Effects of Land Use Change on Grassland Soil Carbon Pools: Does Depth of Measurement Matter?.

Conversion of grassland ecosystems to row-crop agriculture or woody-plant domination often significantly alters soil carbon pools.  However, many of the extant studies that report such changes have focused on shallow (top 20 cm) soils.  Would the conclusions of these studies be different if deeper soil layers were included in the analyses?  We addressed this question by analyzing both shallow and deep soil carbon pools collected from adjacent, paired land use types (woodland vs. grassland and cropland vs. grassland) located at sites across the Great Plains of the U.S.  Our results show that with regard to the conversion of grassland to cropland, the reductions in soil organic carbon (SOC) observed in the top 20 cm of soil are broadly reflective of meter deep profile changes (28.0% and 29.3% less SOC was found in cropland compared to adjacent native grassland, averaged across the 5 sites examined, for the 0-20 and 0-100 cm depth increments, respectively).  However, conclusions about the effects of grass-to-woodland conversion on SOC pools were significantly more sensitive to measurement depth.  Indeed, at two sites in New Mexico, the percent change in SOC accompanying such a conversion went from a loss to a gain of similar proportions when deeper soils were included in the analysis.  These data indicate that deep soil SOC pools may be important and more sensitive to changes in land use than is commonly assumed.