Net Global Warming Potential and Nitrogen Fertigation in Orchards and Vineyards: Opportunities for Mitigation

Permanent cropping systems such as orchards and vineyards offer significant opportunities for mitigation of greenhouse gas emissions. Soils in these systems generally consume the greenhouse gas methane (CH4). Depending on soil and biomass management, the soils and crops can also assimilate carbon from the atmosphere, lowering levels of carbon dioxide (CO2). On the other hand, these systems often emit significant quantities of CO 2, as a result of farm equipment burning fossil fuels. And they typically emit equal or greater quantities of greenhouse gases as nitrous oxide (N 2O), a biogenic gas originating in the soil, which shows increased emissions when nitrogen (N) fertilizers are applied to the soil. This dissertation firstly investigated the net global warming potential of a winegrape vineyard in Oakville, California. It found that minimum tillage could have important effects on soil carbon levels, which could make the system a net greenhouse gas sink for at least 31 years. CO2 emissions from tractor use were the main source of greenhouse gases from the system. N2O was also a significant source, although the majority was not closely associated with events of N fertilization. Nevertheless, N2O derived from N fertilizers is likely to be the principal source of greenhouse gases from more intensively fertilized vineyards, as well as most orchards. In Californian permanent crops the majority of fertilizer is delivered through micro-irrigation systems, a practice termed fertigation. Fertigation offers greater control over N fate but little research has done on its implications for N 2O emissions. Three approaches to nitrogen fertigation were studied in an almond orchard in Belridge, California. Under drip, N2O emissions were less than 0.35% of the N applied, compared to the 1% normally expected with dry fertilizer applications. The use of 20x year-1 high-frequency urea ammonium nitrate applications was not found to produce statistically different N2O emissions from standard practices of 4x year-1. However, high-frequency fertigation of a nitrate-based formulation was seen to cut N2O emissions in half, without apparent increases in nitrate leaching hazards. Soil profile gas samples showed important differences in the origin of N2O from these two fertilizer formulations, illustrating how N2O is produced further below surface with nitrate-based fertilizers and will be consumed at a greater rate before being emitted from the soil surface. Soil microbial tests showed that denitrification capacities of soils increased with frequency of fertigation. Assays under 3% O2 sought to approximate the oxygen-limited conditions of soils under drip irrigation, and results suggested that with ammonium-based fertilizers the majority of N2O emissions from these soils could derive from nitrifier denitrification. Further work was carried out to describe the relevant spatial and temporal patterns of N2O emission under drip fertigation. When temperature was not limiting, N2O emissions from three soils peaked on the first day and followed a regular, exponential course of decline. In this context fitted exponential curves were shown to lead to more accurate emissions estimates than common trapezoidal calculations. An experiment on two soils found that under a soil with 26% clay, the timing of injection of fertilizer within a fertigation had a significant effect on spatial distribution of emissions. The distribution aligned closely with predictions of nitrate presence near surface carried out with the HYDRUS 2D hydrological model. The results of the work under drip fertigation are relevant not only to understanding dominant mechanisms in N2O emission from drip fertigation, but to efforts to model the production of N2O with hydrological programs.