Mapping the Centimeter-Scale Spatial Variability of PAHs and Microbial Populations in the Rhizosphere of Two Plants

Rhizoremediation uses root development and exudation to favor microbial activity. Thus it can enhance polycyclic aromatic hydrocarbon (PAH) biodegradation in contaminated soils. Spatial heterogeneity of rhizosphere processes, mainly linked to the root development stage and to the plant species, could explain the contrasted rhizoremediation efficiency levels reported in the literature. Aim of the present study was to test if spatial variability in the whole plant rhizosphere, explored at the centimetre-scale, would influence the abundance of microorganisms (bacteria and fungi), and the abundance and activity of PAH-degrading bacteria, leading to spatial variability in PAH concentrations. Two contrasted rhizospheres were compared after 37 days of alfalfa or ryegrass growth in independent rhizotron devices. Almost all spiked PAHs were degraded, and the density of the PAH-degrading bacterial populations increased in both rhizospheres during the incubation period. Mapping of multiparametric data through geostatistical estimation (kriging) revealed that although root biomass was spatially structured, PAH distribution was not. However a greater variability of the PAH content was observed in the rhizosphere of alfalfa. Yet, in the ryegrass-planted rhizotron, the Gram-positive PAH-degraders followed a reverse depth gradient to root biomass, but were positively correlated to the soil pH and carbohydrate concentrations. The two rhizospheres structured the microbial community differently: a fungus-to-bacterium depth gradient similar to the root biomass gradient only formed in the alfalfa rhizotron.

根际修复（Rhizoremediation）通过根系发育与根系分泌物来促进微生物活动，借此可强化污染土壤中多环芳烃（PAH）的生物降解过程。根际过程的空间异质性主要与根系发育阶段及植物种类相关，或可解释文献中报道的根际修复效率差异。本研究旨在探究：以厘米尺度解析的整株植物根际空间变异，是否会影响微生物（细菌与真菌）的丰度，以及多环芳烃降解菌的丰度与活性，进而导致多环芳烃浓度出现空间差异。研究通过独立根际箱（rhizotron）装置分别种植紫花苜蓿与黑麦草，待其生长37天后，对比两种差异显著的根际环境。试验中几乎所有添加的多环芳烃均被降解，且在培养期间两种根际环境中的多环芳烃降解菌种群密度均有所提升。采用地质统计估算方法（克里金法，kriging）对多参数数据进行空间制图后发现，尽管根系生物量呈现空间结构性，但多环芳烃的分布却并无此特征。不过在紫花苜蓿的根际环境中，多环芳烃含量的变异程度更高。然而在种植黑麦草的根际箱中，革兰氏阳性（Gram-positive）多环芳烃降解菌的深度梯度与根系生物量的深度梯度呈相反趋势，但与土壤pH值及碳水化合物浓度呈正相关。两种根际环境对微生物群落的构建模式存在差异：仅在紫花苜蓿的根际环境中，才形成了与根系生物量梯度相似的真菌-细菌深度梯度。