Geomorphic controls on SOC along an erosional gradient in a burned montane landscape

Abstract:

The significance of soil organic carbon (SOC) in the global carbon budget and the potential use of SOC in natural climate solutions and Earth system models have been well established. Soil thickness can be a strong predictor of SOC storage. Soil thickness is controlled by the balance between soil production and erosion over millennial timescales such that slow erosion rates yield thicker soils with long particle residence times and more intensely weathered profiles. On the other hand, if erosion exceeds soil production, then thin, patchy, clast-rich soils with little accommodation space for SOC dominate the landscape. On shorter timescales, biological processes interact with soil properties developed over a longer geomorphic timescale to control short-term variations in SOC storage. In SW Oregon, the 2013 Douglas Fire coincided with a prominent knickzone that separates slow and fast eroding terrain. Below this prominent topographic feature slopes are steep and rocky with thin, coarse, minimally altered, and lighter colored soils that contrast the upvalley terrain where slopes are gentle with thicker, more weathered, and darker soils. This setting presents an opportunity to analyze geomorphic controls on SOC across a wide range of erosion rates while other factors are held constant. Preliminary results from hilltop samples exhibit a rapid increase in total profile SOC (grams/m²) with decreasing dimensionless erosion rate, calculated from hilltop curvature, hillslope length, and critical hillslope angle (E*= 2C_HTL_H/S_c). Soil carbon stocks increase more than 2.5 times between E* values of 0.77 and 24. Our site also enables us to test the hypothesis that particles moving slowly through thick, slow-eroding horizons will have prolonged exposure to chemical and physical weathering elements (e.g. water and microbial activity) which have been shown to increase mineral reactivity and physical protection mechanisms conducive to SOC preservation. These findings show that geomorphic theory and field measurements can be combined to predict net SOC gains or losses across complex erosional landscapes. Because erosional landscapes dominate Earth’s surface, understanding how SOC dynamics vary with erosion rate and topographic position is essential for increasing the accuracy of global carbon budgets and climate models.