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

Joy L. Barsotti, Upendra M. Sainju, Andrew W. Lenssen, Zach J. Miller, Patrick G. Hatfield
Sustainable agriculture research 2016 v.5 no.2 pp. 1-14
soil organic carbon, urine, arid lands, grazing, carbon sequestration, silt loam soils, Pisum sativum, sustainable agriculture, weed control, barley, peas, Hordeum vulgare, hay, feces, soil microorganisms, Triticum aestivum, silt, mineralization, crop residues, soil quality, fallow, weeds, pesticide application, sheep, tillage, microbial biomass, spring, spring wheat, cost effectiveness, cropping sequence, Montana
A cost-effective method of weed control compared with herbicide application and tillage is sheep (Ovis aries L.) grazing which may influence soil C fractions by consuming crop residue and weeds and returning C through feces and urine to the soil. Little is known about the effect of sheep grazing on soil C fractions in dryland cropping systems. The objective of this experiment was to examine the effect of sheep grazing compared with tillage and herbicide application for weed control on soil microbial biomass C (MBC), potential C mineralization (PCM), and particulate organic C (POC) in relation to soil organic C (SOC) at 0-5, 5-15, and 15-30 cm depths in a Blackmore silt loam under dryland cropping systems from 2009 to 2011 in southwestern Montana, USA. Treatments were three weed management practices (sheep grazing [grazing], herbicide application [chemical], and tillage [mechanical]) as the main plot 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]) as the split-plot factor arranged in a randomized complete block design with three replications. We hypothesized that sheep grazing on CSW would increase POC, but reduce MBC and PCM compared with tillage and herbicide application on W-P/B-F. The MBC at 0-5 cm was greater in mechanical with CSW than grazing with CSW or mechanical with W-P/B-F in 2010 and 2011. Averaged across years, POC at 0-5 cm was greater in grazing than chemical with W-P/B-F, at 5-15 cm was greater in chemical than grazing with CSW, and at 15-30 cm was greater in grazing than chemical and mechanical with CSW. The PCM/SOC ratio at 15-30 cm was greater in mechanical than grazing with CSW. At all depths, MBC, PCM, and POC decreased from 2009 to 2011 for all treatments, except for MBC in mechanical with CSW at 0-5 cm which increased. Lower proportion of labile than nonlabile organic matter returned to the soil through feces and urine probably reduced soil microbial biomass and activity, but increased coarse organic matter fraction with sheep grazing compared with herbicide application and tillage. Sheep grazing may improve soil health and quality in the long term by enhancing microbial biomass and C storage compared with tillage and herbicide application for weed control under dryland cropping systems.