Tuesday, November 14, 2006

Soil Fractionation Accounting of Carbon Dynamics Following Mesquite Removal in Desert Grasslands.

Jennifer Moore-Kucera, Oregon State University, 4017 Ag And Life Sciences Building, Corvallis, OR 97331-7304, United States of America, Mitchel P. McClaran, University of Arizona, School of Natural Resources,, 325 Biological Sciences East, Tucson, AZ 85721, and Dean A. Martens, USDA-ARS Southwest Watershed Research Center, 2000 E. Allen Road, Tucson, AZ 85719.

Increased velvet mesquite (Prosopis velutina Woot.) in desert grasslands has increased soil organic carbon (C) compared to open grasslands, but the extent of accumulation in soil size fractions and rate of depletion following tree removal are largely unknown. We estimated these unknowns by comparing C content and d13C of bulk soil and physical fractions (light, particulate organic matter (POM), mineral-associated organic matter (MOM), silt and clay) among 3 settings: large mesquite (>15 cm basal diam.), skeleton mesquite (large trees killed by herbicide in 1960s), and open grassland areas. Samples were obtained at 2.5, 7.5, 20 and 40 cm depths in sandy loam soils, Santa Rita Experimental Range, AZ.  We found that (1) the 165% increase of bulk soil C at 2.5 cm depth beneath large mesquite in grassland is incorporated into all soil size fractions except MOM, and the 50% increase at 7.5 cm depth is incorporated into the light, POM and silt fractions only, (2) increased C does not occur in bulk soil or fractions at 20 and 40 cm depths, (3) mesquite is the main source of increased C, (4) >50% of increased C is lost ≤40 y following mesquite removal, (5) depletion of C is slowest in the silt and clay fraction at 2.5 cm and clay fraction at 7.5 cm, (6) C from mesquite becomes incorporated into light, POM, silt and clay fractions at 7.5 and 20 cm depths without increasing total C. These results suggest that mesquite C is very labile and quickly incorporated into most soil size fractions at 7.5 cm depth and POM and clay to 20 cm within 100-150 y of mesquite occupation. This lability probably facilitates the loss of the C accumulated above grassland levels following the removal of the mesquite source.