UC Merced

Loss of organic matter (OM) from soil through degradation can lead to decline in soil structure, depletion of plant nutrients, enhancing mineralization of organic compounds and contribute to climate change through the release of greenhouse gas emissions. To address this, it is important to identify and expand our current understanding of the management practices that ensure that the functions and services of soils in agricultural lands are maintained for the long-term. Sustainable agriculture aims to meet society’s needs for food, feed, and fiber, while protecting natural resources and is primarily guided by the quality of and aims to maintain or improve health of soils. A management practice that directly benefits soil quality, through the enhancement of soil organic matter (SOM), is the use of organic matter amendments (OMA). OMAs are a group of organic materials that are rich in nutrients that can be recycled back to the soil and can have positive effects for improving soil conditions and properties, and are proposed as a climate change mitigation strategy. Through extensive field, laboratory studies, and advanced analytical techniques, I show the potential benefits of recycling organic matter amendments into agricultural soils in California. Three projects were established and make up the three chapters of my dissertation: the first project focused on how the application of two different types of composts in almond orchards influenced soil fertility and nutrient cycling. Main findings from this chapter show that the applications of these OMA were heavily influenced by soil texture. After two years of OMA application there was an improvement in soil fertility. The second chapter focused on how long-term application of biosolids in agricultural soils influences carbon content, specifically focusing on the importance of accounting for deep soil carbon in order to determine a soils climate change mitigation potential in three sites in northern California. Main findings show that application rate of biosolids is not a determining factor when it comes to carbon accounting, but management practices play a bigger role. Additionally, not accounting for deep soil carbon leads to an underestimation of carbon sequestration in biosolid amended soils. Chapter 3 builds off Chapter 2, and investigates the stability of this C and N by determining what pools in the soil they are in. These different pools will provide insight on how vulnerable these reservoirs are to management induced changes which can contribute to C and N losses. Overall findings show that long-term amended soils contribute increase carbon content associated to unprotected pools in surface soils where it is easier for microbes to mineralize. Carbon content in deeper soils show that the carbon is associated to physically protected and mineral associated pools, indicating this carbon to be more persistent. Overall, these findings show that site conditions, measurement methodology, and management practices influence the potential of OMA to be beneficial to soil fertility and climate beneficial.