Tillage-induced soil nitrous oxide fluxes from tow soils in the Manawatu : a thesis presented in partial fulfilment of the requirements for the degree of Master of Applied Science in Agricultural Engineering, Institute of Technology and Engineering, Massey University, Palmerston North, New Zealand

Abstract

Enhanced greenhouse gas emissions of nitrous oxide (N₂O) induced by agricultural practices is believed to be the major anthropogenic source. Studies conducted in New Zealand generally from pasture suggest low N₂O emission, however, there is little information for arable farming systems. Therefore, there is a need for a site-specific assessment of the impact of tillage practices on N₂O fluxes. This paper evaluates tillage system and land use effects on N₂O emissions at two sites using a closed chamber technique. Sites included a Kairanga silt loam where maize/barley was grown continuously for either 17 (K17) or 34 (K34) years, with a conventional tillage system (Kairanga), and an Ohakea silt loam where winter oats and summer fodder maize was double-cropped for five years with conventional (CT) and no-tillage (NT) systems (Massey). At both sites permanent pasture (PP) soil was used as a control. Spatial measurements for all treatments at Massey site showed large inherent variations in N₂O fluxes (a mean CV=119%) which reflected natural soil heterogeneity, and perhaps the measurement technique used rather than the real differences due to the tillage and cropping systems evaluated. N₂O emissions measured from December 1998 to September 1999 from the PP were significantly lower (1.66 kg N₂O-N/ha/year) than the CT and NT plots at 9.20 and 12.00 kg N₂O-N/ha/year respectively. However, there were no differences in N₂O emission rates between the CT and NT treatments. Cumulative coefficient of variation (CV) of treatments ranged from 39 to 140%. Seedbed preparation using power-harrow which was done within few days of ploughing the CT plots reduced N₂O emissions by 65% within the first hour after power-harrowing. However. N₂O emission rates returned to the pre-power harrowing levels one month after power-harrowing. There was strong relationship between log-transformed values of soil moisture content (SMC) and N₂O emissions in all treatments. PP (r = 0.73), CT (r = 0.75) and NT (r = 0.86). Seasonal variation in N₂O emission from the PP was in the order of winter=autumn>summer. Although fluxes in the CT were higher in winter than in the autumn season, there were no differences between the summer and autumn data. Similar to the PP. the seasonal variations in N₂O emission in the NT treatment were in the order of winter>autumn=summer. The estimated annual N₂O emissions from the PP. K17 and K34 (calculated as the mean of all individual closed cover chamber measurements between November 1998 and September 1999) from Kairanga site were similar at 3.24, 3.42 and 2.37 kg N₂O-N/ha/year, respectively. There were large variations in N₂O emissions during the year with the mean flux rates ranging from 0.175 to 13.32, 0.175 to 16.91 and 0.088 to 30.05 kg N₂O-N/ha/year in the PP, K17 and K34 fields, respectively. Although overall comparison of treatment means did not show any discernible differences between management practices, there were signs that the K34 had lower emissions compared to the PP. N₂O fluxes from the K17 and PP field appeared to be influenced by SMC. There is clear indication that low or negligible emissions occur when gravimetric soil water content is less than 30% in the PP. Although N₂O fluxes did not follow the rainfall patterns in the K17 and PP, linear regression analyses indicated low but significant relationship r = 0.46 and 0.53 (0.72 when log-transformed), respectively. In the K34 field. SMC did not seem to govern fluxes which were especially apparent during wet months of April and May. The linear regression analysis using the measured data revealed no relationship (r = 0.12) between the SMC and N₂O fluxes in the K34 treatment. Seasonal grouping of monthly log-transformed N₂O emissions showed significant differences in all treatments. Summer season N₂O emissions in the PP were the lowest than other seasons whereas no discernible differences were observed among other seasons. Although N₂O fluxes during spring and summer were similar in the K17 field, they were significantly lower than the winter and higher than autumn fluxes. There were considerably higher emissions in summer than in autumn in the K34 but seasonal variation between winter and spring was less profound. Spatial variability in N₂O fluxes was large during the year with coefficients of variation (CV) ranging from 10 to 82%, 12 to 99% and 9 to 137% for the PP, K17 and K34 fields, respectively