The reduction of nitrate (NO3-) to gaseous products in aerated environments (oxic denitrification) has been reported sporadically in the past, mostly in pure culture studies, but the extent to which it occurs in soil is not known. We investigated the emission of nitrous oxide (N2O) due to oxic denitrification in bulk soil under continuous aeration in a soil slurry experiment. Out of 19 soils assayed, a subset of five, representing both cultivated and natural or unmanaged ecosystems, showed emissions of N2O-N derived from NO3- ranging from 0.02 to 1.14 ng per gram of soil per hour. This emission occurred immediately after rewetting and continued during the 8 hour observation period. Although much less attention has been given to oxic denitrification compared to its anaerobic counterpart, our study indicates that the phenomenon, specifically the emission of N2O, is active in a variety of soils. The periods during which denitrification in soils can occur may be far more extensive if both aerobic and anaerobic activities are considered, especially in soils that remain moist (but not necessarily anoxic) for long periods.

Composts are increasingly used as soil amendments to enhance soil health and agricultural sustainability. However, their effects on nitrogen (N) availability, and therefore their fertilizer potential, greenhouse gas (GHG) emissions, carbon (C) sequestration, and crop water use efficiency (WUE) are unclear. We conducted a three-year field experiment in sub-surface drip-irrigated tomatoes to investigate the effects of co-compost (FW; food waste and green waste) and green waste compost (GW) on yield and fertilizer N use, as co-composts are gaining research interest as a means of diverting organic waste from landfills and increasing the nutrient value of GW compost. Three FW and GW compost rates (0, 9 t ha-1, or 18 t ha-1) and four fertilizer N levels (0%, 70%, 85% and 100% of recommended rate) were examined, and treatment combinations were chosen to replace a certain fertilizer N input with compost N. 15N-labeled fertilizer was used to determine fertilizer N crop use efficiency (true NUE), while apparent NUE was determined by comparing crop N uptake between treatments and no compost controls. In years two and three, FW and GW sustained and/or increased yield compared to no compost when low fertilizer rates were applied (0% N and 70% N), with lower apparent and true NUE observed in the compost treatments compared to no compost when 70% N and 85% N was supplied. These results suggest that compost served as an N source or primed the mineralization of soil N, potentially replacing fertilizer N crop uptake. Fertilizer N remaining in topsoil post-harvest was greatest for FW compared to GW and controls, while no difference in nitrate leaching potential was found among treatments, except in year two FW had the lowest nitrate leaching potential. These findings did not consistently produce statistically significant effects but show the potential role of compost in immobilizing fertilizer N and priming soil N mineralization. Within the same field experiment, we also evaluated the effects of FW and GW compost on GHG fluxes and cumulative emissions, soil C content, and nematode populations and diversity as a potential biological mechanism for C sequestration. FW and GW composts had no significant effects on the fluxes or annual cumulative emissions of N2O or CH4, likely because emissions under sub-surface drip irrigation were already low and the considerable variation in low fluxes obscured the detection of significant changes. The application of compost significantly increased soil respiration (carbon dioxide (CO2) emissions). The level of fertilizer N played a significant role in regulating N2O and CH4 emissions, as these fluxes decreased with lower levels of fertilizer N. Soil C content increased with the addition of FW and GW composts compared to the no-compost controls in the 0-15 cm soil layer. This increase was not observed in deeper soil layers of 15-30, 30-60, or 60-90 cm. FW and GW compost treatments reduced δ13C values in the topsoil, indicating that the newly added soil C was derived from the compost. Furthermore, compost application increased the presence of certain individual genera of bacterial- and fungal-feeding nematodes compared to the no-compost controls, offering insights into potential biological mechanisms for C decomposition and sequestration. However, no significant treatment effects were observed on individual genera of nematodes. These findings indicate that although compost may not directly lead to reductions of GHG, it holds potential for mitigating climate change by sequestering C in agricultural soils. We also conducted a one-year field experiment in two, surface drip-irrigated, super-high density olive orchards (in Woodland and Stockton, California, hereafter named the MR and ST sites, respectively) to investigate the effects of compost and fertilizer N management on yield and intrinsic WUE (iWUE; the ratio between net CO2 fixation and stomatal conductance). The olive industry in California is rapidly growing, and there is a need for irrigation and N management guidelines to be updated, taking climate change impacts on the state’s diverse microclimates and increased tree density into consideration. At each field site in our trial, two GW compost rates (0 or 9 t ha-1) and three fertilizer N rates (84, 112, or 140 kg N ha-1 at the MR site, and 28, 42, or 56 kg N ha-1 at the ST site) representing a low, medium, and high fertilizer rate were applied. During the growing season, monthly leaf sampling and concurrent soil sampling were conducted for analyzing δ13C, iWUE, and δ18O, which are proxies for plant water status, and soil gravimetric water content (GWC), NH4+, and NO3-. Compost increased yield compared to no-compost controls at the lowest fertilizer N level at the ST site, while no treatment effects on yield were observed at the MR site. Compared to no-compost controls, compost tended to increase olive δ13C and iWUE at lower fertilizer N levels, indicating increased stomatal closure and plant water stress. In contrast, compost decreased δ13C and iWUE at the highest fertilizer N level, indicating that additional N decreased compost-induced plant water stress. These treatment effects on δ13C and iWUE were more significant at the MR site than the ST site, and the overall iWUE was lower at the ST site than the MR site, indicating less plant water stress in the ST site, likely due to higher precipitation, older tree age, and thus larger root zones for water uptake masking treatment effects. The result of compost increasing iWUE, suggesting that the compost increased water stress for olive trees, was unexpected because compost typically improves soil water-holding capacity for crop uptake. However, in our study, soil GWC results contradicted these typical compost effects on soil water, as there was generally a lack of significant treatment effects on GWC at both field sites. There were also no consistent compost or fertilizer N effects on olive δ18O, an indicator for transpiration, and little to no treatment effects on soil NH4+ or NO3−. Soil NH4+ and NO3− did not show significant correlation with plant iWUE or δ18O, whereas soil GWC was positively correlated with iWUE at the MR site. Our results suggest olive iWUE was regulated more by soil water content than plant-available N content.

