生活污泥驱动滩涂土壤肥力发育过程中重金属的环境行为及风险控制

The mudflats formed along the east coast of China can be important alternative sources for arable lands if amended by large input of organic materials. Rich in organic matter and other nutrients, municipal sewage sludge has been considered as a optimum organic source for mudflat soil amendment. Our previous studies showed that sewage sludge could be used as “initial fertility driver” to realize the rapid fertility development in mudflat saline soil. In this technique, one-time input in rational rate of sewage sludge can provide basic fertility to support green manure crop growth, then biomass of green manure was returned in situ. After rotations of green manure crops, the soil fertility of mudflats was rapidly developed, without further input of sewage sludge. However, sewage sludge application in mudflat soil has been greatly limited due to the concern of potential heavy metal contamination in soils. Using sewage sludge in compliance with the national standards for agricultural use could reduce risk of the heavy metal accumulation in mudflats. Nevertheless, it is not clear that whether massive input of sewage sludge would increase migration and bio-availability of heavy metals in mudflats. In this project, field and lab experiments, in combination with analytical and biological methods, are proposed to study the effects of sewage sludge used as initial fertility driver on environmental behaviors and fates of heavy metals. Specifically, the relationship between heavy metal behaviors (including their speciation, transformation, adsorption/desorption characteristics in soil solid phase, migration in soil profile and bioavailability) and soil factors (salinity, pH, organic matter and physical characters) will be analyzed and tracked during whole experimental period. Results of this study are estimated to provide scientific bases for determining the threshold of heavy metals and for seeking environmental risk control approches in mudflats amended by sewage sludge. That “initial fertility driver” technique can better serve the national strategy of arable land requisition-compensation is aslo expected based on this study.

沿海滩涂经大量有机质熟化后可作为重要的后备耕地，对缓解我国人地矛盾及实施国家耕地占补平衡战略具有重要意义。生活污泥富含有机质和养分，将其用于滩涂改良既能驱动土壤肥力快速发育，又能实现污泥资源化利用。污泥农用的重金属环境风险早已引起普遍关注，通过监控生活污泥重金属虽可实施总量控制，但随污泥输入的重金属在滩涂特殊土壤环境中的行为及环境风险目前尚缺乏系统的研究。本项目在前期工作的基础上采用田间定位试验、室内模拟及化学分析相结合的方法，深入研究生活污泥培肥滩涂土壤过程中主要重金属形态转化、剖面迁移分布、固/液界面吸附与解吸及对植物的有效性，并综合分析上述过程与滩涂土壤盐分、pH、离子交换特性、有机物组成、土壤结构特性等重要理化性状及其在培肥过程中变化的关系；在此基础上提出生活污泥快速培肥滩涂土壤过程中重金属环境风险形成与控制机制，为确保重金属环境安全的前提下利用生活污泥培肥滩涂土壤提供科学依据。

我国东部沿海淤长型泥质滩涂经过培肥改良后可作为重要的后备耕地资源，以富含有机质及养分的生活污泥驱动滩涂土壤肥力快速形成与发育，既可为滩涂土壤改良提供优质肥源，又能缓解生活污泥处置压力。本项目通过田间、盆栽及室内模拟试验，较系统地研究了施用生活污泥对滩涂土壤-植物系统中重金属环境行为的影响，并探讨了重金属环境风险的控制途径。获得以下主要结论：1）施用生活污泥可显著改善滩涂盐碱地土壤的理化与生物学性状，主要表现为提高土壤有机质与氮磷养分含量，提高水稳性团聚体含量，增加土壤微生物多样性，减低土壤pH与含盐量。一次性足量施用生活污泥可持续提高滩涂盐碱地土壤肥力，促进大麦、玉米、水稻及甜高粱等作物的生长。2）施用生活污泥提高了滩涂土壤重金属含量，并影响土壤重金属形态分布与迁移特性。随生活污泥施用量增加，土壤重金属活度提高，主要表现为酸溶态（EX）、可还原态（OXI）比例增加，而可氧化态（ORG）与残渣态（RES）比例降低。施用生活污泥提高了土壤渗漏液中重金属浓度，促进其在土层间迁移。3）滩涂土壤重金属生物有效性与施用生活污泥关系密切。施用生活污泥增加了水稻植株对重金属的积累，重金属含量以根系>茎叶>籽粒。本研究条件下，水稻植株对土壤重金属的富集系数（BAFs）均<0.5（其中，Cr、Cu、Ni、Pb<0.1），转移系数（TFs）均<0.5。4）对照土壤与农产品对重金属的限量标准并结合本项目研究结果，符合农用标准的生活污泥用量控制在180t.ha-1以下，土壤与植株重金属含量处于安全可控水平。5）相关分析及冗余分析结果表明，施用生活污泥条件下，滩涂土壤有效态重金属含量或有效度与pH、含盐量及有机质水平关系密切，其数量关系可以通过多元回归方程进行模拟（p<0.01或p<0.001）。6）污泥蚓粪及污泥基生物炭具降低土壤有效态重金属及植株体内重金属含量的作用，可有效降低生活污泥改良滩涂土壤过程中重金属环境风险。.本项目研究结果可为生活污泥改良滩涂土壤过程中重金属环境风险评估与控制提供数据支撑与理论依据。