No abstract

The nitrification inhibitor dicyandiamide (DCD) has been reported to significantly reduce nitrate (NO À 3 ) leaching and nitrous oxide (N 2 O) emissions under urine patches in New Zealand trial conditions. However, results on the efficacy of DCD for increasing pasture production have been inconclusive. As part of a national series of nitrous oxide mitigation research (NOMR) trials, a 3-year study was conducted at a dairy farm in South Otago to examine the effect of DCD on pasture production and N leaching under mowing and/or (simulated) grazing conditions and on N 2 O losses from dung and urine patches. Over the three trial years, DCD significantly inhibited nitrification in urine-amended soil, reducing peak NO À 3 concentrations in the soil by >50% and the N 2 O emission factor from urine by 33%-72%. Nitrate concentrations in drainage under simulated grazing plots with or without DCD applications were low and consequently no effect of DCD was detected. The N 2 O emissions and N leaching results suggested that high rates of denitrification occurred in the imperfectly draining soil, particularly in years 2 and 3 of the study. DCD significantly increased spring pasture dry matter (DM) yields from urine patches by 9%-21% in all years under mowing, but no significant annual effect on DM response was detected under either mowing or grazing. The observed increases in DM response in the urine patches under mowing were not detectable under the variable pasture responses under true grazing conditions.

The purpose of this short-term study was to investigate the effect of increasing fertiliser nitrogen (N) application rates on nitrous oxide (N2O) emissions over the late winter/early spring period from sheep-grazed pasture in Otago rolling hill country. We measured N2O gas emissions from plots on a mottled Fragic Pallic hill soil receiving 0, 100 and 500 kg N/ha.year for 2 years. Plots were sampled weekly for 10 weeks over the 2006 winter/spring period using a static chamber method. Increased N fertiliser rate and the attendant increase in stocking rate significantly increased total N2O emissions (P < 0.05). Total N2O emissions for the measurement period were estimated to be 0.08, 0.13 and 1.36 kg N2O-N/ha (s.e.m, 0.1, 0.18 and 0.45) for the 0, 100 and 500 N treatments, respectively. Our results suggest that high application rates of fertiliser N (i.e. 500 kg N/ha.year) and attendant increased stocking rates may significantly increase emissions of N2O even in dry winter/spring conditions in Otago rolling hill country. These results will assist in the development of best management guidelines for reducing N2O emissions from fertiliser N in hill country.

The purpose of this study was to conduct a preliminary investigation into the effect of increasing fertiliser- and excreta-N inputs on the spatial distribution and rate of potential nitrification activity in hill country pasture land at two sites, Invermay and Ballantrae. High nitrification rates could potentially limit N efficiency by increasing N losses through leaching and denitrification. Nitrification potentials (NP) were measured in camp sites and medium slopes of hill country soils receiving 0 kg N and 500 kg N/ha/yr over the previous 18 months. Nitrification potential was determined by calculating the rate of nitrate production (mg NO3-N/kg soil/h) by linear regression of soil solution concentrations, versus time. Nitrification potential was significantly higher at Invermay than at Ballantrae, which was likely due to a significantly lower soil pH at Ballantrae. At Invermay, NP increased with fertiliser-N application rate and in camp site soils. The fertiliser N effect was not observed at Ballantrae. However, soil NO3-N and NP was significantly greater in soils from camp sites than for soils from medium slopes. Best management practices for fertiliser-N application in hill country should make allowances for these factors to maximise farm efficiency and profitability. Keywords: hill country, nitrification potential, nitrogen fertiliser, stock behaviour, excreta-N, mineral-N, New Zealand