PNNL

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