Nitrous Oxide Emission From Old and New Pastures.

Although intensively managed rotationally grazed pastures should have a relatively high coverage of excreta little information exists about nitrous oxide emission from them.  Annual N₂O-N emission rates for ungrazed (exclosed), grazed and urine spot sites in replicated, rotationally grazed, fertilized (100 kg N ha^-1) old grass (30 yr-old smooth bromegrass, quackgrass, Kentucky bluegrass) and meadow bromegrass (6 yr-old) pastures were compared from 2003 to 2005 in a randomized block design. Average stocking rate of yearling heifers for the pasture types was similar. Gas samples were collected weekly using a non-flow through non steady-state method from early April until freeze-up. Urine sites were marked after urination and monitored for an entire year. Annual N₂O-N loss estimates were calculated for each sampling unit by linear interpolation. NO₃-N and NH₄-N supply rates were monitored within pasture and urine sites using buried cation and anion probes. N₂O-N flux on grazed and urine sites was highly variable. Average annual N₂O-N emission rate for ungrazed and grazed (0.50 vs. 2.04 kg N₂O-N ha^-1) was similar, but lower than urine sites (5.45 kg N₂O-N ha^-1);  on average old grass was higher than meadow bromegrass (P. = 0.05). When analysed by site old grass had significantly lower, similar and higher average annual N₂O-N emission rates compared to meadow bromegrass at ungrazed, grazed and urine sites, respectively. Nitrification of the urine sites in the old grass lagged behind that of the meadow bromegrass as indicated by significantly higher NH₄-N supply rates. This may be an indication that the pathway for nitrous oxide was through a chemo-denitrification process on the old grass pasture. The information represents sites and not entire pasture areas, however the emission rates for both pastures on a carbon dioxide equivalent basis were considered relatively high.