Comparison of tunable diode laser and gas chromatography for measuring nitrous oxide emissions from a paddock during FARMGAS2006 in New Zealand

The New Zealand agricultural sector contributes almost 50% of the overall anthropogenic emissions of the country. As dairy farms are expending, their greenhouse gas contribution needs to be quantified in order to implement mitigation strategies. The FarmGas 2006 measurement campaign was conducted on a commercial dairy-farm in North Canterbury over 3 weeks in October 2006. Typically, the dairy cows were sent in the same paddock for two half-days of grazing and moved to the next paddock. Pivot irrigators were used to compensate for rainfall deficit and ensure an optimum growth of the forage. Nitrous oxide emissions were measured using the flux-gradient technique before, during and after grazing. The eddy diffusivity coefficient was computed using wind and temperature vertical profiles. Nitrous oxide gradients were measured at 2.75 and 1.75 m above the displacement height using a fast-response tunable diode laser (TDL; TGA-100, Campbell Scientific) and an automated gas chromatograph equipped with an electron capture detector (GC/ECD; 6890, Agilent Technologies) coupled to an automated air sampling system. This was the first field deployment of a fast-response TDL for N₂O flux measurement in New Zealand. The TDL measured raw N₂O concentration at 10 Hz, from dried air sampled from the two heights consecutively by routine switching at 3 s intervals, and the values were averaged over 20 minute periods. Calibration was provided by a single 2500 ppm N₂O in N₂ (BOC alpha recipe MA84176). Gases from the same inlet heights as the TDL were dried and sampled simultaneously into a tedlar bag accumulator (one bag per height) over the 20 minute run time and then analysed consecutively. Swapping between a pair of accumulator bag-sets allowed continuous sample collection. The power law response of the GC/ECD was calibrated using a suite of 6 N₂O primary standards (Scott Specialty Gases) and with 2 working standards at 317 and 355 ppb. A calibration run was included with each gradient measurement set. Baseline N2O emission was <100 ng m^-2 s^-1 and rose to <250 ng m^-2 s^-1following grazing by the dairy herd. Emission was characterised by events of high flux lasting several hours such that half of the total N₂O was emitted in about 10% of the time over the duration of the campaign. The GC/ECD gave similar results as the TDL, except that the low emissions were underestimated as its precision was at least an order of magnitude lower. Over 10-d periods the post grazing N₂O emissions were similar for both approaches with 0.55 kg N₂O-N ha^-1, while the 0.13 kg N₂O-N ha^-1 baseline emissions measured by the TDL were 50% lower in the case of the GC/ECD. The irrigator tended to generate instantaneous high N₂O emission peaks compared to rainfall which induced delayed emission peaks. As the maximum precipitation rate of the irrigator was at least twice as much as thus of the rain, we hypothesize that the N₂O emissions correspond to a physical N₂O flush from the soil. The Farmgas field campaign demonstrated the usefulness of measuring continuously the N₂O emissions and pointed out the need to improve the irrigation management is order to limit the N₂O emissions.