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Title: Long-term modeling of soil C erosion and sequestration at the small watershed scale

Author
item IZAURRALDE, R - BATTELLE PACIFIC NW LAB
item WILLIAMS, J - TEXAS A&M UNIVERSITY
item POST, W - OAK RIDGE NAT'L LAB
item THOMSON, A - UNIV. OF MARYLAND
item MCGILL, W - U.NORTH BRITISH COLUMBIA
item Owens, Lloyd
item LAL, R - OHIO STATE UNIVERSITY

Submitted to: Climatic Change
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/18/2006
Publication Date: 1/2/2007
Citation: Izaurralde, R.C., Williams, J.R., Post, W.M., Thomson, A.M., Mcgill, W.B., Owens, L.B., Lal, R. 2007. Long-term modeling of soil C erosion and sequestration at the small watershed scale. Climatic Change. 80:73-90.

Interpretive Summary: Increasing levels of carbon dioxide (CO2) in the atmosphere and the impacts on climate change is a global issue. Agriculture can have major impacts on CO2 emissions depending on whether management practices store carbon (C) in soil or cause C to be released. Two current and competing theories about the impacts of erosion on the availability of C are: C from eroded fields becomes “sequestered” or stored in depressional areas and is rendered unavailable for release as CO2; and erosion events cause aggregate breakdown of physically protected C, and it becomes accessible for oxidation and emission of CO2. This study applied the EPIC (Erosion Productivity Impact Calculator) model evaluate these processes at a small watershed scale, using data from small watersheds at the North Appalachian Experimental Watershed near Coshocton, Ohio. Although the model overestimated the surface soil C compared with observed data, it did prove to be an effective tool for such evaluations. Even though the results of this study did not confirm either of the two theories, they did confirm the importance of erosion-deposition processes in C cycling at the small watershed scale. These results are important to other scientists who are studying C recycling and the impacts of agricultural management practice on those processes.

Technical Abstract: The soil C balance is determined by the difference between inputs (e.g. litter, crop residues, decaying roots, organic amendments, depositional C) and outputs (e.g. soil respiration, dissolved organic C leaching and eroded C). Two competing hypotheses suggest erosion may either increase or decrease output. One hypothesis states that C from eroded fields becomes “sequestered” in depressional areas and thus is rendered unavailable for decomposition. An alternative hypothesis argues that due to aggregate breakdown during erosion events, physically-protected C becomes accessible, thereby increasing oxidation of C and emission of CO2. This study applied the EPIC (Erosion Productivity Impact Calculator) model to evaluate the role of erosion-deposition processes on the C balance at the small watershed scale. The experimental records of three small watersheds (~1 ha) from the USDA North Appalachian Experimental Watershed facility north of Coshocton, OH, were used in the study. Predominant silt loam soils in the area have developed from loess-like deposits over residual bedrock. Soil and crop management in the three watersheds has changed over time. Currently, watershed 118 (W118) is under a corn (Zea mays L.) - soybean (Glycine max (L.) Merr.) no till rotation, W128 is under conventional till continuous corn, and W188 is under no till continuous corn. Predictions of sediment C yields were made through simulation of an entire range of ecosystem processes including plant growth, runoff, and water erosion. A simulated sediment C yield of 39 kg C ha-1 y-1 compared well against an observed value of 31 kg C ha-1 y-1 in W118. EPIC overpredicted the soil C stock in the top 30-cm soil depth in W188 by 21% of the measured value (36.8 Mg C ha-1). Predictions of soil C stocks in the other two watersheds (42.3 Mg C ha-1 in W128 and 50.4 Mg C ha-1 in W188) were off by <1 Mg C ha-1. Although these results do not directly answer any of the two prevailing hypotheses, they do provide insight as to the importance of erosion-deposition processes in the C cycle at the small watershed scale. In future work, the APEX model, the landscape version of EPIC, will be used to study the role of erosion and deposition as sources or sinks of atmospheric C.