UC Davis

To address the complex challenges facing agricultural sustainability, there is need for adaptive land management strategies that promote sustainable agricultural practices while adapting to changing environmental conditions. This research presents three possible approaches to managing agricultural sustainability at the landscape scale, utilizing the Sacramento Valley of California as a research template. This region was a native wetland habitat that was reclaimed for agricultural production and is now dominated by rice agriculture. The unique climate, topography, and soils of this region make it important habitat for endangered species as well. The three chapters of this dissertation discuss sustainability challenges and potential solutions for this research: (1) Methylmercury dynamics in the region’s water, plants and soil during a summer and winter season; (2) Crop rotation as a solution to managing limited water resources and weed pressure; (3) The drivers and consequences of converting rice fields into perennial tree crops. Each of these three sections is introduced in the following paragraphs.Methylmercury (MeHg) is an environmental toxin often produced in and exported from flooded soils. Little is known about MeHg in agricultural wetlands of the Sacramento Valley, where irrigation drain water flows into sensitive wetlands downstream. Soil, grain, and surface water (dissolved and particulate) MeHg and total mercury (THg) were monitored in six commercial rice fields across this region throughout a winter fallow season and subsequent growing season. Both dissolved and particulate mercury fractions were higher in fallow season rice field-water. Total suspended solids and particulate mercury concentrations were positively correlated (r = 0.99 and 0.98 for THg and MeHg, respectively), suggesting that soil MeHg was suspended in the water column and potentially exported. Filtered THg and MeHg concentrations were positively correlated with absorbance at 254 nm (r = 0.47 and 0.58, respectively) in fallow season field water. In the growing season, fields with higher irrigation water MeHg concentrations (due to recycled water use) also had elevated in-field MeHg (r = 0.86, p < 0.05) and grain MeHg concentrations (r = 0.96, p < 0.01). A mass balance analysis shows that soil mercury pools were orders of magnitude larger than surface water or grain mercury pools; however, fallow season drainage and grain harvest were the primary pathways for MeHg export. Based on these findings, efforts to reduce mercury exports in rice field drainage water should focus on reducing discharge turbidity and the build-up of labile carbon pools during the fallow season. Crop rotation is one strategy for adapting agroecosystems to a framework that balances ecological diversity, sustainability, and food production. The Sacramento Valley, one of the most productive rice growing regions in the US, faces sustainability challenges including increasing herbicide resistant weed pressure and water use restrictions. Increasing crop diversity may help address these challenges, but this region has unique soil attributes including high clay content, salinity, alkalinity, and cemented subsurface layers, and the degree to which these soil properties influence crop rotation decisions remain unclear. The objectives of this study were to quantify the extent of crop rotation in this region, compare soil properties for rotated and continuous rice fields, and assess the potential for expanding rotations based on the geographic coverage of influential soil variables. Using satellite derived land cover data for 2007-2021, our analysis shows that only ~5,000 ha are in rotation with rice, while 220,000 ha are in continuous rice production. This land cover information is fused with SSURGO soil maps in a spatial random forest model. The modeling approach indicates that fields with soil pH between 6.5 and 8, EC between 0.5 and 2 (ds m-1), and saturated hydraulic conductivity less than 2 (μm s-1) are more likely to be rotated. However, we estimate that only 11% of the continuous rice area has all three of these soil properties combined, suggesting soil limitations are an important constraint. To better understand barriers to agroecological diversification, this research highlights a method for evaluating land use decisions in relation to spatial variability of soil properties. Broader land use changes in California’s Central Valley over the past 20 years have been dominated by shifts from annual to periannual cropping systems. However, the Mediterranean climate of in this region is prone to drought conditions, which are increasing in frequency due to climate change. The northern portion of the Central Valley is dominated by commercial rice production. Periannual tree crops such as almond and walnut are increasing in this area, however large portions of the valley have unique wetland-basin soil attributes that make growing non-flooded crops difficult. We use a remote sensing land cover information and spatial soil information to quantify the land use changes in the region, and to better understand the effect of soil type. Our analysis shows that almond and walnut have increased in the area over the past 15 years, but that their area is limited (10% of total area). Crop prices, and revenue per hectare are 3-4 fold higher for almond and walnut compared to rice, which has incentivized some growers to plant these crops. However, our random forest model reveals that continuous rice fields and periannual fields have distinct soil types. Clay is the most important variable in the model, and fields with high clay (>40%) are unlikely to be planted in almond or walnut. Many of the fields with high clay are in the interior basins of the rice growing region, where there are no perennial tree crops. This research provides a framework for evaluating land use decision making to promote sustainable land management.