Net exchange of greenhouse gases from soils in an unimproved pasture and regenerating indigenous Kunzea ericoides shrubland in New Zealand

Abstract

Monthly measurements of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) fluxes were made at 3 sites along a sequence of naturally regenerating Kunzea ericoides shrubland in New Zealand, consisting of unimproved pasture (UP), young (8–12 years) Kunzea trees (YK), and old (80 years) Kunzea trees (OK). The CO2 flux at a base temperature of 10°C was highest at the OK site (0.51 g CO2/m2.h) and lowest at the UP site (0.26 g CO2/m2.h). Values of CO2 flux were regulated by soil temperature (Ts) throughout the year, and water availability modified the response to Ts when root-zone water content, (θ), fell below 0.27–0.29 m3/m3 in spring and summer. The soils were mostly CH4 sinks, although there were net CH4 emissions during wet periods at the YK site. The maximum CH4 flux at the YK site was –49.7 μg CH4/m2.h compared with –33.4 μg CH4/m2.h for the UP (and –90.4 μg CH4/m2.h for OK), indicating the potential for rapid recovery of methanotrophic populations in the YK shrubland over 8–12 years. However, on an annual basis our data suggest that CH4 oxidation rates decrease as land reverts from unimproved pasture to shrubland. Methane oxidation rates were strongly dependent on θ and only weakly dependent on Ts. Measurements of N2O fluxes were below the minimum detectable limit throughout the year at the UP and YK sites, and low but dependent on both Ts and θ at the OK site. Annual estimates of soil CO2 flux were 39.9, 23.3, and 21.9 × 103 kg CO2/ha.year at the OK, YK, and UP sites, respectively. All 3 sites were a net sink for CH4, with the highest oxidation rate of –5.1 kg CH4/ha.year at the OK site compared with –1.52 kg CH4/ha.year at the UP site. On a CO2-equivalent basis, the OK site was a greater CH4 sink (–127.3 kg CO2-e/ha.year) than a N2O source (77.5 kg CO2-e/ha.year), demonstrating the potential for soils to oxidise CH4 with forest succession as a possible mitigation strategy for land managers to reduce net emissions.