Vegetation Effects on Soil Organic Carbon Quality in an Arid Hyperthermic Ecosystem

Abstract

Arid lands occupy substantial global land area and thus may play an important role in the terrestrial carbon cycle. This study examined a facet of arid land carbon cycling by examining variation in plant and soil organic carbon quality and the physical partitioning of carbon into aggregate and mineral-associated fractions for soils dominated by Prosopis velutina (mesquite), Larrea tridentata (creosote), and a combination of Bouteloua barbata, Bouteloua aristidoides, Aristida adscensionis, and Cynodon dactylon (mixed grass) vegetation types in an arid hyperthermic ecosystem. We used a combination of density fractionation to quantify physical distribution of organic carbon, in addition to isotopic, thermal, and spectroscopic techniques to quantify carbon quality in each fraction. Data indicated that most of the soil carbon across all vegetation types was concentrated in free light fractions, with little role for aggregate occlusion or mineral adsorption. Differential thermal analysis and derivative thermogravimetry indicated that all vegetation types were dominated by thermally labile material (exothermic peak and mass loss at ∼350°C), with the greatest differences in carbon quality noted among the respective plant materials. Diffuse reflectance Fourier transform infrared spectroscopy also indicated that the most substantial variation in organic carbon chemical quality was among the various plant materials. In particular, the creosote plant material exhibited a distinct high-temperature exothermic peak near 515°C, the greatest specific enthalpy (20 kJ g -1 biomass), and a relatively greater proportion of aromatic components as determined by diffuse reflectance Fourier transform infrared spectroscopy. Furthermore, the data indicated substantial alteration of chemical and thermal properties as organic material progressed from plant material to the soil fractions with a convergence on organic material dominated by polysaccharides and substantial reduction in specific enthalpy in the soil fractions.