Abstract
Abstract. The fate of organic carbon (C) lost by erosion is not well understood in agricultural settings. Recent models suggest that wetlands and other small water bodies may serve as important long-term sinks of eroded C, receiving ~30 % of all eroded material in the US. To better understand the role of seasonally-saturated wetlands in sequestering eroded C, we examined the spatial and temporal dynamics of C and sediment accumulation in a 13-year-old constructed wetland used to treat agricultural runoff. The fate of C sequestered within deposited sediment was modeled using point-sampling, remote sensing, and geostatistics. Using a spatially-explicit sampling design, annual net rates of sedimentation and above-ground biomass were measured during two contrasting years (vegetated (2004) vs. non-vegetated (2005)), followed by collection of sediment cores to the antecedent soil layer, representing 13 years of sediment and C accumulation. We documented high annual variation in the relative contribution of endogenous and exogenous C sources, as well as absolute rates of sediment and C deposition. This annual variation, however, was muted in the long-term (13 yr) sediment record, which showed consistent vertical patterns of uniform C distribution (~14 g kg–1) and δ13C signatures in high depositional environments. This was in contrast to low depositional environments which had high levels of surface C enrichment (20–35 g kg–1) underlain by C depleted (5–10 g kg–1) sediments and an increasing δ13C signature with depth indicating increased decomposition. These results highlight the importance of sedimentation in physically protecting soil organic carbon and its role in controlling the long-term C concentration of seasonally-saturated wetland soils. While significant enrichment of surface sediments with endogenous C occurred in newly deposited sediment (i.e., 125 kg m2 in 2004), fluctuating cycles of flooding and drying maintained the long-term C concentration at the same level as inflowing sediment (i.e., 14 g kg–1), indicating no additional long-term storage of endogenous C. These results demonstrate that constructed flow-through wetlands can serve as important sinks for eroded C and sediment in agricultural landscapes, however, additional C sequestration via enrichment from endogenous sources may be limited in seasonally-saturated wetlands due to rapid decomposition during drying cycles.
Highlights
Organic carbon (C) sequestration in the terrestrial biosphere has become an important research topic due to growing interest in mediating anthropogenic impacts to climate change (Schlesinger, 1999; Lal, 2004a, b; Berhe et al, 2007)
Maynard cycles of flooding and drying maintained the long-term C concentration at the same level as inflowing sediment (i.e., 14 g kg−1), indicating no additional long-term storage of endogenous C. These results demonstrate that constructed flowthrough wetlands can serve as important sinks for eroded C and sediment in agricultural landscapes, additional C sequestration via enrichment from endogenous sources may be limited in seasonally-saturated wetlands due to rapid decomposition during drying cycles
Several studies indicate that the sink/source dynamics of terrestrial ecosystems have substantially changed within the last century with a transition from a C source to sink occurring during the 1930s (Bruno and Joos, 1997; Joos and Bruno, 1998)
Summary
Organic carbon (C) sequestration in the terrestrial biosphere has become an important research topic due to growing interest in mediating anthropogenic impacts to climate change (Schlesinger, 1999; Lal, 2004a, b; Berhe et al, 2007). Global C inventories estimate that between ∼0.5 and 2 Gt C yr−1 are being sequestered and not accounted for in identified C reservoirs (Stallard, 1998), with much of this “missing” C thought to reside as soil organic carbon (SOC) (Smith et al, 2001, 2005). Several studies indicate that the sink/source dynamics of terrestrial ecosystems have substantially changed within the last century with a transition from a C source to sink occurring during the 1930s (Bruno and Joos, 1997; Joos and Bruno, 1998) This source to sink transition is coincident with an era of soil erosion associated with agriculture, resulting in the redistribution of an unknown quantity of SOC into depositional areas within the terrestrial ecosystem (McCarty and Ritchie, 2002). Highlight the potential role of small water bodies
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