Quantifying denitrification losses from a sub-tropical pasture in Queensland, Australia. Use of the 15N gas flux method. [Theme 6: Improved process understanding and new technologies]

This study investigated the influence of different soil moisture contents on N2 and N2O emissions from a sub-tropical pasture in Queensland/Australia using the 15N gas flux method.
Intact soil cores were incubated over 14 days at 80% and 100% water filled pore space (WFPS). Gas samples were taken up to six times per day after application of 15N labelled nitrate, equivalent to 50 kg N ha-1 and analysed for N2 and N2O by isotope ratio mass spectrometry. Fluxes were calculated assuming non-random 15N distribution in the headspace according to Mulvaney and Kurtz (1984) using the labelled pool of nitrate estimated from N2O measurements (Stevens and Laughlin 2001).
The main product of denitrification in both treatments was N2. N2 emissions exceeded N2O emissions by a factor of 1.3 ± 0.3 at 80% WFPS and a factor of 3 ± 0.8 at 100% WFPS. The total amount of N-N2 lost over the incubation period was 13.5±1.0 kg N ha-1 at 80% WFPS and 21.8±1.8 kg ha-1 at 100% WFPS respectively. Over the entire incubation period, N2 emissions remained elevated at 100% WFPS, showing high variation between soil cores, while related N2O emissions decreased. At 80% WFPS, N2 emissions increased constantly over time showing significantly higher values after day five. At the same time, N2O fluxes declined. Consequently, N2:N2O ratios rose over the incubation period in both treatments.
Overall denitrification rates and related N2:N2O ratios were higher at 100% WFPS compared to 80% WFPS, confirming WFPS as a major driver of denitrification. This study highlights denitrification as a major pathway of N loss for sub-tropical pasture systems with a substantial amount of applied fertiliser lost as N2 at high WFPS. The 15N gas flux method proved an effective tool in assessing N losses from fertilised soils. However, its suitability to determine N2 fluxes from soils with lower denitrification rates needs to be confirmed in future studies. The high variation between soil cores emphasises the need for field measurements with a high spatial and temporal resolution in order to capture the dynamics of N2 emissions.

本研究采用15N气体通量法（15N gas flux method），探究了澳大利亚昆士兰州亚热带牧场中不同土壤含水量对N₂和N₂O排放的影响。
研究将原状土芯分别在80%和100%充水孔隙度（water filled pore space, WFPS）条件下培养14天。施加相当于50 kg N ha⁻¹的15N标记硝酸盐后，每日采集气体样本多达6次，并通过同位素比值质谱法（isotope ratio mass spectrometry）分析N₂和N₂O含量。通量计算基于Mulvaney和Kurtz（1984）提出的顶空15N非随机分布假设，使用由N₂O测量值估算的标记硝酸盐库（Stevens和Laughlin 2001）。
两种处理条件下，反硝化作用的主要产物均为N₂。在80% WFPS时，N₂排放量是N₂O的1.3±0.3倍；在100% WFPS时，这一比值为3±0.8倍。培养期间，80% WFPS条件下N-N₂总损失量为13.5±1.0 kg N ha⁻¹，100% WFPS条件下为21.8±1.8 kg ha⁻¹。在整个培养期内，100% WFPS条件下N₂排放量持续较高，且土芯间差异显著，而N₂O排放量则有所下降；80% WFPS条件下，N₂排放量随时间持续增加，第5天后数值显著升高，同时N₂O通量下降。因此，两种处理中N₂:N₂O比值在培养期间均呈上升趋势。
总体而言，100% WFPS条件下的反硝化速率及相关N₂:N₂O比值均高于80% WFPS，证实WFPS是反硝化作用的主要驱动因子。本研究强调，反硝化作用是亚热带牧场系统氮损失的主要途径，在高WFPS条件下，大量施用的肥料以N₂形式损失。15N气体通量法被证明是评估施肥土壤氮损失的有效工具，但它在测定低反硝化速率土壤N₂通量方面的适用性仍需未来研究验证。土芯间的高差异表明，需开展高空间和时间分辨率的田间测量，以捕捉N₂排放的动态变化。