Carbon and Nitrogen Dynamics of a Subtropical Savanna Ecosystem: Consequences of Increased Woody Plant Abundance

Program year: 1994/1995Digitized from print original stored in HDRThe abundance of woody plants has increased in recent history in many grassland and savanna ecosystems throughout the world. Woody plants involved in this successional process are often capable of N₂-fixation, such as Prosopis glandulosa. The biogeochemical consequences of grassland invasion by N₂-fixing woody plants may include increased belowground carbon storage and increased N₂O emissions from the soils, both of which may have the potential to influence atmospheric chemistry and climate. At the Texas Agricultural Experiment Station LaCopita Research Area in the Rio Grande Plains of southern Texas, soil samples were collected under mesquite trees at 0-10 cm and 10-20 cm depths. The mesquite trees were harvested and age was determined for each tree by counting its rings. The soil samples were analyzed for organic carbon and total nitrogen using combustion/gas-chromatography. Regressions were developed for nutrient concentrations and nutrient mass/area vs. tree age. Soil organic carbon and total nitrogen concentrations and mass/area increased significantly with tree age. In the closed canopy woodlands, soils are accumulating total nitrogen at a rate of 2.96 g/m²/yr and organic carbon at a rate of 35.84 g/m²/yr. The increase of nutrients beneath Prosopis canopies suggest that the soils are temporarily sequestering a significant quantity of carbon and nitrogen from the atmosphere, and therefore influencing carbon and nitrogen dynamics of this subtropical ecosystem