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Title: Tracing the source of soil organic matter eroded from temperate forest catchments using carbon and nitrogen isotopes

Here, soil erosion continuously redistributes soil and associated soil organic matter (SOM) on the Earth's surface, with important implications for biogeochemical cycling of essential elements and terrestrial carbon sequestration. Despite the importance of soil erosion, surprisingly few studies have evaluated the sources of eroded carbon (C). We used natural abundance levels of the stable and radioactive isotopes of C ( 13C and 14C) and stable isotope of nitrogen ( 15N) to elucidate the origins of SOM eroded from low-order catchments along the western slopes of the Sierra Nevada of California, USA. Our work was conducted in two relatively undisturbed catchments (low elevation = 1800 m, and high elevation = 2300 m) of the Kings River Experimental Watersheds (KREW) in the Sierra National Forest. Sediment captured in basins at the outlet of each gauged watershed were compared to possible source materials, which included: upland surficial organic horizons (i.e., forest floor) and mineral soils (0–0.6 m) from three landform positions (i.e., crest, backslope, and toeslope), stream bank soils (0–0.6 m), and stream-bed materials (0–0.05 m). We found that most of the organic matter (OM) in the captured sediments was composed of O-horizon material that had high C concentrations. Radiocarbon analyses also showedmore » that the captured OM is composed of modern (post-1950) C, with fraction modern values at or above 1.0. Our results suggest that surface (sheet) erosion, as opposed to channeling through established streams and episodic mass wasting events, is likely the largest source of sediment exported out of these minimally disturbed, headwater catchments. The erosional export of sediment with a high concentration of C, especially in the form of relatively undecomposed litter from the O horizon, suggests that a large fraction of the exported C is likely to be decomposed during or after erosion; hence, it is unlikely that soil erosion acts as a significant net sink for atmospheric CO 2 in these low-order, temperate forest catchments.« less
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  1. Univ. of California, Merced, CA (United States)
  2. U.S. Dept. of Agriculture, Fresno, CA (United States)
  3. Univ. of Nevada, Reno, NV (United States)
  4. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Publication Date:
Report Number(s):
Journal ID: ISSN 0009-2541
Grant/Contract Number:
Accepted Manuscript
Journal Name:
Chemical Geology
Additional Journal Information:
Journal Volume: 445; Journal Issue: C; Journal ID: ISSN 0009-2541
Research Org:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org:
USDOE; National Science Foundation (NSF); USDA
Country of Publication:
United States
54 ENVIRONMENTAL SCIENCES; 60 APPLIED LIFE SCIENCES; Erosion; Radiocarbon; Sediment sources; Sierra Nevada; Soil organic matter; Stable isotopes
OSTI Identifier: