Controls on the Presence and Concentration of Soil Inorganic Carbon in a Semi-Arid Watershed

Abstract

Soil inorganic carbon (SIC) constitutes ~ 40% of terrestrial soil carbon and is an integral part of the global carbon cycle. Whereas rainfall has been identified as the primary factor controlling SIC accumulation in arid and semi-arid regions, the hierarchy of local controls such as parent material and biota on SIC development on distribution and accumulation are 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 range of precipitation (235 mm to 900 mm) along an elevation gradient and soils derive from a wide variety of parent materials (granite, basalt, dust, and alluvium). We collected soil samples to 1 m depth along this elevation gradient to understand the local controls on SIC distributions.  Consistent with previous studies, we found variations in precipitation governed the presence or absence of SIC within the RCEW, with a threshold for SIC accumulation at ~500 mm mean annual precipitation. Below this threshold, SIC pools varied substantially and significantly between sites. SIC content from 71 soil pits and/or augered cores collected between approximately 0-1 m or refusal showed that 90% of the sites (64 sites) contained less than 10 kg m-2 SIC, 7% (5 sites) contained between 10-20 kg m-2, and 3% (2 sites) had between 24 and 29 kg m-2 SIC. The total amount of SIC within RCEW was estimated to be ~ 9 x 105 ± 4 x 104 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.

Keywords: Soil inorganic carbon, semi-arid, carbon storage, modeling, pedogenic carbonate, critical zone, Reynolds Creek Experimental Watershed