Cadmium (Cd) is considered the primary dietary toxic element. Previous studies have demonstrated significant differences in heavy metal accumulation among crop species. However, this information in karst areas with low heavy metal activity is missing. In this study, the uptake and accumulation characteristics of cadmium in soil-crop samples of group 504 in the core karst region of East Asia were analyzed. Cadmium low-accumulating maize and rice were screened using cluster and Pareto analytic methods. In addition, a new method, the species-sensitive distribution model (SSD), was proposed, which could be used to estimate the environmental threshold for cadmium in regional cropland. The results showed that both maize and rice soils in the research area were contaminated with varying degrees of cadmium. The total concentrations of cadmium ω(T-Cd) in maize and rice fields are 0.18-1.32 and 0.20-4.42 mg kg^-1, respectively. The ω(T-Cd) of heavy metals in maize kernels and rice grains is 0.002-0.429 and 0.003-0.393 mg kg^-1, respectively. The bioaccumulation factor (BCF) of cadmium in maize ranged from 0.0079 to 0.9701, with a coefficient of variation of 1.71; the BCF of cadmium in rice ranged from 0.0074 to 0.1345, with a coefficient of variation of 0.99. According to cluster and Pareto analyses, the maize crop varieties with low cadmium accumulation suitable for local cultivation were screened as JHY809, JDY808, AD778, SN3H and SY13, and the rice varieties were DMY6188, GY725, NY6368, SY451 and DX4103. In addition, the environmental cadmium threshold ranges of 0.30-10.05 mg kg^-1 and 0.89-24.39 mg kg^-1 for maize and rice soils, respectively, were deduced in this study. This threshold will ensure that 5-95% of maize and rice will not be contaminated with cadmium in the soil.

The problem of paddy Cadmium (Cd) contamination is currently the focus of global research. Earlier researches have confirmed that utilization of organic fertilizers regulates Cd chemical fraction distribution by increases organic bound Cd. However, environmental behaviours of organic fertilizers in paddy are still lack exploration. Here, we critical reviewed previous publications and proposed a novel research concept to help us better understand it. Three potential impact pathways of utilization of organic fertilizers on the bioavailability of Cd are presented: (i) use of organic fertilizers changes soil physicochemical properties, which directly affects Cd bioavailability by changing chemical form of Cd(II); (ii) use of organic fertilizers increases soil nutrient content, which indirectly regulates Cd supply and bioaccumulation through ion adsorption and competition for ion-transport channels between nutrients and Cd; and (iii) use of organic fertilizers increases activity of microorganisms and efflux of rice root exudates, which indirectly affects Cd bioavailability of through complexation and sequestration of these organic materials with Cd. Meanwhile, dissolved organic matter (DOM) in the rhizosphere of rice is believed to be the key to revealing the effects of organic fertilizers on Cd. DOM is capable of adsorption and complexation-chelation reactions with Cd and the fractionation of Cd(II) is regulated by DOM. Molecular mass, chemical composition, major functional groups and reaction sequence of DOM determine the formation and solubilization of DOM-Cd complexes.