Riverine Carbon Export From Iowa's Tile-Drained Des Moines Lobe.

Research has shown that farmed landscapes engineered for high levels of productivity contribute a disproportionate share of the riverine inorganic carbon exported to the Mississippi River and Gulf of Mexico as bicarbonate. The origin of about 50% of the carbon present in the exported bicarbonate is soil organic matter. In this study, we used alkalinity and total organic carbon (TOC) measurements collected from the tile-drained landscape of Iowa’s 9,389 km² Raccoon River since 1994 to 1) evaluate riverine inorganic and organic carbon concentrations and carbon export patterns; 2) compare current trends to historical conditions; and 3) link carbon transport processes to current land use management, and 4) explore how this data might be used in the context of carbon sequestration efforts and potential trading schemes.  Export of inorganic carbon averaged 106,000 Mg per year and contributes 90% of the carbon flux from the basin.  Distinct periods of seasonal inorganic carbon transport were evident and related to water flux and biological processing.  Alkalinity concentrations are unchanged from 1931-1944 levels (~53 mg/l C), but carbon loads have doubled due to increasing river discharge.  Carbonate-rich glacial deposits and agricultural lime provide a large source of inorganic carbon and results confirm alkalinity export in the Raccoon Basin is transport limited. Riverine carbon export represents only a small fraction of available carbon in typical Des Moines Lobe soils and is much less than carbon harvested with grain.  While fertilization and tillage have possibly helped increase carbon fluxes over the last 70+ years, the overriding factor on carbon export is discharge. Discharge control over carbon export using in-line tile control structures may provide an opportunity for agriculture in terms of quantifying carbon sequestration for potential carbon trading.  Controlling water flux through soils can limit carbon export similar to practices such as reduced tillage and managed rotations.