Sheep (Ovis aries L.) grazing, a cost-effective method of controlling weeds compared to herbicide application and tillage, may influence soil C and N levels by consuming plant residue and returning feces and urine to the soil, but little is known about the practice on soil C and N storage under dryland cropping systems in the northern Great Plains, USA. Three weed control practices [sheep grazing (GRAZ), herbicide application (CHEM), and tillage (MECH)] and three cropping sequences [continuous alfalfa (Medicago sativaL.) (CA), continuous spring wheat (Triticum aestivum L.) (CSW), and spring wheat-pea (Pisum sativumL.)/barley (Hordeum vulgaris L.) hay mixture-fallow (W-P/B-F)] were evaluated on a Blackmore silt loam from 2009 to 2011 in southwestern Montana, USA. Crop yields and soil organic C (SOC), total N (STN), NH4-N, and NO3-N contents at the 0–120 cm depth were quantified. Annualized spring wheat grain and biomass (stems + leaves) yields and C and N contents were greater with CSW than with W-P/B-F, but hay biomass and C content were similar between CA and W-P/B-F. While C and N in aboveground biomass after spring wheat and hay harvest were removed through haying in CHEM and MECH, sheep grazing removed about 99% of these elements in GRAZ. The SOC and STN at 5–15 cm were greater with CSW or W-P/B-F than with CA in GRAZ and MECH, but SOC at 30–60 cm was greater with CA than with CSW in MECH. The NH4-N content at most depths varied among treatments and years, but NO3-N content at 5–120 cm was greater with CSW and W-P/B-F than with CA. Longer duration of sheep grazing during fallow periods due to increased return of C and N through feces and urine or residue incorporation to a greater depth probably increased soil C and N storage at the surface layer with CSW and W-P/B-F in GRAZ and MECH, but increased root biomass likely increased C storage at the subsurface layer with CA in MECH. Absence of N fertilization and/or greater N uptake probably reduced soil NO3-N level with CA than with other cropping sequences. Regardless of treatments, SOC and STN declined from 2009 to 2011, probably due to residue removal from haying and grazing. Moderate sheep grazing during fallow periods can be used to increase soil C and N storage, obtain farm C credit, and sustain dryland crop yields compared to herbicide application for weed control in the semiarid regions.
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