the integrated system (131 vs. 86 Mg ha−1). The highest conventional N2O emissions resulted from fertilization plus irrigation events and the first fall precipitation. In the integrated system, the highest N2O fluxes occurred following harvest and the first fall precipitation. Environmental parameters of soil moisture, soil mineral N, and dissolved organic carbon (DOC) were higher and more spatially variable in the conventional system. Reduced N2O emissions in the integrated system, resulting from low soil moisture, mineral N concentrations, and DOC levels, imply that improved fertilizer and water management strategies can be effective in mitigating greenhouse gas emissions from agriculture.

Crop rotations, organic nutrient amendments, reduced tillage practices, and integration of cover crops are practices that have the potential to increase the sustainability of crop production, yet they also impact nitrous oxide (N2O) emissions. Agricultural soil management has been estimated to contribute 79% of the total N2O emissions in the U.S., and inorganic nitrogen (N) fertilization is one of the main contributors. Nitrous oxide is a potent greenhouse gas that has a global warming potential which is approximately 298 times that of carbon dioxide (CO2) over a 100-year period and is currently the dominant ozone-depleting substance. Few studies have assessed the effects of organic N amendments on direct N2O within the context of a typical dairy forage cropping system. Most research has been limited to studying the effects of one or two sources of N inputs on N2O emissions; however, dairy forage cropping systems often apply manure and have more than two N sources that likely both contribute to N2O emissions. This study investigated how different dairy cropping practices that include differences in crop residues, N inputs (dairy manure and inorganic fertilizer), timing of N amendment applications and environmental conditions influenced N2O emissions from no-till soil planted to corn (Zea mays L.). A two-year field study was carried out as part of the Pennsylvania State Sustainable Dairy Cropping Systems Experiment, where corn was planted following annual grain crops, perennial forages, and a green manure legume crop; all were amended with dairy manure. In the corn-soybean (Glycine max (L.) Merr.) rotation, N sources (dairy manure and inorganic fertilizer) and two methods of manure application (broadcasted and injected) were also compared.Chapter 1 reviews the scientific literature; describing the biotic and abiotic processes of N2O production in soils, summarizing current research on N2O emissions in agricultural systems, and emphasizing the main management and environmental drivers contributing to the emissions. This chapter reviews methods for matching N supply with crop demand, coupling N flow cycles, using advanced fertilizer techniques, and optimizing tillage management. Also, the applicability and limitations of current research to effectively reduce N2O emissions in a variety of regions are discussed.Chapter 2 analyzes the effect of corn production management practices and environmental conditions contributing to N2O in the Pennsylvania State Sustainable Dairy Cropping Systems Experiment. Significantly higher N2O emissions were observed 15-42 days after manure injection and 1-4 days after mid-season UAN application. Manure injection had 2-3 times greater potential for N2O emissions compared to broadcast manure during this time period. Integration of legumes and grasses in the cropping system reduced inorganic fertilizer use compared to soybean with manure or UAN, however, direct N2O emissions were not reduced. The Random Forest method was used to identify and rank the predictor variables for N2O emissions. The most important variables driving N2O emissions were: time after manure application, time after previous crop termination, soil nitrate, and moisture. These field research results support earlier recommendations for reducing N losses including timing N inputs close to crop uptake, and avoiding N applications when there is a high chance of precipitation to reduce nitrate accumulation in the soil and potential N losses from denitrification.Chapter 3 reports the comparison of N2O fluxes predicted with the biogeochemical model DAYCENT compared to measured data from the two-year dairy cropping systems study. Daily N2O emissions simulated by DAYCENT had between 41% and 76% agreement with measured daily N2O emissions in 2015 and 2016. DAYCENT overestimated the residual inorganic N fertilizer impact on N2O emissions in the corn following soybean with inorganic fertilizer and broadcast manure. Comparisons between DAYCENT simulated and measured N2O fluxes indicate that DAYCENT did not represent well organic N amendments from crop residues of perennials and legume cover crops, or manure application in no-till dairy systems. DAYCENT was generally able to reproduce temporal patterns of soil temperature, but volumetric soil water contents (VSWC) predicted by DAYCENT were generally lower than measured values. After precipitation events, DAYCENT predicted that VSWC tended to rapidly decrease and drain to deeper layers. Both the simulated and measured soil inorganic N increased with N fertilizer addition; however, the model tended to underestimate soil inorganic N concentration in the 0-5 cm layer. Our results suggest that DAYCENT overestimated the residual N impact of inorganic fertilizer on N2O emissions and mineralization of organic residues and nitrification happened faster than DAYCENT predicted. Chapter 4 highlights the impact of manure injection and the importance of timing organic N amendments from manures and/or crop residue with crop N uptake to mitigate N2O emissions. More research is needed to better understand the tradeoffs of these strategies in no till dairy cropping systems to help farmers in their operational management decisions. Improving the parametrization of DAYCENT for dairy cropping systems in no-till systems with high surface legume crop residues from perennials and cover crops, will make the model a more useful tool for testing different mitigation scenarios for farmers and policy-designer decision making.