PS 66-130 - Changes in soil C, N, and P storage following woody plant invasion of grassland

Thursday, August 11, 2011
Exhibit Hall 3, Austin Convention Center
Ilsa B. Kantola1, Thomas W. Boutton1, Timothy R. Filley2 and C. Thomas Hallmark3, (1)Ecosystem Science and Management, Texas A&M University, College Station, TX, (2)Earth and Atmospheric Sciences, Purdue University, West Lafayette, IN, (3)Soil and Crop Sciences, Texas A&M University, College Station, TX
Background/Question/Methods

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.

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