Biological denitrification is affected by many environmental factors that control the amount of N 2 and N 2 O entering the atmosphere. This study was conducted to measure the effect of water-filled pore space (WFPS), available C, and soil NO, concentration on total denitrification (N, + N 2 O), using acetylene (C 2 H 2) inhibition, and to ascertain if denitrification could be estimated from N 2 O measurements in the field using an average N,/N 2 O ratio. Repacked cores of four benchmark soils were brought to 60, 75, and 90% WFPS by applying treatments of glucose-C (O, 180, and 360 kg ha-1) and NO,-N (O, 50, and 100 kg ha~'). The cores were incubated at 25 °C, with and without 100 niL C 2 H 2 L-1 , for 5 d during which daily gas samples of the headspace were analyzed for N 2 O and CO 2. Total N loss due to denitrification generally increased as soil texture became finer and WFPS increased. The only exception to this was the C-amended sand, where N losses up to 26 and 66% were recorded at 60 and 75% WFPS, respectively. Denitrification rates at high N concentrations were quite small in the absence of an available C source but increased with increasing available C (glucose). The Nj/N 2 O ratio generally increased with time of incubation after the initial treatment application. The largest ratios (up to 549) were found at the highest available C rate and generally at the highest soil water content. The presence of high NO, concentrations apparently inhibited the conversion of N 2 O to N 2 , resulting in lower N 2 /N 2 O ratios. Using an average Nj/N 2 O ratio for estimation of denitrification from N 2 O field measurements cannot be recommended because of the variation in this ratio due to the many environmental factors altered by field management that influence denitrification and the relative production of N 2 and N 2 O.

We describe a model of N2 and N2O gas fluxes from nitrification and denitrification. The model was developed using laboratory denitrification gas flux data and field‐observed N2O gas fluxes from different sites. Controls over nitrification N2O gas fluxes are soil texture, soil NH4, soil water‐filled pore space, soil N turnover rate, soil pH, and soil temperature. Observed data suggest that nitrification N2O gas fluxes are proportional to soil N turnover and that soil NH4 levels only impact N2O gas fluxes with high levels of soil NH4 (>3 μg N g−1). Total denitrification (N2 plus N2O) gas fluxes are a function of soil heterotrophic respiration rates, soil NO3, soil water content, and soil texture. N2:N2O ratio is a function of soil water content, soil NO3, and soil heterotrophic respiration rates. The denitrification model was developed using laboratory data [Weier et al, 1993] where soil water content, soil NO3, and soil C availability were varied using a full factorial design. The Weier's model simulated observed N2 and N2O gas fluxes for different soils quite well with r2 equal to 0.62 and 0.75, respectively. Comparison of simulated model results with field N2O data for several validation sites shows that the model results compare well with the observed data (r2 = 0.62). Winter denitrification events were poorly simulated by the model. This problem could have been caused by spatial and temporal variations in the observed soil water data and N2O fluxes. The model results and observed data suggest that approximately 14% of the N2O fluxes for a shortgrass steppe are a result of denitrification and that this percentage ranged from 0% to 59% for different sites.

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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