We assessed the effect of biochar incorporation into the soil on the soil-atmosphere exchange of the greenhouse gases (GHG) from an intensive subtropical pasture. For this, we measured N 2 O, CH 4 and CO 2 emissions with high temporal resolution from April to June 2009 in an existing factorial experiment where cattle feedlot biochar had been applied at 10 t ha −1 in November 2006. Over the whole measurement period, significant emissions of N 2 O and CO 2 were observed, whereas a net uptake of CH 4 was measured. N 2 O emissions were found to be highly episodic with one major emission pulse (up to 502 μg N 2 O-N m −2 h −1 ) following heavy rainfall. There was no significant difference in the net flux of GHGs from the biochar amended vs. the control plots. Our results demonstrate that intensively managed subtropical pastures on ferrosols in northern New South Wales of Australia can be a significant source of GHG. Our hypothesis that the application of biochar would lead to a reduction in emissions of GHG from soils was not supported in this field assessment. Additional studies with longer observation periods are needed to clarify the long term effect of biochar amendment on soil microbial processes and the emission of GHGs under field conditions.

Quantifying nitrous oxide (N2O) fluxes, a potent greenhouse gas, from soils is necessary to improve our knowledge of terrestrial N2O losses. Developing universal sampling frequencies for calculating annual N2O fluxes is difficult, as fluxes are renowned for their high temporal variability. We demonstrate daily sampling was largely required to achieve annual N2O fluxes within 10% of the ‘best’ estimate for 28 annual datasets collected from three continents—Australia, Europe and Asia. Decreasing the regularity of measurements either under- or overestimated annual N2O fluxes, with a maximum overestimation of 935%. Measurement frequency was lowered using a sampling strategy based on environmental factors known to affect temporal variability, but still required sampling more than once a week. Consequently, uncertainty in current global terrestrial N2O budgets associated with the upscaling of field-based datasets can be decreased significantly using adequate sampling frequencies.

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 À1 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 À1 day À1 during extended dry periods to less than 2-5 g CH 4 -C ha À1 day À1 when soil water content was high. The calculated annual CH 4 uptake at the site was 3.65 kg CH 4 -C ha À1 yr À1 . 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.

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