| Arsenic (As) is aubiquitous metalloid element and also is one of the toxic elements with contents exceeding the soil limits in some areas of China. The availability or toxicity of exogenous As will decrease with time via interaction with soil colloid, which is described as aging process. However, less information of exogenous As aging in soils, the influences of soil properties, associated mechanisms and the changes of soil microbial community during As aging is available. In this study, soils derived from different parent materials in red soil region and typical soils in different regions were selected. Sodium arsenate solution (Na3AsO4, pentavalent As) used as exogenous As wa sspiked into experimental soils. A simulation aging experiment was carried out to study the variances of available As and As fractionation during the aging and the key influencing factors were analyzed. Based on the obtained data, a semi-mechanism model of exogenous As aging in soils was preliminarily developed and its validation was verified by applying As-contaminated soils in feild. Meanwhile, the high-throughput sequencing technology was used to study the changes of bacterial and archaeal community during aging in two types of soil (yellow soil and the soil derived from purple sandy) with considerable differences in physical-chemical properties. The community adaptation mechanism of bacteria and archaea under As stress was also analyzed. Observations will provide theoretical references for the valid evaluation of exogenous As risk in soil ecosystems and the safety use of As-contaminated soils. The main results are shown as follows:1. The results of exogenous As aging in soils developed from different parent materials indicated that the available As content in the soil derived from purple sandy was significantly higher than others throughout the aging period. This implied a relatively higher risk of As toxicity in soil derived from purple sandy shale in As seriously contaminated subtropical regions such as Hunan Province.2. The results of exogenous As aging in typical soils from different regions showed that the available As contents were consistently high (11.64-71.16%) and the aging approximate balance time were short in Fluvo-aquic soil (FS) and Irrigated desert soil (IDS1). However, the available As contents were consistently low (0.74-31%) and the aging approximate balance time were long in Yellow soil (YS) and Quaternary Red soil (RS1). The processes of the exogenous As aging in different types of soil were distinct. Stepwise regression analysis indicated that the clay content in soils is the key factor controlling the aging approximate balance time. The aging approximate balance time in different types of soils generally decreased in the following order:CS>FS>IDS1>BNS2>RS1>PS>YS>LS1.3. The pseudo-second-order model fit well to the variances of available As during the aging (R2=0.939-0.999, P<0.01). Soil pH and the contents of iron, aluminum and manganese oxides were the key factors controlling As availability, As aging rate, and As contents at the end of equilibrium aging period. Furthermore, aluminum oxides appeared to have intense effects than iron oxides on the variance of available As in red soils containing high content of iron and aluminum oxides, which is probably due to the more stable bidenate binucleate banding form between As and aluminum oxide.4. The changes in the As fractionations is a key factor influencing As-bioavailability in soils.The results indicated that non-specifically absorbed As and specifically absorbed As greatly contributed to the As availability. With As aging time, exogenous As gradually changed into the binding forms of As with amorphous and poorly-crystalline hydrous oxides of Fe, Al (F3) and well-crystalline hydrous oxides of Fe, Al (F4) as well as residual As (F5).5. A semi-mechanism model of As aging was developed taking into account the soil properties and three main fixation processes of metals in soil (As precipitation/nucleation on soil surface, micropore diffusion of As, and As occlusion within organic matter). It was convinced that precipitation/nucleation played a leading role in the aging process of exogenous As in soils. It also made clear the relative importance of H2AsO4- and HAsO42- for precipitation/nucleation of As on soil surface.6. For YS and RS2 soils, the influences of exogenous As stress on the bacterial and archaeal community were distinct during different As aging period. Comparatively, Actinobacteria, Proteobacteria and Chloroflexi showed the higher relative abundance in bacterial phylum populations after exposure to As stress. However, the variances of the bacteria community were different in two kinds of soil. Moreover, the dominant populations of archaea in soils were more resistant to As and stable under As stress than bacteria. It is inferred that the bacteria and archaea in soils may have different community adaptation mechanisms under the stress of As. |
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