Manure and Inorganic N Effects on Soil Greenhouse Gas Emissions Under Irrigated Corn Silage.

Manure from concentrated animal feeding operations is often applied to cropped soils as a substitute for synthetic fertilizer amendments, but the impacts of this practice on soil greenhouse gas (GHG) fluxes, yields, and plant N uptake are uncertain.  We applied dairy manure, enhanced efficiency fertilizer, and urea to a semi-arid irrigated tilled corn silage production system on a clay loam soil for 3 years and monitored GHG emissions. Averaged over years, growing season soil CO₂-C emissions were greater for dairy manure than for urea, SuperU, and check (no N applied) treatments, CH₄-C emissions did not vary among N treatments, and N₂O-N emissions decreased in the order urea=dairy manure>SuperU>check. Averaged over years, non-growing season CO₂-C emissions decreased in the order dairy manure>urea=SuperU=check, CH₄-C emissions did not vary with N treatment, and N₂O-N emissions decreased in the order SuperU=dairy manure, SuperU>urea=check, and dairy manure=urea. Averaged over years, growing season total GHG emissions in CO₂-C equivalents (CO₂+CH₄+N₂O) were greater for dairy manure than any other N treatment due to greater CO₂ fluxes. Cumulative NH₃-N emissions for 91 days following manure application decreased in the order urea>SuperU>check with SuperU=dairy manure and dairy manure=check.  Silage yields and N uptake with dairy manure or inorganic N applications were greater than the check treatment when averaged over years, with no difference between dairy manure, urea, and SuperU. Although total soil GHG emissions were greater with manure than synthetic fertilization, expected increases in soil C sequestration induced by manure addition should compensate for this apparent increased GHG flux.