"Nitrogen inputs, crop N removal, and cumulative N2O emissions were measured from spring 2011 to fall 2012 in three dairy forage production systems receiving liquid and solid manure, as well as synthetic N fertilizer"--Page vii.

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.

Nitrous oxide (N2O) is a powerful greenhouse gas (GHG) produced from soils. It is a natural component of the nitrogen (N) cycle, but emissions are rising due to the increased use of both organic and synthetic N fertilizers in agricultural production systems over the past century. As atmospheric concentrations of N2O continue to climb, its influence on global warming and climate change cannot be denied. Because N2O is produced during soil microbial processes, agricultural production has a great influence on these processes by shaping the soil environment and thus how the microbes are able to process available N. Therefore, knowing the underlying practices responsible for current GHG emissions is necessary to develop best management practices aimed at reducing N2O emissions. Two studies were conducted in a vineyard in Northern California in order to determine how management practices (both conventional and alternative) as well as climatic conditions affect annual and seasonal nitrous oxide emissions within Mediterranean vineyard cropping systems. N2O fluxes were measured using a closed flux chamber method for several days following each management and precipitation event. Furthermore, soil samples were collected to relate N2O emissions with water-filled pore space (WFPS), inorganic N concentration (NO3 and NH4), dissolved organic carbon (DOC), and pH. Cumulative N2O emissions in the no-till (NT) system were greater under both the vine and the tractor row compared to conventional till (CT), with 0.15 ± 0.026 kg N2O-N ha−1 growing season−1 emitted from the CT vine compared to 0.22 ± 0.032 kg N2O-N ha−1 growing season−1 emitted from the NT vine and 0.13 ± 0.048 kg N2O-N ha−1 growing season−1 emitted from the CT row compared to 0.19 ± 0.019 kg N2O-N ha−1 growing season−1 from the NT row. Yet these variations were not significant, indicating no differences in seasonal N2O emissions following conversion from CT to NT compared to long-term CT management. Total annual emissions in the first year, when a leguminous cover crop was planted in the tractor row, totaled 3.92 kg N2O-N ha−1, while emissions in the second year when the tractor rows were fallow showed a 7-fold reduction, reaching only 0.56 kg N2O-N ha−1. During the growing season, fertilization events produced slightly increased emissions compared to the low background values. However, the largest fluxes occurred during the fallow season, in response to the first precipitation event of the year, especially in the tractor rows. Surprisingly, precipitation events in the second year, when the tractor rows were fallow, did not follow the same pattern, indicating the significant influence of the cover crop-derived N on annual N2O emissions. By and large, these studies show that agricultural management practices do have a direct influence on N2O emissions, and that their interaction with climatic conditions greatly influences total N2O emissions within a Mediterranean vineyard cropping system. Furthermore, the vastly increased N2O emissions following leguminous cover crop addition highlights the need to assess the environmental impacts of leguminous cover crops in systems where a crop does not directly follow legume incorporation.

Nitrogen is indispensable to all life on Earth. However, humans now dominate the nitrogen cycle, and nitrogen emissions from human activity have real costs: water and air pollution, climate change, and detrimental effects on human health, biodiversity, and natural habitats. Too little nitrogen limits ecosystem processes, while too much nitrogen transforms ecosystems profoundly. The California Nitrogen Assessment is the first comprehensive account of nitrogen flows, practices, and policies for California, encompassing all nitrogen flows—not just those associated with agriculture—and their impacts on ecosystem services and human wellbeing. How California handles nitrogen issues will be of interest nationally and internationally, and the goal of the assessment is to link science with action and to produce information that affects both future policy and solutions for addressing nitrogen pollution. This book also provides a model for application of integrated ecosystem assessment methods at regional and state (subnational) levels.