Controls on the presence and storage of soil inorganic carbon in a semi-arid watershed

Soil inorganic carbon (SIC) constitutes-40-50% of the terrestrial soil carbon and is an integral part of the global carbon cycle. Rainfall is a primary factor controlling SIC accumulation; however, the distribution and hierarchy of controls on SIC development in arid and semi-arid regions is poorly understood. The Reynolds Creek Experimental Watershed (RCEW) in southwestern Idaho is an ideal location to study factors influencing SIC because it spans a wide mean annual precipitation range (235 mm to 900 mm) along a 1,425 to 2,111 m elevation gradient and has soils derived from a wide variety of parent materials (granite, basalt, dust, and al-luvium). We collected soil samples along this elevational gradient to understand local controls on SIC distri-butions. SIC content was quantified at 71 soil pits and/or augered cores collected between approximately 0-1 m depth or until refusal. Consistent with previous studies, we found variations in precipitation governed the presence or absence of SIC; field measurements of the top 1 m of soils confirm little or no SIC in soils receiving > 500 mm in mean annual precipitation. Below this 500 mm threshold, SIC pools varied substantially and significantly between sites. Results showed that 90% of sites (64 sites) contained less than 10 kg m- 2 SIC, 7% (5 sites) contained 10-20 kg m- 2, and 3% (2 sites) contain between 24 and 29 kg m- 2 SIC. The total SIC within RCEW was estimated at-5.17 x 105 Mg. After precipitation, slope consistently ranked as the second most important predictor of SIC accumulation in random forest analysis. Wind-blown dust likely contributed to SIC accumulation; prior work indicates an average dust flux rate in RCEW of about 11 +/- 4.9 g m- 2 year -1. This study provides an initial model predicting SIC distribution and accumulation in a shrub-dominated dryland watershed.