The conceptualization of agricultural sustainability has become increasingly complex. Sustainable food production must account for multiple societal imperatives, including meeting the nutritional needs of a growing population, adapting to climate change and mitigating its effects as well as preserving biodiversity in support of natural ecosystems. Ecosystem services refer to the multiple benefits, which humanity obtain from ecosystems. To address challenges facing agriculture, sustainable practices have been developed through scientific research. My dissertation explores the case of cover cropping for perennial systems, as one such science-based practice developed to support agricultural sustainability. Cover crops are plant species that are seeded in addition to the main food crop in agro-ecosystems. Prior literature demonstrates that cover crops can improve nutrient cycling and other soil properties, and can also bring other services, such as refuge for beneficial insects and the suppression of weed species. Despite considerable scientific evidence, there remains low adoption of cover crops in global agriculture. I believe that major gaps between the establishment of scientific evidence and the actual uptake of sustainable practices hinders progress. In the case of cover crops, this lag may in part be due to remaining data gaps hampering the successful establishment and use of the practice. However, I suggest that lack of adoption is due less to large gaps in knowledge, and more a result of conflicting ecosystem service valuation systems among stakeholders. In Chapter 1, I present the results of a farmers’ survey specific to almond production in California. The purpose of this survey was to better understand the factors that affect the decision to use cover crops for almond farmers. Results indicate that while most farmers recognize the many ecosystem services of the practice, concerns over water use and economic costs of the practice can dissuade farmers from adopting the practice. However, the perception of cover crop outcomes was not uniform among farmers. Our results reveal distinct systems of ecosystem service valuation, determined by farm size and regions of California (Sacramento Valley, North San Joaquin Valley and South San Joaquin Valley). In Chapter 2, I conducted a systematic review of cover crop literature to explore scientific rationales for cover cropping. Our analysis of researchers’ knowledge pathways indicates two distinct systems in the valuation of cover crop services: a biological management approach, which considers ecological conservation as inherently valuable, irrespective of immediate profitability, and a nutrient management approach, which prioritizes resource conservation for sustainable crop production. Our results highlight different conceptualizations of agricultural sustainability within literature. Chapter 3 explores how cover crop ecosystem service frameworks vary across commodity types. Our analysis reveals different systems of ecosystem service valuation specific to commodity types and their diverse approaches to agricultural sustainability. Certain commodities (i.e., olive and vineyard systems) focused on cover crop outcomes specific to water use services whereas others prioritized biological control services. These findings emphasize the need to reflect stakeholder priorities in the design of ecosystem service frameworks, to inform their decision to cover crop. This work supports the development of data-based decision-making systems, which reflect diverse approaches to agricultural sustainability.

