红树林根系对沉积物砷赋存转化的影响机制

Mangroves are woody plants grow on the coastal ecotone in tropical and subtropical latitudes, providing important ecological roles including flood protection and maintenance of estuarine /coastal wetland ecological systems. Arsenic (As) is a ubiquitous and highly toxic metalloid, while the toxicity, mobility and bioavailability of As are decided by its speciation and formation. Mangrove sediments serve as both a sink and source for As, with increasing levels of As were found in mangrove wetlands. Mangroves developed specific roots to adapt the special habitat, while the specific root behaviors have significant influences on As occurrence and transformation in sediments. In this project, combined field sampling and greenhouse cultureling experiments are taken to study responses of root anatomy, radial oxygen loss (ROL), low-molecular-weight organic acids (LMWOAs) exuded by root to As exposion, togother with the influnces of those changes of rhizosphere microenvironment on As occurrence and transformation. Besides, the contents and speciations of As in root iron plaques and As regionalization in mangrove roots are researched to illustrate the biogeochemical process of As occurrence and transformation in the ‘sediment-rhizosphere-root surface-root’ interface. In addtion, a comprehensive attribution analysis is taken to provide better understanding on sediment As occurrence and transformation. Results of this study will provide useful information to improve our understanding of the influences of mangrove roots on As occurrence and transformation in sediments, which provide theoretical basis and technical support for the prevention and control of As pollution in mangrove ecosystems, also with the potential of mangroves for As phytoremediation in mangrove wetlands.

红树林是生长在热带、亚热带的海陆交汇带的木本植物群落，其在防浪护岸、维护河口和滨海湿地生态系统平衡具有重要作用。砷（As）是一种普遍存在、高毒性的类金属元素，其毒性取决于赋存含量和形态。当前红树林湿地的As污染日趋严重，其沉积物同时作为As的汇和源。红树植物具有特异性的根系和根系行为，从而对沉积物中As赋存、转化产生重要影响。本项目利用野外采样与温室模拟试验相结合，研究As污染下红树植物根系解剖结构、根系泌氧、根系分泌物的响应及其造成的根际微环境改变、根系微环境改变对沉积物As赋存形态的影响、根表铁膜对As的吸附和络合机制、红树植物根部As区域化分布特征，系统性阐明沉积物中As在红树林非根际-根际-根表-根部间的赋存和转化，并通过进行红树林湿地沉积物As赋存转化的综合归因分析，以揭示红树林根系对沉积物As赋存转化的影响机制，为红树林湿地As污染防控和植物修复提供科学理论依据。

砷（As）的毒性取决于含量和赋存形态。本项目通过对红树植物桐花树幼苗进行0~150mg kg－1 浓度梯度As 胁迫实验，研究了As在红树林非根际-根际-根表-根部间的赋存转化机制。研究表明：（1）As在沉积物中以无机As(V)为主，根际As、As(V)、As(III)含量分别为非根际的1.01～2.07倍、1.06～1.56倍、1.45～2.00倍，As具有由非根际向根际富集的趋势；As在幼苗中以无机As(V)为主累积在根部，As向地上部分转移的能力很低，推测与幼苗根表铁膜有关。根表铁膜含量在中低浓度As处理中上升，与沉积物根际As、As(V)、As(III)呈极显著正相关（p＜0.01），表明铁膜吸附As，将其滞留在植物体外以减轻毒害；（2）As胁迫促进根系分泌低分子量有机酸（LMWOAs)，通过络合重金属，以减轻As对根系的毒害；根际pH值与非根际相比降低了0.2～0.5个单位，与沉积物根际As、As(V)、As(III)呈极显著正相关（p＜0.01），表明较低的pH环境促进了As的溶解，增强了As的生物有效性，这可能是As由非根际向根际富集、迁移的原因之一。（3）As胁迫会引起沉积物根际及非根际微生物多样性和群落结构的变化。中低浓度As胁迫下，根际土壤微生物菌群丰富度逐渐增加，出现大量耐As的微生物；而高浓度As使微生物群落多样性下降并产生毒性，不耐As的微生物逐渐减少；沉积物微生物优势群落为Proteobacteria（变形菌门）、Chloroflexi（绿弯菌门）及Bacteroidetes(拟杆菌门)；（4）通过模拟红树林沉积物As形态、微生物和LMWOAS之间的根际相互作用，得出影响沉积物中As(V)含量大小顺序为：草酸＞苹果酸＞柠檬酸；沉积物As赋存形态在LMWOAs作用下随时间增加由As(III)逐渐转变As(V)，降低了其毒害作用；微生物对As在植物根际-沉积物界面迁移作用因LMWOAs类型不同而表现各异。.因此，研究结果将有助于阐明As在红树林湿地的赋存和转化机制，为红树林湿地As污染防控和修复提供科学依据。