UC Davis

Considerable uncertainty exists on the topic of soil organic carbon (SOC) sequestration in agroecosystems with Mediterranean climate due to the wide range in practices, crops, and soils . These vulnerable yet productive agroecosystems are characterized by low soil organic carbon content. Thus, these soils are considered potential sinks to sequester SOC. To efficiently stabilize SOC, it is well established that the main mechanisms of SOC stabilization and destabilization across soil depths are edaphic factors that affect soil management practices. The first chapter of this dissertation focused on reviewing the main mechanisms of soil organic carbon stabilization within different fractions of soil organic matter, particularly, particulate organic matter (POM) and mineral associated organic matter (MAOM) with the aim of optimizing C sequestration through soil management practices, such as cover cropping. The impact of different cover crop management practices such as species selection, termination time and termination method on long term SOC stabilization are discussed. This chapter sheds light on future research avenues to provide more informed decisions on cover crop management practices that target SOC sequestration in croplands with Mediterranean climate. Given the complexity of soil biogeochemical functions and the escalating need to manage soils for multiple outcomes, we focused on another key nutrient required by plants, which is Phosphorus (P). P has multiple benefits spanning from the metabolic scale, where P plays a crucial role as an energy carrying molecule to its largest role increasing plant yield and root proliferation. Hence, the second chapter focuses on investigating the possibility to use readily available and easy-to-measure soil data such as soil color, clay, and SOC to estimate P sorption index (PSI), which is considered as an important parameter to rationalize P amendments and avoid the environmental risks related of surface water eutrophication, which is primarily due to P overfertilization. Our results showed that there is a great potential to use readily available soil data to predict P sorption capacity. Soil color parameters, soil redness (a) and soil yellowness (b) were found to be closely related to PSI as well as clay content and SOC content. Regression models built using the aforementioned set of predictors performed reasonably well when considering all soil types including Entisols, Mollisols, Vertisols, Alfisols and Ultisols (R2=0.62). The model fit increased (R2= 0.72) when highly developed soils with very high PSI were removed. Results suggest there is great potential to capitalize on the use of standard measurements from characterization by laboratories and observations from the field or soil survey to guide regional patterns in P sorption and availability. Lastly, the third chapter of this dissertation highlights the regional variability of soils’ capacity to stabilize SOC within the fine fraction. In this chapter, we explored variability in soil C saturation deficit, defined as the difference between the theoretical maximum C and current C stored within MAOM along a chronosequence of soils having variable degree of soil development (weakly developed soils of the alluvial fan, moderately developed soils in low terraces, and highly developed soils in high terraces). Results showed a remarkable difference in soil C saturation deficit between the three soil regions, the key difference being mass proportion of fine fraction (clay and silt), and the dominant clay mineralogy. Soils having significantly higher clay and silt with predominantly 2:1 clays had significantly higher soil C saturation deficit. Furthermore, our results showed that for soils far from the saturation limit, increases in soil C inputs didn’t significantly alter soil C saturation deficit. However, for soils saturated or nearly saturated with C, the addition of carbon significantly reduced soil C saturation deficit. Important variation in the most reactive SOC fractions (coarse POMc, fine POMc, and MAOMc) were observed along the soils of the chronosequence having variable degree of soil C saturation deficit. These results emphasize the importance of place-based soil management practices depending on the unique soils’ inherent properties in order optimize soil C sequestration.