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| Titel |
Silicate weathering and CO2 consumption within agricultural landscapes, the Ohio-Tennessee River Basin, USA |
| VerfasserIn |
S. K. Fortner, W. B. Lyons, A. E. Carey, M. J. Shipitalo, S. A. Welch, K. A. Welch |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 9, no. 3 ; Nr. 9, no. 3 (2012-03-06), S.941-955 |
| Datensatznummer |
250006835
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| Publikation (Nr.) |
 copernicus.org/bg-9-941-2012.pdf |
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| Zusammenfassung |
| 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 (0.0065 to 0.383 km2) 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. Median DSi yields
(2210–3080 kg km−2 yr–1) were similar to the median of annual
averages between 1979–2009 for the much larger Ohio-Tennessee River Basin
(2560 kg km−2 yr–1). Corn watersheds, which only had surface
runoff, had substantially lower DSi yields (<530 kg km−2 yr–1)
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. 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 20 % lower than estimates derived from
carbonic acid weathering alone for the Ohio-Tennessee River Basin between
1979–2009. Globally, this may mean that younger landscapes with soils
favorable for agriculture are susceptible to fertilizer-enhanced silicate
weathering. Increases in silicate weathering, however, may be offset by
shifts in hydrology resulting from agricultural land management practices or
even from soil silica losses in response to repeated acidification. |
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