Three-season rotation of chicory-tobacco-peanut with high biomass and bioconcentration factors effectively remediates cadmium-contaminated farmland

Traditional phytoremediation is one approach to remediate heavy metal pollution. In developing countries, the key factor in promoting practical application of phytoremediation in polluted soils is selecting suitable plants that are tolerant to heavy metals and also produce products with economic value. Therefore, a field experiment was conducted with a three-season chicory-tobacco-peanut rotation to determine effects on remediation of cadmium (Cd)-contaminated farmland in China. All crops had strong Cd accumulation capacity, with bioconcentration factors of 6.61 to 11.97 in chicory, 3.85 to 21.61 in tobacco, and 1.36 to 7.0 in peanut. Yield of total dry biomass reached 32.4 t ha-1, and the Cd phytoextraction efficiency was 10.3% per year. Aboveground tissues of the three crops accounted for 83.9 to 91.2% of total biomass in the rotation experiment. Cd content in peanut grain and oil met the National Food Safety Standard of China (0.5 mg kg-1, GB 2762-2017) and the Food Contaminant Limit of the European Union (0.1 mg kg-1, 18,812,006). Therefore, in addition to phytoremediation of Cd-contaminated soils, the chicory-tobacco-peanut rotation system can also produce economic benefits for local farmers.

Phosphorus fertilization regimes and rates alter Cd extractability in rhizospheric soils and uptake in maize (Zea mays L.)

Understanding cadmium (Cd) extractability and transfer in soil-plant system is crucial for the evaluation of the remediation effect of Cd-contaminated soils. However, knowledge on the effects of different phosphorus (P) fertilizers on Cd uptake in plants, root morphology, and Cd extractability in rhizosphere soils remains very limited. In this study, a five-year field experiment was conducted to evaluate the impacts of four P fertilizers (i.e. calcium superphosphate, calcium magnesium phosphate, monopotassium phosphate, and compound fertilizer) on Cd uptake in maize (Zea mays L.), root morphology, and Cd extractability in rhizospheric acidic soils contaminated with Cd. The results showed that compared to the control, the contents of rhizospheric DTPA-Cd were respectively 18-40% and 8-29% lowered by the calcium magnesium phosphate and monopotassium phosphate, but 21-59% and 10-36% elevated by the calcium superphosphate and compound fertilizer. Similar effects of P fertilizers were observed on exchangeable Cd. Furthermore, the altered levels of the DTPA-Cd and exchangeable Cd in the rhizospheric soils were greater than those in the non-rhizospheric soils. Moreover, different P fertilization regimes altered the contents of Cd in maize tissues (roots, stems, leaves, and grains), and the alterations were closely related to the variation of DTPA-Cd and exchangeable Cd in the rhizospheric soils. Meanwhile, different P fertilization regimes enhanced root morphological parameters (root length, surface area, and volume), and the activities of urease and surcase. In general, the lowest concentrations of soil DTPA-Cd and Cd in maize tissues were found in the treatments with calcium magnesium phosphate. This study has demonstrated that the calcium magnesium phosphate can be used as a potential amendment agent for the acidic Cd-contaminated soils cultivated with maize.

Consistent inter-annual reduction of rice cadmium in 5-year biannual organic amendment

Organic fertilizers may contain cadmium (Cd) and the transformation of organic materials in soil also has a role in soil-plant Cd distribution, both of which lead to Cd accumulation in plant edible parts. However, the advisability of applying organic fertilizer to remediate soils that are moderately and slightly contaminated with Cd has not been clarified. In this study, we investigated the impacts of an organic amendment (chicken manure) on the Cd concentration in rice grains (Cd_R) and the soil chemical properties over a five year period (10 rice seasons) within a slightly contaminated paddy soil in Hunan Province, subtropical China. We found that the Cd_R was reduced by 28%-56% as a result of the organic amendment. The within-year reduction in Cd_R was higher in late rice (43%-56%, averaging 51%) than in early rice (28%-45%, averaging 38%); however, the inter-annual reduction in Cd_R was fairly stable (40%-49%), which suggests that chicken manure amendment has a long-term and persistent remediation potential. The concentrations of DTPA-extractable Cd and exchangeable plus water-soluble Cd fractions in soil were reduced, whereas soil pH and the concentrations of soil organic C and its labile fractions increased. These results indicate a lower apparent phytoavailability of Cd in soil following organic amendment. A two-variable empirical model using DTPA-Cd extracted from the soil at the full heading stage of rice and a climatic factor (total precipitation during the rice growing season) showed great potential in effectively predicting Cd_R. Our study suggests that Cd phytoavailability in soil (indexed by DTPA-extractable and exchangeable Cd) and climatic factors (such as temperature and precipitation) may control inter-annual reductions in Cd_R following organic amendment in slightly contaminated paddy soils.