A comment on scaling methane emissions from vegetation and grazing ruminants in New Zealand

Keppler et al. (2006, Nature 439, 187-191) showed that plants produce methane (CH₄) in aerobic environments, leading Lowe (2006, Nature 439, 148-149) to postulate that in countries such as New Zealand, where grazed pastures have replaced forests, the forests could have produced as much CH₄ as the ruminants currently grazing these areas. Estimating CH₄ emissions from up to 85 million ruminants in New Zealand is challenging and, for completeness, the capacity of forest and pastoral soils to oxidise CH₄ should be included. On average, the CH₄ emission rate of grazing ruminants is estimated to be 9.6 ± 2.6 g m^-2 year^-1 (±standard deviation), six times the corresponding estimate for an indigenous forest canopy (1.6 ± 1.1 g m^-2 year^-1). The forest's soil is estimated to oxidise 0.9 ± 0.2 g m^-2 year^-1 more CH₄ than representative soils beneath grazed pasture. Taking into account plant and animal sources and the soil's oxidative capacity, the net CH₄ emission rates of forest and grazed ecosystems are 0.6 ± 1.1 and 9.8 ± 2.6 g m^-2 year^-1, respectively.