Surface erosion and sediment accumulation in the Sierra Nevada: Using stable isotopes to source eroded material in captured sediments

Background/Question/Methods

Topsoil and associated soil organic matter is continually distributed over the landscape laterally by soil erosion. Deposition and stabilization of eroded soil organic matter in low lying terrestrial and aquatic environments can induce a sink for atmospheric carbon dioxide. However, determining the sources that make up the deposited material affecting the amount and stability of carbon (C) stored is not well studied due to the amount of mixing that occurs in soils. We used stable isotopic ratios and elemental concentrations of C and nitrogen (N) of captured sediments from below two gauged low- order catchments (low elevation, 1800 m and high elevation, 2300 m) of the Kings River Experimental Watersheds (KREW) in the Sierra National Forest. The captured sediments were compared to possible source materials including upland forest floor and mineral soils (0- 0.6 m ) from three landform positions (crest, backslope, toeslope), and stream bank soils (0- 0.6 m). 

Results/Conclusions

We found that stable isotope analysis of the captured sediments showed a combination of upland materials present consisting of material from all landscape positions and stream banks with the main contributor being forest floor at both elevations (Low elevation: sediment δ¹⁵N= -2.4‰, δ¹³C : -27.5‰; forest floor δ¹⁵N = -3.5‰, δ¹³C = -26‰; and upland mineral soils δ¹⁵N = 0.1‰, δ¹³C = -24.8‰; High elevation: sediment δ¹⁵N = -2.5‰, δ¹³C = -26.5‰; forest floor δ¹⁵N = -3.2‰, δ¹³C = -25.2‰; and upland mineral soils δ¹⁵N = 1.9‰, δ¹³C = -24.1‰, entire watershed averages). Elemental C and N show soils, stream banks, and sediments were more closely related in terms of elemental carbon and nitrogen (25, 18, 29 C:N respectively) than the forest floor (C:N of 49). Our isotope results show that surface erosion is likely the main source of sediment exported out of these catchments. Elemental ratios suggest that transformations are occurring during transport or after deposition. Knowing the sources of deposited material will enable increased facilitation of C storage in Sierra Nevada forests to mitigate the effects of climate change.