UC Davis

AbstractAgricultural soils are net sources of atmospheric carbon globally. However, they have the potential to be carbon sequesters if managed using climate-smart agriculture (CSA) management practices that can improve global soil organic carbon (SOC) stocks. These practices are necessary to limit climate change and safeguard soil productivity and global food security. Soil also incorporates a wide range of environments that can comprise diverse microbial communities. Microbes serve important roles in decomposition of C and mineralization of N and are regulated by a variety of abiotic and biotic factors that can be influenced by different fertilizers, such as mineral and organic fertilizers. Additionally, soils serve as a primary sink and source of trace metals, especially in agricultural settings. Agricultural practices can lead to substantial heavy metal addition to soils through the use of fertilizers and pesticides. Whether soils serve as sinks or sources of trace metals depends on a wide range of factors that dictate their retention capacities for these metals. The first chapter of this dissertation explores the effects of tree crop agriculture on soil carbon sequestration, with an emphasis on citrus and avocado groves in Southern California. It was found that tree crop agriculture may be a viable means of balancing both food production and carbon sequestration. The second chapter examines how mineral versus organic fertilizer treatments compare regarding their effect on nitrogen cycling, nitrogen uptake, and microbial communities in avocado and citrus groves. Contrary to expectations, there were no substantial differences observed within the target variables, although this outcome may be due to not much time having passed after fertilization treatments. The third chapter investigated the effect of long-term mineral versus organic fertilizer application on trace metal concentrations in soil through sequential extractions. Both the DTPA and BCR extraction results suggest that organic fertilizer treatments increase bioavailability of Cu. Zn bioavailability was also found to increase with organic fertilizer per the DTPA extraction results and the XRF data, and Cr bioavailability was found to increase with organic fertilizer per the BCR extraction results. Overall, this dissertation demonstrates the potential of tree crop agriculture for soil carbon sequestration and sets the foundation for future research comparing mineral and organic fertilizers on soil properties, with specific insights on how these fertilizers differ regarding trace metal bioavailability.