Soil-atmosphere exchange of nitrous oxide and methane in New Zealand terrestrial ecosystems and their mitigation options: a review

Abstract

The two non-CO2 greenhouse gases (GHGs) nitrous oxide (N2O) and methane (CH4) comprise 54.8% of total New Zealand emissions. Nitrous oxide is mainly generated from mineral N originating from animal dung and urine, applied fertiliser N, biologically fixed N2, and mineralisation of soil organic N. Even though about 96% of the anthropogenic CH4 emitted in New Zealand is from ruminant animals (methanogenesis), methane uptake by aerobic soils (methanotrophy) can significantly contribute to the removal of CH4 from the atmpsphere, as the global estimates confirm. Both the net uptake of CH4 by soils and N2O emissions from soils are strongly influenced by changes in land use and land management. Quantitative information on the fluxes of these two non-CO2 GHGs is required for a range of land-use and land-management ecosystems to determine their contribution to the national emissions inventory, and for assessing the potential of mitigation options. Here we report soil N2O fluxes and CH4 uptake for a range of land-use and land-management systems collated from published and unpublished New Zealand studies. Nitrous oxide emissions are highest in dairy-grazed pastures (10–12 kg N2O–N ha−1 year−1), intermediate in sheep-grazed pastures, (4–6 kg N2O–N ha−1 year−1), and lowest in forest, shrubland and ungrazed pasture soils (1–2 kg N2O–N ha−1 year−1). N deposited in the form of animal urine and dung, and N applied as fertiliser, are the principal sources of N2O production. Generally, N2O emissions from grazed pasture soils are high when the soil water-filled pore-space is above field capacity, and net CH4 uptake is low or absent. Although nitrification inhibitors have shown some promise in reducing N2O emissions from grazed pasture systems, their efficacy as an integral part of farm management has yet to be tested. Methane uptake was highest for a New Zealand Beech forest soil (10–11 kg CH4 ha−1 year−1), intermediate in some pine forest soils (4–6 kg CH4 ha−1 year−1), and lowest in most pasture (<1 kg CH4 ha−1 year−1) and cropped soils (1.5 kg CH4 ha−1 year−1). Afforestation /reforestation of pastures results in increases in soil CH4 uptake, largely as a result of increases in soil aeration status and changes in the population and activities of methanotrophs. Soil CH4 uptake is also seasonally dependent, being about two to three times higher in a dry summer and autumn than in a wet winter. There are no practical ways yet available to reduce CH4 emissions from agricultural systems. The mitigation options to reduce gaseous emissions are discussed and future research needs identified.