保障蔬菜安全生产中十分重要的是研究推导出适用于蔬菜安全生产的土壤镉阈值。在高自然背景地区，可提取态镉的土壤阈值对于食品安全和环境管理显得尤为重要。本研究采用物种敏感性分布法，耦合土壤老化、生物有效性、以及土壤性质影响，推导土壤全量镉和可提取态镉的阈值。在5个文献数据库中，利用指定的关键字对已发表论文中的数据进行筛选，共筛选出90种符合条件的蔬菜品种。利用土壤-植物系统中镉迁移转化模型对生物积累数据进行归一化处理后，种内变异性降低18.3 ~ 84.4%。利用物种敏感性分布曲线推导出保护95%的物种不超标的土壤镉浓度，即HC5值。进而提出了基于外源添加法推导的土壤镉阈值，该阈值可以用基于土壤有机质和pH的连续性方程表示。全量镉和EDTA可提取态镉的阈值分别为0.23 ~ 0.61 mg kg^-1和0.09 ~ 0.25 mg kg^-1。利用田间试验数据对推导的阈值模型进行验证。大部分的预测值都小于由田间试验计算出的HC5值，说明本阈值是保守可行的。综上所述，本研究为建立蔬菜种植土壤镉阈值提供了以保障蔬菜产品质量安全的科学依据。

镉（Cd）不仅是环境中毒性最大的重金属之一，而且在土壤中具有较低的环境容量，控制镉进入土壤刻不容缓。土壤中镉的来源众多，其中大气沉降已被证明是中国大规模土壤镉污染的主要来源。因此，确定镉的人为排放、大气沉降和土壤中镉累积之间的定量关系，对于缓解镉的非点源污染具有非常重要的意义。因此，本研究以中国长江中下游平原为例，基于2000-2020年发表文献中的大气排放-沉降-土壤累积数据集，研究了大气排放、大气沉降和土壤Cd累积之间的关系。结果表明，近十年来，土壤Cd仍在累积，平均累积率从9.45下降到8.86 μg kg^-1 yr^-1，依然远超全国平均水平（4.0 μg kg^-1 yr^-1）。大气沉降通量对土壤Cd增量的贡献在22-29%之间，大气沉降通量从0.54下降到0.48 mg m^-2 yr^-1，但仍高于全国平均水平。在一个省级行政区域内，大气沉降和排放高度相关，接近1.0的比例，而多种因素增加了其不确定性。由于控制技术和环境政策的影响，大气重金属排放因子可能处于动态变化状态。作为镉排放的主要来源，2000-2010年至2010-2020年期间，有色金属生产每吨烟尘排放量下降了64.7%。尽管各种环境政策已经出台，但镉的大气排放仍然不容乐观。这项工作为合理控制镉的大气排放和土壤污染提供了科学依据。

Received  11 November, 2017    Accepted  28 January, 2018

   The variations of grain cadmiun (Cd) concentrations, translocation factors (TFs) of Cd from roots to shoots/grains of six rice cultivars, characterized with different Cd-sensitivities in polluted soil were studied, the selected rice cultivars were Xiangzao 17 (R1), Jiayu 211 (R2), Xiangzao 42 (R3), Zhuliangyou 312 (R4), Zhuliangyou 611 (R5), and Jinyou 463 (R6), respectively. The Cd subcellular distribution and Cd binding characteristics on subcellular fractions of rice root cell wall (CW) were further investigated. The results showed that the rice grain Cd contents varied significantly, with a maximum variation of 47.0% among the cultivars, the largest grain Cd content was observed with cultivar R1 (Cd-sensitivity cultivar) and the smallest with R5 (Cd-tolerance cultivar). The translocation factors of Cd from roots to shoots (TF_shoot) and roots to grains (TF_grain) varied greatly among the cultivars. In general, the TF_grain of the cultivars followed the order of R1>R2>R3>R4> R6-R5. The Cd concentration (mg kg^–1 FW) in the fraction of root CW, the fraction of cell wall removing pectin (CW-P) and the fraction of cell wall removing pectin and hemicellulose (CW-P-HC) of the cultivars generally followed the order of CW-P>CW>CW-P-HC; the ratios of Cd concentration (mg kg^–1 FW) in the fraction of CW-P to that of CW were mostly more than 1.10, while the ratios of Cd concentration in the fraction of CW-P-HC to that of CW were mostly less than 0.60, indicating that Cd was mainly stored in the hemicellulose of the root CW. The ratios of Cd of CW-P-HC to CW generally followed the descending order of R1~R2>R3>R4>R5~R6 for the cultivars, which implied that hemicellulose is probably the main subcellular pool for transferring Cd into rice grain, and it restrains the translocation of Cd from shoot to the grain, especially for the Cd-tolerance cultivars (R5 and R6), the compartmentation of more Cd in hemicellulose in root CW is probably one of the main mechanisms for Cd tolerance of rice cultivars.

Ecological risk assessment of metals in soils is important to develop the critical loads of metals in soils. Phytotoxicity is one of the endpoints for ecological risk assessment of soils contaminated with metals. The sensitivity of eight Chinese plant species (bok choy, mustard, tomato, green chilli, paddy rice, barley, spinach and celery) to copper (Cu) and nickel (Ni) toxicity in two Chinese soils was investigated to assess their potential use for ecological risk assessment in the region. The results showed that bok choy and mustard were the two most sensitive species to Cu and Ni toxicities. Assessment of metal accumulation by the plants demonstrated that bok choy shoot had the highest bioconcentration factor (BCF, the ratio of metal concentration in plant shoots to metal concentration in soil). Given the importance of bok choy to agricultural production in Asia, it is therefore important that these sensitive plant species are included in species sensitivity distributions for ecological risk assessment of Cu and Ni in soils.

A field plot experiment in a calcareous soil with wheat and maize rotation was carried out for 2 yr. The study aimed to investigate the effects of biosolids (sewage sludge or chicken manure) application on nitrogen (N) and phosphorus (P) accumulation in soils and to develop a model for the effects of biosolids application on available P (Olsen-P) accumulation in soils, by which the quantities of biosolids that can be safely applied to agricultural soils were estimated. The results showed that heavy application of biosolids to agricultural soils based on the N requirement of a wheat-maize rotation cropping system will oversupply P. Soil total N was increased by 0.010 g kg-1 at application rate of 1 ton sewage sludge per hectare. The high ratio of N to P in grains of wheat and maize (from 4.0 to 7.6) and low ratio of N to P in biosolids (<2) led to more surplus P accumulated in soils. Although plant yields and P uptake by plants increased with increasing quantities of applied biosolids in soils, there was still an average 2.87 mg kg-1 increase in Olsen-P in the plough layer treated with biosolids for every 100 kg P ha-1 surplus. A predictive model was developed based upon the initial Olsen-P in soils, P input rates, crop yield, soil pH, and cultivation time. From the model, it is suggested that sewage sludge could be applied to calcareous soils for 12 yr using the recommended application rate (9 tons ha-1 yr-1). The field results will be helpful in achieving best management of biosolids application for agricultural production and environmental protection.