| On September 8,2008, an iron ore tailings dam failed in the Taershan mining area in Xiangfen, Shanxi Province, China. In this study, the heavy metal concentrations and physicochemical properties of the soil affected by tailings spilled from the Taershan Dam were determined and assessed using a combination of soil survey, experimental study, and correlation analysis. Based on the soil problems, different measures were evaluated for the effects on soil use and improvement in the disaster area affected by Taershan tailings spill. The main results were as follows:(1) Heavy metal determination and assessment:The concentrations of soil total heavy metals including Fe, Mn, and Zn were significantly higher in the tailings spill coverage site than in the non-coverage site; however, the opposite trend was observed in soil Cr and Ni concentrations, namely coverage site< non-coverage site. The concentrations of available trace elements such as Fe and Cu were higher in the coverage site than in the non-coverage site; the differences reached a highly significant level According to the national environmental quality grade II standard for soil, single-factor pollution index assessment showed that the study area was only polluted with Cd and Cu; mild pollution with Cd and Cu was detected in soil at 5.88% and 2.94% of sampling points, respectively. Nemerow pollution index assessment revealed that the study area was relatively clean. Hakanson potential ecological risk index assessment indicated that the study area was exposd to low-level ecological risk. Soil physicochemical analysis showed that both field capacity and available P were significantly higher in the non-coverage site than in the coverage site; however, soil bulk density and compaction showed the opposite trend, namely coverage site> non-coverage site. Nutrient deficiency and deterioration of soil physical properties were the major obstacles to soil in the disaster area affected by dam failure.(2) Effect of fertilization on plant growth and yield:Application of P fertilizer, N fertilizer, or irrigation improved the growth of Macleaya cordata plants in the immature soil formed after coverage with tailings spill. Plant height, aboveground biomass, stem diameter, total root length, root weight, active absorption area, and root total absorption area were improved to varying degrees. Meanwhile, superoxide dismutase and peroxidase activities were enhanced, whereas malondialdehyde content was decreased. The effect of N fertilization alone was inferior to P fertilization alone, while the effect of fertilization under water shortage was inferior to that under water supply. Significant Iinteraction was observed between water supply, N application, and P application. P was the primary limiting factor for biomass formation of M. cordata plants grown in the immature soil formed after coverage with tailings spill. Soil enzyme activities and microbial number in the rhizosphere of M. cordata plants were significantly positively correlated with plant growth condition and root activity. This relationship indicated that soil urease and phosphatase activities in the rhizosphere were improved mainly because the developed root system facilitated rapid propagation of microbial communities in the rhizosphere.A certain amount of fertilizer application, particularly P and manure, improved grain yield of Broomcorn millet and contributed to the enhancement of soil microbial and enzymatic activities. Grain yield of B. millet was increased by the application of K fertilizer plus P, N, and NP, or the manure application alone. The effect of P was better than N, while the best effect was obtained with NP. In the treatment with manure application alone, the number of panicles per pot did not decrease; it was even slightly higher than that in the treatment with NPK application. However, thousand-grain weight and the number of grains per panicle were markedly lower in the former than in the latter treatment. This indicated that manure application alone had no significant effect during early growth, tillering and panicle formation; in the late growth stage, fertilizer deficiency might have caused the reduction in the number of grains per panicle and thousand-grain weight, ultimately resulting in the decreased grain yield of B. millet.(3) Effect of plant growth on soil quality:The content of >0.5 mm water-stable aggregates was above 90% in the soil after the planting of sweet clover, alfalfa and crown vetch. The planting of the three legumes was therefore beneficial to increase the amount of soil aggregate structure, thereby coordinating soil water, nutrient, air, and heat conditions, and ultimately improving soil properties and fertility. The effect of different legumes for improving organic matter and total N in the subsurface soil showed the trend of:crown vetch > alfalfa> sweet clover. In the soil profile, the vertical distribution of soil available P was consistent with that of total N and organic matter. The available P decreased with soil depth: in the first two layers (surface 0-20 cm and subsurface20-40 cm), large decreases occurred in soil available P and the differences were significant for various treatments; from the third layer (40-60 cm) to deeper depth, the decrease in soil available P was markedly diminished, which might be attributed to the low mobility of P in the soil. Under mild Cd pollution due to coverage with tailings spill (Cd concentration,0.80 mg/kg), Conyza canadensis plants maintained good growth potential and accumulated more heavy metals in shoots. After adding 10 mg/kg Cd on the basis of mild Cd pollution, the growth of C. canadensis plants was inhibited; however, shoot and root Cd contents were significantly increased. In two treatments with Cd pollution, increasing manure application contributed to the growth of C. canadensis; meanwhile, it markedly increased the removal rate of soil Cd, without significantly changing plant Cd content. Adding EDTA increased Cd content in C. canadensis plants, while plant biomass was negatively affected.(4) Major soil problems and countermeasures:The main soil problems in the disaster area affected by the Taershan tailings dam failure event in Xiangfen are as follows.First, the soil surface is covered with a large amount of tailings spill, which has resulted in soil nutrient deficiency, particularly N and P deficiency. Increasing N, P, or manure application can achieve good effects in soil improvement. It is better to increase the application of N and P fertilizers on the basis of planting barren-resistant pioneer plants such as M. cordata and B. millet. This will not only benefit the maturation of the "root-soil system", but will also gain good earnings in the current season.Second, during the disaster relief, compaction by large machinery has resulted in deterioration of soil physical properties. For example, excessively high compactness is not conducive to plant growth, particularly the downward growth of roots. This problem can be addressed by deep plowing with increased thickness of tillage and application of manure or soil conditioner.Third, the tailings spill contains heavy metals. Thus, the soil is exposed to a certain ecological risk in the coverage area of tailings spill. However, based on the national environmental quality grade II standard for soil, the assessment results indicate low-level ecological risk in the study area, which can be continuously used for agricultural production. Increasing manure application or adding EDTA will improve Cd removal from the soil by C. canadensis in the Taershan iron mining area in Xiangfen associated with low concentrations of Cd pollution. C. canadensis has good prospects for use as a hyperaccumulator of Cd to remediate Cd-polluted soil in the disaster area affected by Taershan tailings dam failure. |
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