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Soil Biology & Biochemistry

Soil Organic Carbon Composition in a Northern Mixed-Grass Prairie

Effects of Grazing

^a Dep. of Renewable Resources, 1000 E. University Ave., Univ. of Wyoming, Laramie, WY 82071-3354
^b Canadian Forest Service, Natural Resources Canada, Pacific Forestry Centre, 506 West Burnside Rd., Victoria BC, Canada V8Z 1M5
^c USDA-ARS, High Plains Grasslands Research Station, Rangeland Resource Research Unit, 8408 Hildreth Rd., Cheyenne, WY 82009
^d Environment and Natural Resource Institute, Univ. of Alaska, 3211 Providence Dr., Anchorage, AK 99508

^* Corresponding author (gfv{at}uwyo.edu)

Growing interest in the potential for soils to provide a sink for atmospheric C has prompted studies of effects of management on the amount and nature of soil organic C (SOC). In this study, we evaluated effects of different grazing management regimes (light grazing [LG], heavy grazing [HG], and non-grazed exclosures [EX]) on amount and composition of SOC at the USDA–ARS High Plains Grasslands Research Station (HPGRS), Cheyenne, WY. Soils (0–5 cm) from each treatment were analyzed for total C and N contents and lignin composition. Soil organic C and N contents were significantly greater in LG (SOC–13.8 Mg ha^–1; total N–1.22 Mg ha^–1) than HG (SOC–10.9 Mg ha^–1; total N–0.94 Mg ha^–1) or EX (SOC–10.8 Mg ha^–1; total N–0.94 Mg ha^–1). From CuO oxidation studies, significantly greater (P < 0.05) total lignin (Vanillyl [V] + Syringyl [S] + Cinnamyl [C] compounds) contents were noted in EX (21 g kg^–1 SOC) than LG (12 g kg^–1 SOC) and HG (15 g kg^–1 SOC) soils. The lignin composition of humic (HA) and fulvic (FA) acids indicated that HA under LG contained significantly greater V and S than HG or EX. Fulvic acids contained S-depleted lignin compared with HAs and FAs from HG, which contained significantly greater V and C than FAs extracted from LG and EX. Nuclear magnetic resonance (NMR) spectra of HA and FA, however, did not vary significantly among the three grazing treatments. Results from CuO oxidation and NMR spectroscopy emphasized the familiar problem that determining the nature of soil organic matter (SOM) is a difficult task and sometimes different analytical techniques provide different information about the nature of SOM. Nonetheless, results of this study indicate that LG is the most sustainable grazing management system for northern mixed-grass prairies.

Abbreviations: CPMAS, cross polarization magic angle spinning • CuO, cupric oxide • DD, dipolar dephasing • EX, exclosures • FA, fulvic acid • HA, humic acid • HG, continuous heavy grazing • HPLC, high performance liquid chromatography • LG, continuous light grazing • NMR, nuclear magnetic resonance • SOC, soil organic carbon • SOM, soil organic matter • SPE, solid phase extraction • VSC, vanillyl-syringyl-cinnamyl groups