Methane uptake in New Zealand planted forest soils

Context New Zealand’s 1.6 million ha of Pinus radiata represent a potentially substantial but unaccounted for methane (CH4) sink in the nation’s greenhouse gas inventory. Quantifying this sink is crucial as it offers an opportunity to offset emissions from agriculture and other sectors. Aim To obtain the first in-situ data on CH4 fluxes in P. radiata forests across New Zealand and to evaluate the implications for the country’s CH4 budget. Methods Monthly CH4 flux measurements were collected over a 12-month period at three forest sites: Kaingaroa, McLeans Island, and Orton Bradley Park. Key results All sites were CH4 sinks, with average uptakes of 2.8 kg CH4 ha−1 year−1 at Kaingaroa, and 2.5 kg CH4 ha−1 year−1 at the other two sites. While fluxes varied spatially and temporally, no significant correlations were found between CH4 flux and the measured environmental variables (soil water content, soil temperature, and relative gas diffusivity), possibly due to unmeasured within-site variability, low bulk density, and high gas diffusivity of the soils – meaning that gas diffusion was not limited, or insensitivity of CH4 cycling over the temperature and moisture ranges present at the sites. Extrapolated nationally, P. radiata forests are estimated to absorb 113.6 kt CO2-e of CH4 annually. Conclusions Scaled nationally, CH4 oxidation in planted forest soils could offset a large proportion of emissions from sectors such as rail transport and natural gas production. Implications This study shows that consideration should be made for incorporating this sink into New Zealand’s greenhouse gas budgets along with other CH4 sinks and sources in P. radiata forests to better inform climate change mitigation policies.