Topographic Controls on the Thickness of Mobile Regolith and Total Soil Organic Carbon in Complex Terrain

Arid and semi-arid regions comprise over 40% 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. The aim of this study was to develop a total soil carbon model using curvature, thickness of mobile regolith (TMR), and aspect in order to predict the distribution of soil carbon across complex terrain within a semi-arid environment. We excavated 45 randomly selected soil pits vertically down to saprolite to acquire TMR, in a first order watershed within the Reynolds Creek Critical Zone Observatory located in Southwestern Idaho. TMR varied as an inverse linear function of curvature (r2 value = 0.89) and was shown to be predicable across lithology and ecosystems. Soil thicknesses were plotted against total soil carbon for both the north and south-facing aspects. A quadratic polynomial function fit well with r2 values of 0.89 and 0.90, respectively. Across the watershed, total soil carbon on the north-facing aspect was 7,172 Mg C or 54% of the total soil carbon, despite comprising only 37% of the total land area. South-facing aspect totaled 6,115 Mg C, or 46% of the total soil carbon. If samples were collected to a maximum thickness of 1 m or 0.3 m and extrapolated using kriging techniques (which is typical of other inventories and models) total soil carbon has the potential to under- and overestimate carbon as much as 50% to 133% and 26% to 1167% with aspect and microtopography, respectively. By requiring sampling down to saprolite, our model has the potential to provide more accurate total carbon pools than other models. Our findings indicate that a significant amount of carbon is stored deep in a part of the critical zone that may be less sensitive to loss but leads to an underestimation of total soil carbon stores on complex terrain.