DISSERTATION RESEARCH: Woody plant encroachment into grasslands: Impacts on landscape-scale patterns of soil carbon, nitrogen, and phosphorus storage and dynamics

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Lands that receive small amounts of rain or snow occupy approximately 40% of the Earth's land surface. Large portions of these areas are experiencing a replacement of plants that are mostly grasses with increased numbers of trees and shrubs. This change generally increases the amount of carbon found in both plants and soils, and impacts how other elements that are important for plant growth, such as nitrogen and phosphorus, are used; carbon accumulation may also impact climate. Nitrogen and phosphorus are nutrients that often limit the rates at which plants grow, and the supply of one or the other can affect other elements. Despite the importance of carbon, nitrogen and phosphorus in plants and soils, very few studies have been conducted that consider all three elements together in dry landscapes, where the numbers of trees and shrubs are increasing. This project will measure patterns of storage of carbon, nitrogen, and phosphorus within the soils in areas where trees and shrubs have largely replaced grasses. The results will provide knowledge on why trees and shrubs are replacing grasses and what these changes mean for those environments. The information gained by this research will be used to improve mathematical models that describe the interactions of climate and nutrient cycles that in turn can inform the conservation and management of these dry lands. Educational materials about the research will be prepared for the general public in cooperation with the Texas A&M AgriLife Extension program.Modification of soil nutrient pool sizes following woody proliferation has long been of interest to grassland, savanna, and desert ecologists. However, most previous studies examining nutrient dynamics following woody encroachment have been confined to small spatial scales, limited to the uppermost portions of the soil profile, and focused primarily on C and/or N. This research will quantify soil organic C, total N, and total P throughout the entire soil profile in order to make the first assessment of landscape-scale C:N:P soil stoichiometry following woody plant encroachment into grassland. Specific objectives are: (1) Examine whether vegetation cover change alters the 3-dimensional spatial patterns of soil C, N, and P storage at the landscape scale; and (2) Test whether soil P scales isometrically with respect to C and N, and whether these isometric patterns change with soil depth in N-fixer encroached systems. Nutrient stores will be quantified in spatially-specific soil cores taken to a depth of 1.2 meters in a subtropical savanna landscape where N-fixing woody plants have encroached into grasslands during the past century. Results will offer new perceptions on the effects of woody encroachment on interactions between C, N, and P cycles in arid and semi-arid ecosystems across the globe, and enhance our ability to represent these interactions in linked biogeochemistry-climate models.