Sheep Grazing Enhances Coarse Relative to Microbial Organic Carbon in Dryland Cropping Systems

Sheep (Ovis aries L.) grazing, a cost-effective method of weed control compared with herbicide application and tillage, may influence soil C fractions by consuming crop residue and weeds and returning C through feces and urine to the soil. We examined the effect of three weed management practices (sheep grazing, herbicide application, and tillage) and two cropping sequences (continuous spring wheat [Triticum aestivum L.] [CSW] and spring wheat-pea [Pisum sativum L.]/barley [Hordeum vulgare L.] mixture hay-fallow [W-P/B-F]) on soil microbial biomass C (MBC), potential C mineralization (PCM), and particulate organic C (POC) in relation to soil organic C (SOC) at the 0- to 30-cm depth from 2009 to 2011 in southwestern Montana. The MBC at 0 to 5 cm was greater with tillage on CSW than tillage on W-P/B-F in 2009 and 2011, but was greater with herbicide application on CSW than tillage on CSW in 2010. The POC at 0 to 5 cm and 15 to 30 cm was greater with sheep grazing than herbicide application on CSW and W-P/B-F, but at 5 to 15 cm was greater with grazing on CSW. The MBC, PCM, and POC at all depths decreased from 2009 to 2011. Crop residue incorporation into the soil increased MBC with tillage on CSW. Lower proportions of labile than nonlabile organic matter through feces and urine probably reduced MBC at the soil surface, but increased POC with sheep grazing compared with herbicide application on CSW and W-P/B-F. Sheep grazing may increase coarse soil organic matter compared with microbial biomass in dryland cropping systems.