The response of soil bacterial communities from farmland ecosystems to cadmium (Cd) pollution, in which a steep concentration gradient of more than 100 mg/kg has naturally formed, has not previously been fully reported. In this study, a field investigation was conducted in a typical severe Cd-polluted farmland ecosystem, and the bacterial community response to the steep Cd gradient was analyzed. The results showed that Cd concentration sharply decreased from 159.2 mg/kg to 4.18 mg/kg among four sampling sites alongside an irrigation canal over a distance of 150 m. Bacterial diversity and richness were significantly lower in highly polluted sites, and random forest analysis indicated that Cd gradient played a decisive role in reducing alpha diversity. Redundancy analysis (RDA) and co-occurrence network indicated that the synergistic effects of pH, Cd, and phosphorus were the main drivers shaping community structure. The functional results predicted by BugBase suggested that the bacterial community may adapt to the harsh environment by recruiting Cd-resistant microbes and improving oxidative stress tolerance of the whole community. Cd-resistant microorganisms such as Burkholderia, Bradyrhizobium, and Sulfurifustis, which directly or indirectly participate in diminishing oxidative damage of Cd, may play essential roles in maintaining community stability and might be potential bacterial resources for the bioremediation of Cd pollution.

Cadmium (Cd) speciation ratio in arable land determines the Cd exposure risk and Cd uptake in crops. However, the driving mechanisms of Cd speciation change on the vertical scale of paddy fields remain poorly understood. In this study, the effects of plow layer and plow pan on Cd speciation distribution were investigated in a long-term Cd-contaminated rice ecosystem. The Cd accumulative effect within rice grain was enhanced with high levels of activated Cd speciation ratios in soils. Activated Cd speciation ratios were higher in plow layer soils, while stabilized Cd speciation ratios were elevated in plow pan soils. Soil physicochemical properties and soil microbes synergistically affected the Cd speciation changes in different ways between the two soil layers. Soil pH and organic elements in plow layer environment directly hindered the transformation of stabilized Cd speciation, while in plow pan environment, soil pH and organic elements indirectly decreased activated Cd speciation ratios and resulted in the accumulation of stabilized Cd speciation via regulating the predominant bacterial taxa. This study will improve our understanding of how soil environments regulate Cd speciation distributions in rice ecosystems and help to seek effective remediation methods of Cd-contaminated paddy fields to reduce the Cd accumulation in rice.

Effects of iron and sulfur redox states in the soil caused by different growth seasons on Cd uptake by rice remain unclear. In this study, three early rice cultivars and three late rice cultivars were cultivated in a double-cropping system in Cd-contaminated paddy fields. The total Cd accumulation of early rice cultivars was 20.5-51.1 µg plant^-1, and the Cd concentration in grains was 0.19-0.73 mg kg^-1, significantly lower than those of late rice cultivars by 8-15 times and 3-9 times, respectively. The filling and mature stages were identified as the most crucial stages of Cd uptake by both early and late rice cultivars. The growth season of early rice cultivars was characterized by more abundant rainfall and lower soil Eh than that for late rice cultivars. Therefore, the abundances of Fe-reducing bacteria (FeRB, 36.9-39%) and S-reducing bacteria (SRB, 1.77-2.79%) were higher during the filling and mature stages of early rice. They primarily belonged to the Clostridium, Geobacter, and Desulfuromonadales genera. Stimulation of FeRB and SRB activity promoted Fe(III) and S reduction and increased the content of Fe²⁺ and S^2- in rhizosphere soil. This promoted the binding of Cd to amorphous Fe oxides and sulfides or Fe sulfides, thereby decreasing the available Cd content. Moreover, the Cd in the iron plaque (IP) and Cd transfer from IP to roots were lower in early rice. These findings suggest that maintaining high moisture content in the soil during the filling and mature stages, especially for late rice cultivars, could efficiently reduce Cd uptake by rice planted in contaminated soil.

Bioremediation of Cd- (cadmium) contaminated soil using Cd-tolerant fungus is considered an eco-friendly and cost-effective technique. In this study, we isolated one fungal strain that was hyper-tolerant to Cd from a highly polluted river and conducted pot experiments to evaluate its effects on bioremediation. We found that the fungal strain belonging to the genus, Purpureocillium lilacinum, tolerated 12,000 mg/L Cd. SEM manifested that Cd can be bioaccumulated on the crumpled mycelial surface, generating plenty of metal precipitation particles. In addition, pot experiments showed that the inoculation of P. lilacinum N1 could reduce the total Cd content in soil (2.09% in low contaminated soil and 12.56% in high contaminated soil) and greatly promote plant growth (2.16~3.13 times). Although the Cd concentration of plants was increased by 112.8% in low contaminated soil and decreased by 9.5% in highly contaminated soil with the inoculation of P. lilacinum N1, the total uptake of Cd by plants was greatly improved—1.84~3.6 times higher than that in CK groups. All our results suggest that P. lilacinum N1 is a valuable candidate for the bioremediation of Cd-contaminated soils because of its dual effects on the total Cd content in soil and Cd uptake in plants.