Alteration of nitrous oxide emissions from floodplain soils by aggregate size, litter accumulation and plant–soil interactions

Semi-terrestrial soils such as floodplain soils are considered potential hot spots of nitrous oxide (N2O) emissions. Microhabitats in the soil – such as within and outside of aggregates, in the detritusphere, and/or in the rhizosphere – are considered to promote and preserve specific redox conditions. Yet our understanding of the relative effects of such microhabitats and their interactions on N2O production and consumption in soils is still incomplete. Therefore, we assessed the effect of aggregate size, buried leaf litter, and plant–soil interactions on the occurrence of enhanced N2O emissions under simulated flooding/drying conditions in a mesocosm experiment. We used two model soils with equivalent structure and texture, comprising macroaggregates (4000–250 µm) or microaggregates (<250 µm) from a N-rich floodplain soil. These model soils were planted with basket willow (Salix viminalis L.), mixed with leaf litter or left unamended. After 48 h of flooding, a period of enhanced N2O emissions occurred in all treatments. The unamended model soils with macroaggregates emitted significantly more N2O during this period than those with microaggregates. Litter addition modulated the temporal pattern of the N2O emission, leading to short-term peaks of high N2O fluxes at the beginning of the period of enhanced N2O emission. The presence of S. viminalis strongly suppressed the N2O emission from the macroaggregate model soil, masking any aggregate-size effect. Integration of the flux data with data on soil bulk density, moisture, redox potential and soil solution composition suggest that macroaggregates provided more favourable conditions for spatially coupled nitrification–denitrification, which are particularly conducive to net N2O production. The local increase in organic carbon in the detritusphere appears to first stimulate N2O emissions; but ultimately, respiration of the surplus organic matter shifts the system towards redox conditions where N2O reduction to N2 dominates. Similarly, the low emission rates in the planted soils can be best explained by root exudation of low-molecular-weight organic substances supporting complete denitrification in the anoxic zones, but also by the inhibition of denitrification in the zone, where rhizosphere aeration takes place. Together, our experiments highlight the importance of microhabitat formation in regulating oxygen (O2) content and the completeness of denitrification in soils during drying after saturation. Moreover, they will help to better predict the conditions under which hot spots, and “hot moments”, of enhanced N2O emissions are most likely to occur in hydrologically dynamic soil systems like floodplain soils.

半陆生土壤（如漫滩土壤（floodplain soils））被认为是一氧化二氮（N₂O）排放的潜在热点区域。土壤中的微生境（microhabitats）——包括团聚体内部与外部、碎屑圈（detritusphere）以及根际（rhizosphere）——被认为可促进并维持特定的氧化还原条件。然而，目前学界对这类微生境的相对影响及其相互作用如何调控土壤中N₂O的产生与消耗仍缺乏完整认知。因此，本研究通过中宇宙（mesocosm）实验，在模拟淹水-干燥循环条件下，评估了团聚体粒径、埋藏落叶以及植物-土壤相互作用对N₂O排放增强现象的调控效应。本研究采用两种结构与质地一致的模拟土壤，分别取自富氮漫滩土壤的大团聚体（4000–250 µm）与微团聚体（<250 µm）；将这些模拟土壤分别进行种植篮柳（Salix viminalis L.）、混入叶片凋落物或不施加任何改良处理。经过48小时淹水后，所有处理组均出现了N₂O排放增强的阶段。其中未做改良的大团聚体模拟土壤在此阶段的N₂O排放量显著高于微团聚体模拟土壤。添加凋落物会改变N₂O排放的时间动态模式，在排放增强阶段初期引发短期的高N₂O通量峰值。篮柳的存在会显著抑制大团聚体模拟土壤的N₂O排放，掩盖了团聚体粒径的影响效应。将通量数据与土壤容重、含水量、氧化还原电位及土壤溶液组成数据相结合分析表明，大团聚体为空间耦联的硝化-反硝化（nitrification–denitrification）过程提供了更有利的环境，而该过程尤其有利于N₂O的净产生。碎屑圈中有机碳的局部富集最初会刺激N₂O排放；但最终，过剩有机质的呼吸作用会使系统转向以N₂O还原为N₂为主导的氧化还原条件。同理，种植组土壤的低排放速率可通过两种机制得到最佳解释：一是根系分泌的低分子量有机物质支持了缺氧区域的完全反硝化，二是根际通气区域的反硝化作用受到抑制。综上，本实验凸显了微生境形成对调控土壤饱和后干燥过程中氧气（O₂）含量与反硝化完整性的重要意义。此外，本研究结果将有助于更精准地预测，在漫滩土壤这类水文动态变化的土壤系统中，N₂O排放增强的热点区域与“热点时刻”最有可能出现的环境条件。