湿地植物根系泌氧及其在自然基质中的扩散效应研究进展

湿地植物根系径向泌氧(ROL)是构造根际氧化-还原异质微生态系统的核心要素,其扩散层为好氧、厌氧微生物提供了良好生境并促进其代谢活动,使湿地植物根际成为有机物降解、物质循环及生命活动最为强烈的场所,已有成果证明湿地植物根系ROL的强弱与污染物的去除效果密切相关。因此,开展湿地植物根系ROL及其在自然基质中的扩散效应研究,对于了解湿地植物根系ROL机理及其根际氧环境特征,进而发挥湿地植物的污染去除功能具有十分重要的意义。基于此,首先归纳了湿地植物根系ROL特征及其受影响机制的研究现状,而后从种属差异、时空分布及对微生物的影响等方面对根系ROL在自然基质中的扩散效应国内外研究成果进行了总结,最终根据研究现状与不足对今后的研究方向进行了简要展望。创新之处在于:1)提出影响根系氧供给及氧输送释放通道的环境、生物等因素,阐述了其对根系ROL的影响机制;2)着重阐述了目前研究较少提及的根系ROL扩散效应测定方法。

Research advancements on the radial oxygen loss in wetland plants and its diffusion effect in natural sediments

Radial oxygen loss (ROL) generated by wetland plants is the fundamental factor that maintains the heterogeneous oxidation-reduction microcosm in sediments. The subsequent diffusion layer provides fine microhabitats for both aerobic and anaerobic microorganisms and stimulates their metabolism, which makes the rhizosphere a superior active site for various processes including organism degradation, material circulation, and vigorous life activities. The intensity of ROL in the roots of wetlands plants is known to considerably affect the efficacy of pollutant removal. It is thus of special importance to investigate ROL in wetland plants and its diffusion effect to better understand the actual rhizosphere oxygen distribution and the underlying mechanisms of ROL generation, as well as to optimize the utilization of wetland plants for pollutant removal. The present review, therefore, aims at (1) summarizing the research advancements on the characteristics of ROL and the related influential impact factors on root oxygen supply; (2) deciphering the current knowledge on the diffusion effect of rhizosphere oxygen in terms of species differences, spatio-temporal distribution, and its regulating effects on microorganisms; and (3) raising perspectives on future research issues based on the pros and cons of quantification approaches to ROL. The existing research has comprehensively compared the ROL in different types of wetland plants in artificial substrates such as agar medium and nutrient solutions. The results have established a reference for choosing wetland plants for pollutant removal, suggesting that the level of ROL is closely related to oxygen supply and whether the oxygen transportation pathway is unobstructed. Meanwhile, the oxygen supply is mainly related to factors including irradiance, ventilation, temperature, and the biomass of shoots, and the inner oxygen transportation in plants and its external release are mainly related to the oxidation-reduction condition in substrates, root age, iron plaque, nitrogen in substrates, and the levels of sulfide and organic acids. Due to the restraints of measuring approaches, the above-mentioned work did not test the inner oxygen concentrations of roots at the same time, so it is difficult to disclose the mechanisms of oxygen supply in roots. Further, wetland plants commonly grow in sediments where the oxidized rhizosphere is the combined outcome of both ROL and depletion by sedimentary organic matter. The current knowledge may not be sufficient to indicate the real root oxygen release of wetland plants in sediments.Since the ROL diffusive layer is normally a few millimeters thick, and owing to the complexity of sediment, research on the diffusion effect of ROL in wetland plants has progressed slowly. With the application of techniques like microelectrodes, micro-optodes, and planar-optodes, as well as other one- and two-dimensional methods, it has become feasible to measure the micro-spatial and temporal distribution of real root oxygen circumstances. This has demonstrated that the rhizospheric oxygen concentrations at different root positions have different patterns in wetland plants than in artificial substrates. However, the features of oxygen diffusion in different substrates, especially the continuous oxygen changes under long-term scales, are still poorly known, and studies on the effect of ROL on microbial communities at the rhizospheric scale are rare. The feasibility of the non-destructive in situ detection of ROL is undoubtedly the current bottleneck. Therefore, it is important to advance methodological studies, study the spatio-temporal characteristics of rhizosphere oxygen conditions in wetland plants and the impact of oxygen diffusion on rhizosphere microbes, and elucidate the regulating mechanisms of ROL on microorganisms to offer theoretical support for maximizing the capacity of wetland plants in pollutant removal. The innovative points of the present review are that (1) it describes the environmental and biological factors associated with the oxygen supply, transportation, and release pathway and the underlying mechanisms affecting these; and (2) emphasizes the currently available detection approaches for measuring rhizosphere oxygen diffusion.