Can all agriculturally productive soils store an unlimited amount of carbon if we “just use regenerative agriculture practices?” Do tillage and cover cropping increase soil carbon in high-value annual cropping systems California’s mediterranean climate? Does the production of dairy manure compost result in nitrate leaching? Can nitrate be attenuated through natural processes present in some soils? This dissertation addresses these questions that the author tackled during his doctoral research at UC Davis. State agency funding made this research possible through: 1) A California Department of Food and Agriculture (CDFA) – Healthy Soils Program (HSP) – Demonstration Project – Type A and B and 2) A California Department of Water Resources (CDWR) – Funded Project – ‘Dairy Manure Composting in California’. The main findings of his work are in the following pages, but the main theses follow. Much of the existing research on cover cropping and no-till, suggests that no-till increases soil carbon. My data takes that into question and does not support the positive correlation of no-tillage, alone or combined with cover cropping, to organic carbon accumulation in the top 90 cm of soil in the semi-arid to arid mediterranean climate of California. Second, given the reductive effect sub-surface irrigation has already had on nitrous oxide emissions, there does not appear to be a further substantive effect, lowering emissions as a result of no-tillage or cover cropping. There is an excess of dairy manure and the nitrogen and phosphorus it contains can be harmful to the environment if allowed to enter groundwater. Composting has been suggested as an improvement on existing practices which often include static piling. But questions remain about whether composting likely to result in nitrate leaching into groundwater. My research shows that nitrate is produced by composting. However, I also found that soil conditions found in the subsurface attenuated this leached nitrate to background levels through denitrification. I posit that the same would be true at many sites with heterogeneous soil textures including subsurface clay layers. These findings do align with much of the published work on the mechanisms driving denitrification but have not been demonstrated in this way before. The above findings add to the great body of research in Soils and Biogeochemical processes and summarize a great deal of the research I accomplished during my tenure as a graduate student at UC Davis.

This thesis explores different approaches to enhancing soil health and agricultural productivityin California's Central Valley, a region facing significant challenges due to intensive farming practices, soil degradation, and the need for sustainable management strategies. The document is organized into three key studies, each addressing different aspects of soil management and their implications for short-term sustainability.

Background

Methodology/results

Conclusions/significance

Subsurface drip irrigation (SDI), because it can precisely deliver nutrients close to plant roots, could lead to carefully determined applications of fertilizer to meet crop needs and less risk of nitrate (NO₃ ^-) leaching to groundwater. Appropriate fertilizer applications, however, depend on an accurate assessment of the spatial distribution of the main plant macronutrients (N, P and K) in the soil profile before planting. To develop nutrient sampling guidelines, we determined the spatial distributions of preplant nitrate (NO₃ ^-), bicarbonate extractable phosphorus (Olsen-P) and exchangeable potassium (K) in the top 20 inches (50 centimeters) of subsurface drip irrigated processing tomato fields in three of the main growing regions in the Central Valley of California. Nutrient distribution varied with depth (P and K), distance from the center of the bed (NO₃ ^-) and growing region (NO₃ ^- and K). No depletion of NO₃ ^-, Olsen-P or K in the root feeding areas close to the drip tape was detected. Preplant NO₃ ^- ranged considerably, from 45 to 438 pounds N per acre (50 to 491 kilograms/hectare), the higher levels in fields with consecutive crops of tomatoes. A sampling protocol that growers could use, developed from analysis of the distribution results, provided reliable estimates of preplant NO₃ ^- as well as P and K in all surveyed fields.