Winter Time Nitrous Oxide Emission From Grass Receiving Whole and Separated Dairy Slurry Or Fertilizer In a Mild Maritime Climate.

There is increasing evidence that biological activity continues in soils under cold and even frozen conditions during winter months when crops are not actively growing.  This study examined nitrous oxide emissions during winter months from a multi-year trial in a moist maritime climate of coastal British Columbia, Canada. The trial consisted of a tall fescue stand which had received nutrient applications from commercial fertilizer (FERT), whole dairy manure (MAN) and the separated liquid portion of whole dairy manure (SEP) for ten years. Solids were removed from the SEP manure by settling and decanting which reduced slurry viscosity, enhanced soil infiltration and reduced the organic N fraction. Whole dairy manure and the separated liquid portion were applied at high (HI) and low (LO) total nitrogen rates. Nitrous oxide emissions were measured over winters from 2008/2009 to 2011/2012 beginning at last grass harvest in the fall until first nutrient application in the spring. Annual total nitrogen applications for the growing seasons associated with the four winters for FERT, SEP_LO, MAN_LO, SEP_HI and MAN_HI averaged 400, 396, 399, 547 and 577 kg N/ha respectively. Nitrous oxide emissions showed considerable variation between winters. Generally emissions were low during cool, wet weather typical of the region. Significantly higher emissions followed freeze-thaw events. Averaged over four winter periods nitrous oxide emissions were highest for MAN_HI followed by SEP_HI and FERT which in turn were significantly greater than SEP_LO and MAN_LO. Nitrous oxide emissions as a percent of previous growing seasons total nitrogen application were significantly higher for MAN_HI and FERT (0.184 and 0.182% of applied N, respectively) compared to SEP_HI, SEP_LO and MAN_LO (0.139, 0.140 and 0.112% of applied N, respectively). Emissions associated with winter periods and freeze thaw events should be added to the annual emissions attributed to application of manure and commercial fertilizer.