Determining Total Soil Carbon Storage in the Critical Zone Using Topography and Lithology

Arid and semi-arid regions comprise over 41% of the terrestrial ecosystems on Earth and are considered to be one of the most susceptible to environmental change. Estimating the amount and distribution of soil carbon in these regions is challenging due to their high degree of spatial heterogeneity. We aim to develop a total soil carbon model using tangential curvature, total mobile regolith depth, and aspect in order to predict the distribution of soil carbon in complex terrain within a semi-arid environment. We excavated 45 randomly selected soil pits down to saprolite in a first order watershed within the Reynolds Creek Critical Zone Observatory located in Southwestern Idaho. Total mobile regolith depth was measured vertically and plotted against tangential curvature to produce an inverse linear relationship function, predicting soil depth to an r2 value of 0.86. Soil depths were plotted against total soil carbon for both the north and south-facing aspects. A quadratic polynomial function fit well with r2 values of 0.90 and 0.89, respectively. Across the watershed, total soil carbon on the north-facing aspect was 8.02 x1011 Kg C or ~38% of the total soil carbon, despite comprising only ~33% of the total land area. South-facing aspect totaled 9.87 x1011 Kg C, or ~46% of the total soil carbon. East and west aspects were excluded. If samples were collected to a maximum depth of 1 m, total soil carbon would be underestimated by ~1.6 times the amount that we predict (1.31 x1012 to 2.12 x1012 Kg C, respectively). By sampling down to saprolite, our model has the potential to provide more accurate total carbon pools than other models, which focus only on the upper 1 m of soil. Our findings indicate that a significant amount of carbon is stored deep in critical zone that may be less sensitive to loss and that estimates of soil carbon significantly underestimate total soil carbon stores on complex terrain.