Dynamics of Soil Aggregation, Organic Carbon Pools, and Greenhouse Gases in Integrated Crop-Livestock Agroecosystems in the Texas High Plains

In the Texas High Plains (THP), marked by limited water availability and low soil fertility, management (i.e. tillage, irrigation, crop selection) has great potential to impact soil quality factors, in particular soil organic matter (SOM) and carbon (SOC), and in turn the global C cycle. Soil organic C in whole soil and physically isolated pools can be indicative of a soil’s potential for C sequestration and changes due to management. Measurement of greenhouse gas (GHG) fluxes (CO2 and N2O), can provide insight into soil microbial activity, and allow for tracing losses of SOC. Further analysis of C functional groups using mid-infrared (MidIR) Fourier Transform spectroscopy, a recently (~20 years) developed method, can allow for classification of the C within aggregate fractions. Combination of these methods can provide a detailed outlook of the C interactions and response to management practices in semi-arid systems. The purpose of this research was to thoroughly examine the impacts of alternative agroecosystems on SOC as it relates to aggregation and contributions to GHG concentrations. Conventional production in this area typically consists of continuous cotton (CTN), although the implementation of alternative agroecosystem management practices such as integrated crop-livestock (ICL) systems is growing. Studied systems ranged from those established in 1997 to 2007 and represented various management practices. Specifically, seven systems were selected for monitoring of soil quality factors including mean weight diameter (MWD), aggregate proportions and SOC content, and total nitrogen (TN) content. Additionally, two systems were also utilized for monitoring of GHG fluxes. To examine these changes soil analysis was done at the whole soil level as well as within free aggregates (Elliott, 1986) and intra-aggregate (Six et al., 2000) fractions using physical dispersion. Stability of SOC was examined using the novel technique, MidIR spectroscopy, which can be used to identify C functional groups. Measurements of soil GHG fluxes, specifically CO2 and N2O, were done to aid in the estimation of the global warming potential in these semi-arid systems. Chapters 2 and 3 focus on the impacts of land management practices, including conventional and alternative agroecosystems, in seven systems located in the THP. Chapter 2 examines the changes over time as well as the differences between an ICL and conventional CTN system. Significant increases in SOC were measured within the ICL system, while no significant change was measured in the CTN. In general, MWD and SOC was greatest in systems which utilized alternative management practices (i.e. no-till, perennial vegetation, rotational cropping). Chapter 3 examined the impacts of multiple agroecosystems and associated vegetation components on SOC, aggregate stability, and nutrient content. The complexity of the systems made determination of distinctive impacts difficult. However, similar to the findings in Chapter 2, alternative management techniques resulted in increased SOC content and mean weight diameter. Chapter 4 focuses on fluxes of GHG from two of the systems identified in Chapter 3. This chapter compares fluxes of CO2 and N2O from five vegetation components managed as either irrigated or dryland systems. It was determined that perennial vegetation management resulted in significantly greater fluxes of CO2. In the case of N2O, fluxes were episodic and greatest in bermudagrass, following significant rainfall events but did not contribute significantly to global warming potential. Soil moisture, temperature, and SOC content were the major driving factors for GHG emissions. Chapter 5 examines the use of MidIR to characterize C functional groups from aggregates obtained in Chapter 2. Analysis indicates that SOC within intra-aggregate particulate organic matter was significantly different from all remaining fractions and that further separation based on C functional groups was possible in the intra-aggregate particulate organic matter and silt+clay fractions. The level of degradation associated with the intra-aggregate microaggregate fraction resulted in no significant difference in absorbance spectrum based on vegetation management. Chapter 6 compares the fractionation process when done on field-moist soils (for DNA extraction) and air-dried soils (for SOC analysis). Significant correlation was measured in fractions which produced significantly different results due to pre-fractionation conditions. This correlation may be improved by the inclusion of soil moisture at time of sampling and allow for the estimation of water stable aggregates using fractionation of field-moist soils.