Changes in soil labile-carbon indicated by the ratio of microbial biomass-carbon to total organic-carbon in a semi-arid grassland undergoing desertification

Changes in the abundance and spatial distribution of biologically-available (labile) organic carbon fractions in soils can alter the productivity of arid and semi-arid ecosystems, often leading to desertified conditions. Early indicators of this shift are inevitably linked to microbial activity, which responds more rapidly to environmental changes than does total organic-C stored in the soil. To study changes in labile-C in a Chihuahuan desert grassland in southern New Mexico, microbial biomass-C (C{dollar}sb{lcub}rm mic{rcub}{dollar}) and total soil organic-C (C{dollar}sb{lcub}rm org{rcub}{dollar}) were measured along a gradient of long-term grazing impacts, with an ungrazed site chosen to represent steady state conditions with respect to soil organic matter.; Microbial biomass-C estimated using the chloroform fumigation-extraction method ranged from 28.4 {dollar}mu{dollar}g C g{dollar}sp{lcub}-1{rcub}{dollar} to 423.2 {dollar}mu{dollar}g C g{dollar}sp{lcub}-1{rcub}{dollar} for three soil sample categories that included (1) beneath grass canopies, (2) beneath shrub canopies, and (3) in non-vegetated interspaces. Abundances of total soil organic-C ranged from 1.57 mg C g{dollar}sp{lcub}-1{rcub}{dollar} to 6.53 mg C g{dollar}sp{lcub}-1{rcub}{dollar}, and were generally lower in highly-impacted sites. Using the C{dollar}sb{lcub}rm mic{rcub}{dollar} to C{dollar}sb{lcub}rm org{rcub}{dollar} ratio as an indicator of labile-C, values at all sites were near the steady state level beneath grass canopies, suggesting that abundances remain unchanged with higher grazing-related impacts. Beneath shrub canopies, ratio values were generally higher than the steady state level, an indication that soil labile-C fractions are accumulating. In bare soil interspaces, ratio values along the gradient indicated increasingly significant losses of labile-C with higher grazing impacts relative to the steady state level.; The C{dollar}sb{lcub}rm mic{rcub}{dollar} to C{dollar}sb{lcub}rm org{rcub}{dollar} ratio is a better indicator of soil labile-C in this semi-arid ecosystem than either variable alone. The C{dollar}sb{lcub}rm mic{rcub}{dollar} to C{dollar}sb{lcub}rm org{rcub}{dollar} ratio may thus provide a tool for monitoring changes in soil productivity that occur with desertification. (Abstract shortened by UMI.)...