Many grass-dominated ecosystems around the world have experienced woody plant encroachment over the last century due to livestock grazing, fire suppression, and/or changes in climate and atmospheric chemistry. In the Rio Grande Plains of Texas, subtropical thorn woodlands dominated by N-fixing tree legumes have largely replaced grasslands and altered the biogeochemistry of this region. The purpose of this study was to assess the impact of this grassland-to-woodland transition on the size and distribution of soil C, N, and P pools across five landscape elements of a mesquite-invaded grassland. Soil samples (0-10 cm) were collected in remnant grasslands and near the centers of woody plant clusters, groves, drainage woodlands, and playas ranging in age from 14 to 134 years in a subtropical savanna parkland in southern Texas. Soil organic C and total N were measured by elemental analysis, and total P was measured by lithium fusion followed by phospho-molybdate colorimetry.
Results/Conclusions
Concentrations of soil C, N, and P increased linearly in all wooded landscape elements with time following woody encroachment. Mean soil organic C increased between 320 and 460% in wooded areas compared to remnant grasslands. Similarly, soil total N increased 285-535% and total P increased 160-340% in wooded areas. The most dramatic increases were observed in the lowland drainages and playas, while C, N, and P increased at slower rates in upland woody clusters and groves. Woody plant invasion likely increases C and N pools in the soil through increased NPP and symbiotic N-fixation by legume species, respectively. Soil P likely increases because the more deeply rooted woody plants are acquiring P from deep in the soil profile and transferring it into the upper portion of the profile via litterfall and root turnover. Since N and P are generally the most limiting nutrients in terrestrial ecosystems, increased stores of these elements are likely to alter rates of microbial processes, plant-microbe and plant-plant interactions, and successional dynamics in this ecosystem.