Tower-based N2O flux measurements following liquid dairy manure applications over a forage field

As a contribution to the development and verification of a farm GHG emission calculator, a roving tower-based GHG flux measurement system was developed to monitor GHG emissions from agricultural fields in relation to management practices. In the spring of 2004, the roving GHG flux system was installed in a mixed alfalfa grass forage field belonging to a large dairy farm of 900 animal units located in Richmond (ON, Canada). Liquid dairy manure was applied without incorporation in the spring and after each forage cut. The N₂O emissions were monitored using the flux-gradient technique for a year. The site had a large fetch of more than 250 m in all directions except for a 45 degree section centred east of the tower. Criteria related to the tunable diode laser, ultrasonic anemometer, wind direction, turbulence were used to screen the data. Daily fluxes N₂O were calculated after filling gaps. The N₂O emissions were short-lived after each application of liquid dairy manure lasting about a week. The peak emissions were in the range of 10-15 mg N₂O-N m^-2 d^-1. The background N₂O emissions between the dairy manure applications were below 0.5 mg N₂O-N m^-2 d^-1 and did not exhibit any rainfall-driven emission pattern. The cumulative emissions following each liquid application were small, being below 0.4 kg N₂O-N ha^-1. During the following spring thaw, the emissions peak was 3 mg N₂O-N m^-2 d^-1, with cumulative emission over the period of 0.4 kg N₂O-N ha^-1.