Pristine New Zealand forest soil is a strong methane sink

Methanotrophic bacteria oxidize methane (CH4) in forest soils that cover similar to30% of Earth's land surface. The first measurements for a pristine Southern Hemisphere forest are reported here. Soil CH4 oxidation rate (F-CH4) averaged 10.5+/-0.6 kg CH4 ha(-1) yr(-1), with the greatest rates in dry warm soil (up to 17 kg CH4 ha(-1) yr(-1)). Methanotrophic activity was concentrated beneath the organic horizon at 50-100 mm depth. Water content was the principal regulator of F-CH4 (r(2)=0.88) from the most common value of field capacity to less than half of this when the soil was driest. Multiple linear regression analysis showed that soil temperature was not very influential. However, inverse co-variability confounded the separation of soil water and temperature effects in situ. Fick's law explained the role of water content in regulating gas diffusion and substrate supply to the methanotrophs and the importance of pore size distribution and tortuosity. This analysis also showed that the chambers used in the study did not affect the oxidation rate measurements. The soil was always a net sink for atmospheric CH4 and no net CH4 (or nitrous oxide, N2O) emissions were measured over the 17-month long study. For New Zealand, national-scale extrapolation of our data suggested the potential to offset 13% of CH4 emissions from ca. 90 M ruminant animals. Our average F-CH4 was about 6.5 times higher than rates reported for most Northern Hemisphere forest soils. This very high F-CH4 was attributed to the lack of anthropogenic disturbance for at least 3000-5000 years and the low rate of atmospheric nitrogen deposition. Our truly baseline data could represent a valid preagricultural, preindustrial estimate of the soil sink for temperate latitudes.