Vegetation, Land Use History, and Patterns of Soil Organic Carbon in the Southern Great Plains.

Livestock grazing, woody plant proliferation, and efforts to manage shrub encroachment interact to alter storage of soil organic carbon (SOC) via changes in plant species composition and rates of ecosystem processes.  We evaluated the impact of these phenomena on the storage and vertical distribution of SOC in mixed-grass savanna in north-central Texas. Soil cores (to 1.5 m) were collected from two vegetation types (under mesquite [Prosopis glandulosa] tree canopies and in adjacent grassland areas) on sites subjected to contrasting land use histories: (1) livestock grazing, and (2) woody plant removal by root-plowing.  At each site, soils were sampled in treated areas (i.e., grazed or root-plowed) and in untreated control areas.  Soil cores were divided into 10 cm depth increments and analyzed for bulk density and SOC.  At the grazed and ungrazed sites, the grazing x vegetation type x soil depth interaction influenced SOC storage in the upper 1.5 m of the soil profile.  Grazing reduced SOC by 10-25% throughout the upper 60 cm of the profile in grass-dominated areas, but had little impact on SOC beneath mesquite canopies.  At the root-plowed site and its control counterpart, interactions between root-plowing and vegetation type and between root-plowing and soil depth determined the distribution and amount of SOC.   Root-plowing increased SOC by 15-30% in the upper 20 cm of the profile under both grassland and mesquite, but had little impact below that depth.  At both study sites, soils beneath mesquite stored 10-80% more SOC in the upper 10 cm of the profile than those under grassland; however, vegetation type had little effect on SOC at depths >10 cm.  Therefore, plant cover and land use history in the southern Great Plains were important determinants of SOC storage at soil depths <20 cm, but had little impact on SOC located deeper in the profile.