Submitted to: Soil Science Society of America Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/20/2005
Publication Date: 5/30/2006
Citation: Ganjegunte, G.K., Vance, G.F., Preston, C.M., Schuman, G.E., Ingram, L.J., Stahl, P.D., Welker, J.M. 2006. Soil organic carbon composition in a northern mixed-grass prairie: effects of grazing. Soil Science Society of America Journal. 69:1746-1756. Interpretive Summary: Proper grazing management has been shown to increase soil organic carbon in Northern Great Plains rangelands. Heavy stocking rates have resulted in increased soil organic C during years of average or above average precipitation but has resulted in significant shifts in plant community composition. Heavy grazing pressure has caused shifts from C3 dominated plant communities to those dominated by C4 grasses which are shallower rooted. During a 10 year period that included 5 years of severe drought, the heavy stocking rate treatments lost nearly 30% of the soil organic C stored during the previous 11 years of treatment; whereas, the non-grazed and lightly grazed did not show any loss in soil organic C. Characterization of the soil organic C under these three grazing treatments exhibited significant differences in fulvic acid lignins were higher under the heavy grazing compared to the other grazing treatments. Total lignin was greater under the non-grazed treatment compared to the light and heavy treatments. The soil organic matter composition data suggest that the light grazing treatment is the most sustainable grazing management strategy for the northern mixed-grass prairies. Being able to store or at least maintain soil organic C in the soil during drought periods is important from a climate change standpoint and also from a nutrient cycling and soil water storage standpoint, these characteristics enable greater sustainability of the system.
Technical Abstract: A study was conducted at the USDA-ARS High Plains Grasslands Research Station (HPGRS), Cheyenne, WY to evaluate soil organic carbon (SOC) composition in light (LG) and heavily (HG) grazed sites and in 60 year old non-grazed exclosures (EX). 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). Significantly greater 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, whereas FA extracted from HG contained significantly greater V and C than that extracted from LG and EX. Whereas 13C NMR spectra of HA and FA did not vary significantly among the three grazing treatments, overall the HA spectra had significantly greater alkyl, methoxyl, and aromatic C than FA and FA spectra had significantly greater O-alkyl and di-O-alkyl C than HA, suggesting HA is more recalcitrant and aromatic than FA. Isotopic signatures (delta13C and delta15N) for HA and FA indicated that humic substances were derived from both C3 and C4 plants and that N was more recalcitrant in HA. The results of this study indicated that light grazing might be the most sustainable grazing management system for northern mixed-grass prairies.