Environmental factors controlling temporal and spatial variability in the soil‐atmosphere exchange of <scp><scp>CO₂</scp>, <scp>CH₄</scp></scp> and <scp><scp>N₂O</scp></scp> from an Australian subtropical rainforest

Abstract

Abstract The temporal variations in CO 2 , CH 4 and N 2 O fluxes were measured over two consecutive years from February 2007 to March 2009 from a subtropical rainforest in south‐eastern Queensland, Australia, using an automated sampling system. A concurrent study using an additional 30 manual chambers examined the spatial variability of emissions distributed across three nearby remnant rainforest sites with similar vegetation and climatic conditions. Interannual variation in fluxes of all gases over the 2 years was minimal, despite large discrepancies in rainfall, whereas a pronounced seasonal variation could only be observed for CO 2 fluxes. High infiltration, drainage and subsequent high soil aeration under the rainforest limited N 2 O loss while promoting substantial CH 4 uptake. The average annual N 2 O loss of 0.5 ± 0.1 kg N 2 O‐N ha −1 over the 2‐year measurement period was at the lower end of reported fluxes from rainforest soils. The rainforest soil functioned as a sink for atmospheric CH 4 throughout the entire 2‐year period, despite periods of substantial rainfall. A clear linear correlation between soil moisture and CH 4 uptake was found. Rates of uptake ranged from greater than 15 g CH 4 ‐C ha −1 day −1 during extended dry periods to less than 2–5 g CH 4 ‐C ha −1 day −1 when soil water content was high. The calculated annual CH 4 uptake at the site was 3.65 kg CH 4 ‐C ha −1 yr −1 . This is amongst the highest reported for rainforest systems, reiterating the ability of aerated subtropical rainforests to act as substantial sinks of CH 4 . The spatial study showed N 2 O fluxes almost eight times higher, and CH 4 uptake reduced by over one‐third, as clay content of the rainforest soil increased from 12% to more than 23%. This demonstrates that for some rainforest ecosystems, soil texture and related water infiltration and drainage capacity constraints may play a more important role in controlling fluxes than either vegetation or seasonal variability.