Dynamic replacement and loss of soil carbon on eroding cropland

Abstract

Abstract. Links between erosion/sedimentation history and soil carbon cycling were examined in a highly erosive setting in Mississippi loess soils. We sampled soils on (relatively) undisturbed and cropped hillslopes and measured C, N, 14C, and CO2 flux to characterize carbon storage and dynamics and to parameterize Century and spreadsheet •4C models for different erosion and tillage histories. For this site, where 100 years of intensive cotton cropping were followed by fertilization and contour plowing, there was an initial and dramatic decline in soil carbon content from 1870 to 1950, followed by a dramatic increase in soil carbon. Soil erosion amplifies C loss and recovery: About 100% of the original, prehistoric soil carbon was likely lost over 127 years of intensive land use, but about 30% of that carbon was replaced after 1950. The eroded cropland was therefore a local sink for CO2 since the 1950s. However, a net CO2 sink requires a full accounting of eroded carbon, which in turn requires that decomposition rates in lower slopes or wetlands be reduced to about 20% of the upland value. As a result, erosion may induce unaccounted sinks or sources of CO2, depending on the fate of eroded carbon and its protection from decomposition.For erosion rates typical of the United States, the sink terms may be large enough (1 Gt yr -l, backof-the-envelope) to warrant a careful accounting of site management, cropping, and fertilization histories, as well as burial rates, for a more meaningful global assessment.

Storage and Turnover of Organic Matter in Soil

“…In terms of total watershed C stock, recent empirical and modeling studies conclude that erosion-and the balance between its effects on productivity and decomposition in eroding and depositional sites-tends to lead to an increase in stock, even if there are local decreases in the eroding sites themselves (Stallard 1998;Smith et al 2005;Berhe et al 2007;Harden et al 1999).…”

“…In general, the factors that control (enhance) destabilization promote disturbances that expose SOM or otherwise foster a physical environment more advantageous to microbial and meso-faunal degradation of SOM. Examples of natural and anthropogenic soil disturbance include tilling (Six et al 1999), freeze/thaw and shrink/swell cycles (Denef et al 2001), erosion and mass wasting (Harden et al 1999), bioturbation (Stork and Eggleton 1992), windthrow (Kramer et al 2004), and fire . The degradation of a substrate can also act as a mechanism for further destabilization through the production of more labile byproducts.…”

Modeling carbon dynamics in vegetation and soil under the impact of soil erosion and deposition

“…It is often implicitly assumed that the SOC that is lost is released to the atmosphere [DeFries et al, 1999;Houghton et al, 1999;Hurtt et al, 2002]. In fact, in addition to the atmospheric pathway, erosion can also contribute to SOC loss from uplands [Slater and Carleton, 1938;Webber, 1964;Tiessen et al, 1982;Geng and Coote, 1991;Cihacek and Swan, 1994;Lal, 1995;Harden et al, 1999]. Some of the eroded soil, carbon, and nutrients are redistributed across the landscape, and some are transported and deposited to waterlogged environments, such as reservoirs, lakes, wetlands and oceans [Lal, 1995;Stallard, 1998;Smith et al, 2001].…”

“…[2] Conversion of lands from a native state to an agricultural use usually leads to a 20-40% reduction in soil organic carbon (SOC) storage [Donigian et al, 1994;Paul et al, 1997;Buyanovsky and Wagner, 1998a;Harden et al, 1999]. It is often implicitly assumed that the SOC that is lost is released to the atmosphere [DeFries et al, 1999;Houghton et al, 1999;Hurtt et al, 2002].…”

Modelling a century of soil redistribution processes and carbon delivery from small watersheds using a multi-class sediment transport model

“…To date, few soil and carbon erosion models integrate detailed transport processes. There have been attempts to address this issue using single-point models with varying degrees of complexity (Harden et al, 1999;Manies et al, 2001;Liu et al, 2003;Billings et al, 2010). These models apply prescribed carbon erosion and/or deposition rates and simulate the resulting effects on the soil organic carbon (SOC) profile using CEN-TURY (Parton et al, 1988) parameterizations.…”