Grazing and Ecosystem Carbon Storage in the North American Great Plains |
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Authors: | Justin D Derner Thomas W Boutton David D Briske |
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Institution: | (1) High Plains Grasslands Research Station, USDA-ARS, 8408 Hildreth Road, Cheyenne, WY 82009, USA;(2) Department of Rangeland Ecology and Management, Texas A&M University, College Station, TX 77843-2126, USA |
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Abstract: | Isotopic signatures of 13C were used to quantify the relative contributions of C3 and C4 plants to whole-ecosystem C storage (soil+plant) in grazed and ungrazed sites at three distinct locations (short-, mid- and
tallgrass communities) along an east–west environmental gradient in the North American Great Plains. Functional group composition
of plant communities, the source and magnitude of carbon inputs, and total ecosystem carbon storage displayed inconsistent
responses to long-term livestock grazing along this gradient. C4 plants primarily Bouteloua gracilis (H.B.K.) Lag ex Steud.] dominated the long-term grazed site in the shortgrass community, whereas the ungrazed site was co-dominated
by C3 and C4 species; functional group composition did not differ between grazed and ungrazed sites in the mid- and tallgrass communities.
Above-ground biomass was lower, but the relative proportion of fine root biomass was greater, in grazed compared to ungrazed
sites at all three locations. The grazed site of the shortgrass community had 24% more whole-ecosystem carbon storage compared
to the ungrazed site (4022 vs. 3236 g C m−2). In contrast, grazed sites at the mid- and tallgrass communities had slightly lower (8%) whole-ecosystem carbon storage
compared to ungrazed sites (midgrass: 7970 vs. 8683 g C m−2; tallgrass: 8273 vs. 8997 g C m−2). Differential responses between the shortgrass and the mid- and tallgrass communities with respect to grazing and whole-ecosystem
carbon storage are likely a result of: (1) maintenance of larger soil organic carbon (SOC) pools in the mid- and tallgrass
communities (7476–8280 g C m−2) than the shortgrass community (2517–3307 g C m−2) that could potentially buffer ecosystem carbon fluxes, (2) lower root carbon/soil carbon ratios in the mid- and tallgrass
communities (0.06–0.10) compared to the shortgrass community (0.20–0.27) suggesting that variation in root organic matter
inputs would have relatively smaller effects on the size of the SOC pool, and (3) the absence of grazing-induced variation
in the relative proportion of C3 and C4 functional groups in the mid- and tallgrass communities. We hypothesize that the magnitude and proportion of fine root mass
within the upper soil profile is a principal driver mediating the effect of community composition on the biogeochemistry of
these grassland ecosystems. |
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Keywords: | δ 13C C isotope signatures carbon storage grazing Great Plains soil organic carbon |
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