酸性硫酸盐土的形成、 特性及其生态环境效应

酸性硫酸盐土(ASS)是全球沿海周边广泛分布的土壤种类,其铁、 硫元素的生物地球化学过程在全球物质循环过程中具有重要地位,但ASS也是最低质的土壤类型之一。ASS成土母质常形成于富含有机质、 海水浸泡的江口、 海湾等热带亚热带滨海环境,经异化细菌还原海水硫酸盐而形成四方硫铁矿(FeS)、 硫复铁矿(Fe3S4)、 黄铁矿(FeS2)等多种还原态Fe-S矿物沉淀物。ASS成土母质形成过程中的生物活动、 化学反应相当活跃,还原态Fe-S矿物沉淀物将环境中游离的金属、 稀土元素以及痕量元素固定下来,实现海水净化、 金属富集作用。因自然条件变化或者人为干扰等影响,ASS成土母质中的还原态Fe-S矿物被氧化而形成ASS。富含还原性硫化铁矿物的成土母质经一系列复杂反应,被氧化形成氢氧化铁、 酸、 硫酸盐等最终产物,同时伴生多种铁、 硫生物中间产物,以及强酸土壤环境。强酸环境下,铝、 镉、 锰、 砷、 铬等有毒金属的活性大幅提高,而磷、 钾、 锌、 硼等必需营养元素含量显著降低,严重危害实地动植物生长。另一方面,ASS中的酸和活化的重金属随雨水、 径流、 毛细管等途径进入河流、 地下水,威胁周边生态安全。目前,ASS的形成机理已基本被揭示,以及ASS发育过程中的生态环境效应已基本清晰。然而,我国早期学者主要关注ASS的铁、 铝、 硫含量水平,以及ASS发育农田的改良应用,对于ASS的发育过程、 生态功能及风险等尚未形成系统的认识。近年来,随着耕地面积不断萎缩,开发改良ASS等低产田块是提高我国粮食产量水平的重要措施。因此,为了合理开发利用ASS,尽量降低ASS的生态风险,亟需对ASS的形成机理、 发育过程、 土壤特性、 生态环境效应进行全面综述。本文首先对ASS的形成条件与过程进行综述,进一步梳理了ASS中硫的演变和铁的地球化学过程,并着重阐述了ASS的酸性特点,最后对ASS的生态环境效应进行了讨论。结合我国研究现状,展望了进一步研究ASS的主要问题,旨在为科学开发和利用酸性硫酸盐土提供参考。

Formation,characteristics and ecological effects of acid sulfate soil

Acid sulfate soil (ASS) is one of the nastiest soils in the word because of its strong acidity.Fe and S biogeochemical cycling in ASS plays an important role in global material recycling.The parent materials of ASS are often form tropical and subtropical estuary, bay and other coastal environment, where is submerged and rich in organic matter. Dissimilation bacteria reduce seawater sulfate to form mackinawite(FeS), greigite(Fe3S4), pyrite(FeS2) and other reduced Fe-S sediments. Sedimentation of reduced Fe-S minerals coprecipitates free metals, rare earth elements and trace elements in the environment, purifies seawater and enrichs metals. ASS is formed through oxidation of reduced Fe-S minerals in soils by natural process or anthropogenic interventions. The oxidation of Fe-S minerals ultimately results in products such as Fe-hydroxides, acidity and sulfate, involves a series of complex reactions, with several Fe and S intermediate and strong acidic soil. In strong acid soils, the mobilities of Al, Cd, Mn, As, Cr and other potentially toxic metals are significantly high, while the bioavailabilities of P, K, Zn, B and other essential nutrients are significantly low, which adversely affects animal and plant growth. On the other hand, the mobilised acid and toxic elements can enter nearby waterways and groundwater through rain, runoff, seepage and other ways, threatening the surrounding ecological security.The formation mechanism of ASS has been disclosed, and its ecological effect has been understood. In the early studies, Chinese researchers have focused on concentrations of iron, aluminum and sulfur in ASS and the remediation and applications of ASS farmland,few studies on the developmental processes, the ecological functions and risks of ASS are conducted. Recently, as arable land shrinking, improving the yields of ASS and other low-yielding fields become important measures to increase Chinaapos;s grain production.For the rational exploit and utilize of ASS with minimum ecological risks, the formation mechanism, soil development, soil characteristics and ecological effects of ASS are reviewed. The forming condition and development of ASS were summarized, sulfur dynamics and geochemistry and partitioning of iron in ASS were presented, and the distinctive acid characteristics of the soil were discussed.The perspectives of ASS in China were proposed to provide references for full unitilization of ASS.