Soil carbon and nitrogen and greenhouse gas emissions affected by sheep grazing under dryland cropping systems
Barsotti, Joy Lynn
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Sheep grazing to control weeds during fallow may influence soil C and N and greenhouse gas emissions by consuming crop residue and returning feces and urine to the soil. An experiment was conducted to evaluate the effect of sheep grazing compared to tillage and herbicide application for weed control on soil total C, total N, NH 4-N, and NO 3-N contents at the 0-120 cm depth from 2009 to 2011 and greenhouse gas (CO 2, N 2O, and CH 4) emissions from May to October, 2010 and 2011 under dryland cropping systems in western Montana. Treatments were three fallow management practices (sheep grazing [GRAZ), herbicide application [CHEM], and tillage [MECH]) and three cropping sequences (continuous alfalfa [CA], continuous spring wheat [CSW], and spring wheatpea/barley hay-fallow [W-P/B-F]). Soil samples were collected with a hydraulic probe after crop harvest and greenhouse gas samples at 3 to 14 d intervals with a static chamber. Soil total C was greater in CSW and W-P/B-F than in CA at 5-30 cm but was greater in CA and CSW than in W-P/B-F at 60-90 cm. Soil total N and NO 3-N contents were greater in CSW and W-P/B-F than in CA at 5-120 cm. Soil NH 4-N content varied with treatments and years. Soil temperature and water content at 0-15 cm were greater in CHEM with W-P/B-F and GRAZ with CA than in other treatments. Greenhouse gas fluxes peaked immediately following substantial precipitation (>12 mm) and/or N fertilization, regardless of treatments. Total CO 2 flux from May to October was greater in GRAZ with CA but N 2O flux was greater in CHEM and GRAZ with CSW than in other treatments in 2010 and 2011. Total CH 4 flux was greater in CA than in CSW and W-P/BF in 2011. Net global warming potential and greenhouse gas intensity were greater in CHEM with CSW than in other treatments. Continuous spring wheat increased soil C and N storage and available N at subsurface layers compared to other cropping sequences. Because of higher N 2O emissions and lower C sequestration rate, global warming potential and greenhouse gas intensity increased under continuous spring wheat with herbicide application for weed control.