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The impacts of long-term cultivation on soil degradation in the San Luis Valley, Colorado




Daniels, Judith Marie, author
Kelly, Eugene F., advisor
Butters, Gregory, committee member
Melzer, Susan E., committee member
von Fischer, Joe, committee member

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Essentially all agricultural lands globally are under pressure to meet the food demands of an additional 2 billion people over the next 20 years. All of the agroecosystems possess limitations that constrain their ability to optimize production, however, these limitations are magnified in semi-arid regions where permanent, seasonal or periodic moisture deficiency results in evaporation and transpiration rates that exceed precipitation. Traditional cultivation practices that utilize modern technology have resulted in substantial amounts of soil loss through wind and water erosion, decreased soil organic matter, reduction in soil water-holding capacity, and alterations to the microbial community composition. Cultivation also affects soil chemical processes and conditions (e.g., pH, cation exchange complexes, electric conductivity, and sodium adsorption ratio) that can lead to further soil degradation. Changes in one or more of these properties often have direct or indirect effects on the fertility of soils, which influence resiliency and soil health. While research has clearly established the most common modifications to soil systems from cultivation, further investigation is needed in semi-arid regions to identify the critical links between physical, chemical, and biological properties that regulate resiliency and soil degradation. In establishing these critical links, I evaluated the importance of parent material (basalt versus granite) in assessing the impacts on the physical, chemical, and biogeochemical soil properties as a function of cultivation, specifically sprinkler and flood irrigation. I also distinguished microbial community composition by parent material and land use and identified key soil properties that regulate changes in microbial community structure by sampling native and cultivated soils in the San Luis Valley (SLV), located in the South Central part of Colorado. The SLV is a high elevation semi-arid agroecosystem with basalt and granite substrates, that receives 177 mm of precipitation annually and the potential evapotranspiration that exceeds 1016 mm. The SLV has also has a 150-year history of irrigated agriculture practices, which add an additional 153 to 1226 mm of water during the growing season. This alters the natural climate and possibly results in some degree of land degradation. Overall, the results indicate the importance of parent material (basalt vs. granite), as a soil forming factor in assessing the impact of cultivation on soil degradation processes. The initial clay percent in the native soils was 20% for basalt and 18% for granite. The additional accumulation of clay from irrigation was slightly higher for basalt soil, (22%) and 20% for granite soils. Soils derived from basalt have greater quantities of the major cations while soils derived from granite have lower quantities and a poor nutrient status. Soils derived from basalt have greater percentage of soil organic carbon in the soil surface horizons than soils derived from granite. The uncultivated soils derived from basalt classify as saline-sodic while those derived from granite were consistently non-saline, non-sodic. As a function of irrigation, the nutrient concentrations of calcium, magnesium, sodium, potassium, chloride and sulfate were reduced in basalt soils while concentrations increased in granite. In addition, the greatest accumulation of clay and soil organic carbon occurred in granite soils with flood irrigation which resulted in similar concentrations as the basalt soils. Also, basalt soils re-classified as non-saline and non-sodic while those derived from granite remain consistently non-saline non-sodic. These results demonstrate a convergence among the basalt and granite soil properties as a function of land use. Using the ester-linked fatty acid methyl ester (EL-FAMEs), which evaluates differences among soil microbial community composition based on the condition variables of parent material (granite and basalt) and treatments (control, sprinkler, and flood). The results indicated that total microbial biomass and the stress ratios differed between basalt and granite with flood irrigation and the most variation was observed in the basalt-flooded soils. The fungi-to-bacteria ratios were the same in basalt and granite soils and both irrigation types (sprinkler and flood). Arbuscular Mycorrhizal (AM) Fungi did not differ between basalt and granite, however, the concentrations of AM fungi increased in irrigated soils, suggesting alfalfa and pasture hay grasses nurture root biomass. The correlations analysis identified pH, magnesium, sodium, potassium, chloride, and organic carbon as being the primary soil properties associated with the microbial communities in both soils and treatment types. The results from the sensitivity model for microbial communities in granite soils indicated changes in these soil properties were more pronounced pH, magnesium, sodium, potassium, chloride, and soil organic carbon in both sprinkler and flood irrigation. While the microbial communities in basalt soils were sensitive to pH and soil organic carbon in both irrigation practices; the responses were negligible compared to granite soils. Physical soil properties were not significant in determining correlations or sensitivities among the microbial communities. Overall, my data revealed the importance of communally evaluating the physical, chemical, and biological properties in determining the key properties that collectively regulate resiliency and indicate soil degradation. The key indicators in this study are soil texture, bulk density, clay, soil organic matter, sodium, chloride, sulfate, and AM Fungi microbial communities, which provide a benchmark for quantifying the magnitude and directional change of soils in cultivated systems with respect to their native counterparts. The findings revealed that long-term cultivation in the SLV has not degraded the soils according to the indices used. The parameters used this study improve the understanding of long-term irrigation impacts on agroecosystems in arid and semi-arid regions by linking the substrate properties with the soil-forming factors and irrigated water quality. This study provides the key information that can be used as a matrix by which to evaluate the impacts of climate change and a growing global population in other water-limited regions.


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