CONTROLS ON THE PRESENCE AND AMOUNT OF SOIL INORGANIC CARBON IN A TRANSITIONAL SEMI-ARID WATERSHED

Soil inorganic carbon (SIC) constitutes approximately 40% of terrestrial soil carbon and it is an integral part of the global carbon cycle; however, knowledge on the controls of its storage and flux is limited. The precipitation and storage of inorganic carbon within soils is controlled by the soil forming factors: climate, organisms, relief, parent material, and time among which rainfall is widely regarded as the strongest control on SIC presence or absence (Jenny, 1941). In arid and semi-arid regions where SIC accumulates, however, the hierarchy of controls on the amount of SIC is complex. The Reynolds Creek watershed in southwestern Idaho is an ideal location to study the factors controlling both the presence and variations in SIC accumulations. We collected samples from sites under a wide range of soil forming regimes. Vegetation at sampled sides included various sagebrush species, bitterbrush, greasewood, and juniper. We sampled soils formed on different parent materials: granite, basalt, other volcanics, and alluvium. Our results show that the threshold for SIC accumulation is approximately 500mm of mean annual precipitation, and variability in SIC concentrations below that is significant. The highest SIC concentrations range from 15 to 27kg/m² with stage II/III carbonate development and are found in basaltic and terrace soils. A chronosequence of four terraces in the lower watershed (282-296mm of rainfall) contains generally increasing amounts of SIC with elevation above the active channel highlighting the role of time and soil development. While parent material is thought to be a significant predictor for SIC concentrations, it ultimately had limited power to predict in our analyses. This discrepancy potentially results from limited data on the accumulation and composition of wind-blown dust deposited in the soils of the watershed. Additionally, measurements from both fine-grained and gravelly soils suggest that approximately 15% of SIC in the watershed is in the form of clast coatings. While previous work ignores the gravel fraction or simply combines it with the fine fraction, these results indicate that the gravel fraction is a significant part of the SIC pool and must be considered in future work.