Sheep Grazing Effect On Greenhouse Gas Emissions Under Dryland Cropping System.

Sheep grazing is an inexpensive method of weed and pest control but little is known about its effect on greenhouse gas emissions under dryland cropping systems. We evaluated the effect of two fallow management practices (sheep grazing [Grazing] and no-tilled herbicide application [Chemical] for weed control) and three cropping sequences (continuous spring wheat [CSW], spring wheat-Austrian winter pea/hay barley-fallow (SW-P/HB-F), and continuous alfalfa (CA) on soil surface greenhouse gas (CO₂, N₂O, and CH₄) fluxes and temperature and water content to a depth of 15 cm. Measurements were made from May to October 2010 in a silt loam soil in southwestern Montana. Soil temperature increased from May to August and then declined while water content responded to substantial precipitation events in May, June, and September.  Greenhouse gas fluxes peaked following substantial precipitation during increased temperature. The CO₂ flux was greater in the chemical than in the grazing treatment in May and June and greater in CA than in other cropping sequences at most measurement dates. The N₂O flux was greater in CSW than in other cropping sequences in May and June. The CH₄ flux varied with fallow management at various measurement dates. Total CO₂ flux from May to October was greater in CA than in other cropping sequences, regardless of fallow management. Total N₂O flux followed the order: CSW>SW-P/HB-F>CA. Total CH₄ flux was not influenced by treatments. While enhanced microbial activity due to increased soil temperature and water content probably increased greenhouse gas fluxes, greater root respiration likely increased CO₂ flux in CA than in other cropping sequences. Increased N fertilization probably increased N₂O flux in CSW than in other cropping sequences. Sheep grazing had minimum effect on greenhouse gas emissions compared with no-tilled herbicide application for weed control.