Organic carbon composition in a northern mixed-grass prairie: Effects of grazing

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 N4.94 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.