Nitrous oxide emissions from New Zealand agriculture: research to refine the national inventory

摘要

Ratification of the Kyoto Protocol will commit New Zealand to reduce its greenhouse gas emissions to 1990 levels, on average, during the first commitment period. About 55% of the total greenhouse gas emissions derive from New Zealand's pastoral agriculture, which typically involves year-round grazing of grass/clover pastures and relatively low N fertiliser use. Direct emissions from N excreted on grazed pastures are therefore the single largest source of anthropogenic N_2O. New Zealand currently applies a single country-specific emission factor for this source (EF_(3 PR&P) = 1%). Further refinement of this emission factor is a priority. N_2O emissions for animal urine were determined over a 4 to 5 month period, for 4 soils, in 3 regions. Nitrous oxide emission ranged from < 0.5% to 2.6% of the urine N applied, in average soil drainage classes ranging from well to poorly drained soils. Based on these results, separate values for EF_(3 PR&P) were used for pastoral soils of different drainage class, and the resulting weighted average emission factor (0.94%) was very similar to the currently used EF_(3 PR&P). The national N_2O inventory was further refined by using regional animal population statistics and New Zealand specific values for N excretion rate that were calculated using a nutrient balance model. The N excretion rates estimated for 1990, 1999 and 2010 were on average 33%, 51% and 58% higher than values currently used in New Zealand's inventory calculations. When using these higher N excretion rates in combination with the EF_(3 PR&P) values for soils of different drainage class, N_2O emissions for 1999 and 2010 were estimated to be 10 and 20-30% higher than in 1990. From Monte Carlo numerical simulation, including a log-normal distribution for the most influential parameter, EF_(3 PR&P), we were 95% sure that New Zealand's direct and indirect N_2O emission from sheep, beef and dairy cattle excreta is currently between 23 and 81 (mean = 44) Gg/yr. The range of uncertainty is nearly four-fold and skewed like that of drainage since 74% of New Zealand's pastoral soils are well drained, while only 9% are poorly drained.