Sources of nitrous oxide from intensively managed pasture soils

Rainfall and irrigation trigger large pulses of the powerful greenhouse gas N2O from intensively managed pastures, produced via multiple, simultaneously occurring pathways. These N2O pulses can account for a large fraction of total N2O losses, demonstrating the importance to determine magnitude and source partitioning of N2O under these conditions.

Researchers investigated the response of different pathways of N2O production to wetting across three different textured pasture soils.  Soil microcosms were fertilised with an ammonium nitrate (NH4NO3) solution which was either single or double 15N labelled, wetted to four different water-filled pore space (WFPS) levels, and incubated over two days.

The use of a 15N pool mixing model together with soil N gross transformations enabled the attribution of N2O to specific pathways, and to express N2O emissions as a fraction of the underlying N transformation. Denitrification and nitrification mediated pathways contributed to the production of N2O in all soils, regardless of WFPS. Denitrification was the main pathway of N2O production accounting for > 50 % of cumulative N2O emissions even at low WFPS. The contribution of autotrophic nitrification to N2O emissions decreased with the amount of wetting, while the contribution of heterotrophic nitrification remained stable or increased. Following the hole-in-the-pipe model, 0.1 to 4% of nitrified N was lost as N2O, increasing exponentially with WFPS, while the percentage of denitrified N emitted as N2O decreased, providing critical information for the representation of N2O/WFPS relationships in simulation models.

Findings demonstrated that the wetting of pasture soils promotes N2O production via denitrification and via the oxidation of organic N substrates driven by high carbon and N availability upon wetting. The large contribution of heterotrophic nitrification to N2O emissions should be considered when developing N2O abatement strategies, seeking to reduce N2O emissions in response to rainfall and irrigation from intensively managed pastures.

The associated data for the study are available here.

降雨与灌溉会触发集约化管理牧场中强效温室气体氧化亚氮（N₂O）的大量脉冲式排放，该排放通过多种同时发生的途径产生。这类氧化亚氮脉冲式排放可占总氧化亚氮损失量的较大比例，凸显了在此类条件下确定氧化亚氮排放规模与源解析的重要性。
研究人员针对三种不同质地的牧场土壤，探究了氧化亚氮生成的不同途径对土壤增湿的响应。实验中，土壤微宇宙培养体系（soil microcosms）经硝酸铵（NH₄NO₃）溶液施肥，该溶液采用单标记或双标记15N方式标记，将土壤调至四种不同的充水孔隙度（WFPS）水平，并进行为期两天的培养。
结合15N池混合模型（15N pool mixing model）与土壤氮素总转化过程分析，研究人员可将氧化亚氮的生成归因为特定途径，并将氧化亚氮排放量表示为对应氮素转化过程的占比。无论充水孔隙度水平如何，所有供试土壤的氧化亚氮生成均由反硝化作用与硝化作用介导的途径共同驱动。反硝化作用是氧化亚氮生成的主要途径，即使在低充水孔隙度条件下，其贡献也占累积氧化亚氮排放量的50%以上。自养硝化作用对氧化亚氮排放的贡献随土壤增湿程度升高而降低，而异养硝化作用的贡献则保持稳定或有所上升。依据管道漏洞模型（hole-in-the-pipe model），有0.1%~4%的硝化态氮以氧化亚氮形式损失，且该比例随充水孔隙度呈指数增长；而反硝化态氮以氧化亚氮形式排放的占比则随充水孔隙度升高而下降，这为模拟模型中氧化亚氮与充水孔隙度的关系表征提供了关键依据。
研究结果表明，牧场土壤增湿可通过反硝化作用，以及增湿后高碳氮有效性驱动的有机氮底物氧化途径促进氧化亚氮生成。在制定氧化亚氮减排策略以降低集约化管理牧场因降雨与灌溉产生的氧化亚氮排放时，应考虑异养硝化作用对氧化亚氮排放的显著贡献。
本研究的相关配套数据可在此处获取。