Impacts of arid land shrub expansion on soil carbon cycling from carbon-14 ages and carbon and oxygen isotope ratios of pedogenic calcite and organic matter

In deserts, the distribution and turnover of soil organic matter (SOM) determines patterns of nutrient availability and productivity, providing a fundamental measure of ecosystem integrity. However, the response of arid land carbon (C) pools to desertification and future climate change are poorly constrained.; Over the past century, overgrazing and drought in New Mexico's Jornada Basin promoted the replacement of perennial black grama (Bouteloua eriopoda) grasslands by woody shrubs, primarily mesquite (Prosopis glandulosa). The associated ecosystem-scale changes have included reduced vegetation cover, increased soil erosion, and a redistribution of soil resources. This study examines patterns of C storage and cycling within a relict grassland community and desertified mesquite dunes at the Jornada Basin.; Stable C isotopes, {dollar}sp14{dollar}C dating, and C/N analyses were used to identify changes in active (light fraction--LFC) and stable (heavy fraction--HFC) SOM following mesquite expansion. The {dollar}deltasp{lcub}13{rcub}{dollar}C of LFC and HFC provided evidence of recent shrub expansion and an earlier (ca. 9.5-5 kyr BP) woodland/shrub-to-grass transition. Spatial patterns of SOM derived from mesquite were similar to the distribution of fine roots. {dollar}sp{lcub}14{rcub}{dollar}C activities of LFC indicated that root litter is an important source of SOM at depth. C/N ratios of LFC decreased under mesquite, perhaps signaling greater N mineralization potential. Turnover rates of surface LFC pools in grass (7 yrs) and mesquite (11 yrs) soils and for HFC pools at depth ({dollar}sim{dollar}135 to 875 yrs), suggest that mesquite may enhance soil C storage relative to grasses.; The isotopic composition of coexisting pedogenic carbonate and SOM were compared to examine short ({dollar}<{dollar}100 yrs) and longer-term contributions of vegetation, soil disturbance, and diagenesis on the isotopic signal of carbonate. {dollar}sp{lcub}14{rcub}{dollar}C ages and {dollar}deltasp{lcub}13{rcub}{dollar}C values indicated that grassland carbonates have been isotopically homogenized by erosion and reburial of pre-Holocene carbonate ({dollar}rmdeltasp{lcub}13{rcub}C cong -4.19perthous; deltasp{lcub}18{rcub}O cong -6.90perthous).{dollar} As a result, the carbonate isotopes do not record mid-Holocene climate/vegetation changes, which are inferred from {dollar}deltasp{lcub}13{rcub}{dollar}C values of SOM. Mass balance estimates indicated that at least 80% of the dune carbonate (in C horizons) has been isotopically altered by dissolution and recrystallization processes under mesquite (i.e. {dollar}<{dollar}100 yrs). This carbonate is currently precipitating from soil waters derived primarily from winter rainfall, as evidenced by carbonate {dollar}deltasp{lcub}18{rcub}{dollar}O values.; The net carbon balance associated with desertification was estimated from the distribution and amount of C in biomass and soils from grasslands and shrub communities. Relative to grasslands, only mesquite communities gained biomass C. Both mesquite and creosote-bush (Larrea tridentata) communities lost C as SOM. In contrast, tarbush (Flourensia cernua) sites contained less biomass C and more SOM. At an ecosystem-level, total C storage (26-28 kg C m{dollar}sp{lcub}-2{rcub}{dollar}) appears to remain relatively unchanged. C flux estimates indicate that CO{dollar}sb2{dollar} fertilization has had little affect on SOM storage ({dollar}le{dollar}3-4% of total) in the Jornada.; Desertification processes effect basic changes in the physical and biological structure of arid land ecosystems which diminish the capacity of these regions to support human needs. This study indicates that changes in arid land productivity (via. nutrient availability) may be linked to patterns of SOM production and decomposition In this context, the sustainable use (or restoration) of arid land environments may be best achieved by managing spatial and temporal distr...