Submitted to: Biogeosciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/20/2011
Publication Date: 3/6/2012
Citation: Fortner, S., Lyons, W., Carey, A., Shipitalo, M.J., Welch, S., Welch, K. 2012. Silicate weathering and CO2 consumption within agricultural landscapes, the Ohio-Tennessee River Basin. Biogeosciences. 9(3):941-955. Interpretive Summary: When natural areas are brought into agricultural production the movement of water can be altered. That is, planting of crops can change how fast water infiltrates into the soil and how much and how fast water runs off into streams compared to when the areas were under natural vegetation. The shift to agricultural crops can also affect how fast the minerals in the soil weather. When minerals containing the element silicon are weathered, dissolved silica can be found in the water that leaves the fields. Thus, by measuring the amount of dissolved silica in water leaving fields under various management practices the influence of these practices on weathering of soil minerals can be assessed. While this is not important from an agricultural production standpoint, the weathering of silicate minerals can result in a reduction in the amount of carbon dioxide, a greenhouse gas, in the atmosphere. In this study we found that the addition of fertilizers and manures containing the ammonium form of nitrogen to agricultural fields and the decomposition of the litter in forests contributed to the release of dissolved silica. If this is not taken into account the amount of carbon dioxide used in the weathering process may be over estimated. Therefore, the amount of ammonium nitrogen containing fertilizer used on agricultural fields should be considered to get a complete assessment of the impact of farming practices of carbon dioxide levels in the atmosphere.
Technical Abstract: Myriad studies have shown the extent of human alteration to global biogeochemical cycles. Yet, there is only a limited understanding of the influence that humans have over silicate weathering fluxes; fluxes that have regulated atmospheric carbon dioxide concentrations and global climate over geologic timescales. Natural landscapes have been reshaped into agricultural ones to meet food needs for growing world populations. These processes modify soil properties, alter hydrology, affect erosion, and consequently impact water-soil-rock interactions such as chemical weathering. Dissolved silica (DSi), Ca2+, Mg2+, NO3-, and total alkalinity were measured in water samples collected from five small (0.65 to 38.3 ha) gauged watersheds at the North Appalachian Experimental Watershed (NAEW) near Coshocton, Ohio, USA. The sampled watersheds in this unglaciated region include: a forested site (70+ year stand), mixed agricultural use (corn, forest, pasture), an unimproved pasture, tilled corn, and a recently (<3 yr) converted no-till corn field. The first three watersheds had perennial streams, but the two corn watersheds only produced runoff during storms and snowmelt. For the perennial streams, total discharge was an important control of dissolved silicate transport, with no statistical distinction among watersheds. Median DSi yields (22.1-30.8 kg/ha/yr) were similar to the median of annual averages between 1979-2009 for the much larger Ohio-Tennessee River Basin (25.6 kg/ha/yr). Corn watersheds, which only had surface runoff, had substantially lower DSi yields (<5.3 kg/ha/yr) than the perennial-flow watersheds. The lack of contributions from Si-enriched groundwater largely explained their much lower DSi yields with respect to sites having baseflow. A significant positive correlation between the molar ratio of (Ca2+ & Mg2+)/alkalinity to DSi in the tilled corn and the forested site suggested, however, that silicate minerals weathered as alkalinity was lost via enhanced nitrification resulting from fertilizer additions to the corn watershed and from leaf litter decomposition in the forest. This same relation was observed in the Ohio-Tennessee River Basin where dominant landuse types include both agricultural lands receiving nitrogenous fertilizers and forests. However, greater gains in DSi with respect to alkalinity losses in the Ohio-Tennessee River Basin than in the NAEW sites suggested that soils derived from younger Pleistocene glacial-till may yield more DSi relative to nitrogenous fertilizer applications than the older NAEW soils. Because silicate weathering occurs via acids released from nitrification, CO2 consumption estimates based on the assumption that silicate weathers via carbonic-acid alone may be especially over-estimated in fertilized agricultural watersheds with little baseflow (i.e. 67% overestimated in the corn till watershed). CO2 consumption estimates based on silicate weathering may be as much as an average of 8% lower than estimates derived from carbonic acid weathering alone for the Ohio-Tennessee River Basin between 1979–2009.