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

The soil C balance is determined by the difference between inputs (e.g., plant litter, organic amendments, depositional C) and outputs (e.g., soil respiration, dissolved organic C leaching, and eroded C). There is a need to improve our understanding of whether soil erosion is a sink or a source of atmospheric CO2. The objective of this paper is to discover the long-term influence of soil erosion on the C cycle of managed watersheds near Coshocton, OH. We hypothesize that the amount of eroded C that is deposited in or out of a watershed compares in magnitude to the soil C changes induced via microbial respiration. We applied the erosion productivity impact calculator (EPIC) model to evaluate the role of erosion-deposition processes on the C balance of three small watersheds (similar to 1 ha). Experimental records from the USDA North Appalachian Experimental Watershed facility north of Coshocton, OH were used in the study. Soils are predominantly silt loam and have developed from loess-like deposits over residual bedrock. Management practices in the three watersheds have 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. Simulations of a comprehensive set of ecosystem processes including plant growth, runoff, and water erosion were used to quantify sediment C yields. A simulated sediment C yield of 43 +/- 22 kg C ha(-1) year(-1) compared favorably against the observed 31 +/- 12 kg C ha(-1) year(-1) in W118. EPIC overestimated the soil C stock in the top 30-cm soil depth in W118 by 21% of the measured value (36.8 Mg C ha(-1)). Simulations 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). Simulated eroded C re-deposited inside (30-212 kg C ha(-1)year(-1)) or outside (73-179 kg C ha(-1) year(-1)) watershed boundaries compared in magnitude to a simulated soil C sequestration rate of 225 kg C ha(-1) year(-1) and to literature values. An analysis of net ecosystem carbon balance revealed that the watershed currently under a plow till system (W128) was a source of C to the atmosphere while the watersheds currently under a no till system (W118 and W188) behaved as C sinks of atmospheric CO2. Our results demonstrate a clear need for documenting and modeling the proportion of eroded soil C that is transported outside watershed boundaries and the proportion that evolves as CO2 to the atmosphere.