Straw removal reduces Cd availability and rice Cd accumulation in Cd-contaminated paddy soil: Cd fraction, soil microorganism structure and porewater DOC and Cd.

The impacts of straw removal on rice Cd absorption, behaviour of Cd and microbial community in rhizosphere soil were investigated in paddy fields over two consecutive seasons. The results of the experiments in two fields revealed that straw removal promoted the transformation of soil Cd from acid-extractable and oxidisable fraction to residual fraction and reduced soil DTPA-Cd content with the reduction in DOC and Cd ions in soil porewater, thereby decreasing Cd content in rice. Specifically, the Cd content in brown rice was below 0.2 mg·kg-1 when all rice straw and roots were removed in the slightly Cd-contaminated soils. The α-diversity of soil microbial communities was less influenced by continuous straw removal, ß-diversity was altered and the relative abundances of Anaeromyxobacter, Methylocystis and Mycobacterium microbes were increased. Redundancy analysis and network analysis exhibited that soil pH predominantly influenced the microbial community. Path analysis revealed that the Cd content in brown rice could be directly influenced by the soil Total-Cd and DTPA-Cd, as well as soil pH and OM. Straw removal, including roots removal, is an economical and effective technique to reduce Cd accumulation in rice plants.

A rice-chicory rotation pattern ensures safe grain production and phytoremediation of cadmium-contaminated paddy fields: A four-year field experiment in southern China.

Cadmium (Cd) contamination in paddy systems is a serious problem, and a strategy must be devised that ensures safe grain production and rapid remediation of soil Cd contamination. To investigate the remediation potential of crop rotation and its effect on Cd accumulation in rice, a four-year (seven-season) rice-chicory rotation field trial was conducted on a moderately acidic Cd-contaminated paddy soil. Rice was planted in summers, followed by straw removal, and chicory, a Cd-enrichment plant, was planted during winter fallows. Rotation effects were compared with those with rice only (control). Rice yields between the rotation and control were not significantly different, whereas Cd concentrations in rice tissues decreased in the rotation. Cd concentration in brown rice of the low-Cd variety decreased to less than 0.2 mg/kg (national food safety standard) from the third season onward, whereas in the high-Cd variety, it decreased from 0.43 mg/kg in the first season to 0.24 mg/kg in the fourth season. The highest Cd concentration in chicory aboveground parts was 24.47 mg/kg, with an enrichment factor of 27.81. Chicory had high regenerative capacity and was repeatedly harvested for biomass in multiple mowings, with average aboveground biomass over 2000 kg/ha in a single mowing. Theoretical phytoextraction efficiency (TPE) of one rice season with straw removal was 0.84%-2.44%, whereas the highest TPE of one chicory season reached 8.07%. The seven seasons of rice-chicory rotation extracted up to 407 g/ha Cd from soil with a TPE exceeding 20%. Therefore, rice-chicory rotation and straw removal can effectively reduce Cd accumulation in subsequent rice crops, without interrupting production and simultaneously rapidly remediating Cd-contaminated soil. Thus, the production potential of light to moderately Cd-contaminated paddy fields can be realized with crop rotation.

[Effects of Straw Removal Measure on Soil Cd Bioavailability and Rice Cd Accumulation].

A field experiment was conducted in a lightly Cd-contaminated rice field in Ningxiang City, Hunan Province, to study the effects of straw removal measures on the soil Cd bioavailability and rice Cd accumulation. The results showed that:① two consecutive seasons of straw removal measures (T1-T4 treatments) effectively increased soil pH by 0.04-0.58 units, reduced soil organic matter by 0.68%-25.87%, and reduced the Cd content of rhizosphere soil by 3.76%-12.78%. ② The proportions of Cd in the acid-extractable fraction and oxidizable fraction decreased, and the proportion of Cd in the residual fraction increased. Furthermore, straw removal measures significantly reduced the bioavailability of Cd in rhizosphere soil, and the Cd contents in TCLP, DTPA, and CaCl2 extracts all significantly decreased compared with those in CK. ③ The straw removal measure could significantly reduce the content of DOC and Cd in soil pore water; and the contents of Cd in soil pore water decreased by 4.54%-40.00% and 2.75%-67.34% under the straw removal measure (T1-T4) for two consecutive seasons, respectively, indicating that DOC was one of the key factors affecting the content of Cd in soil pore water. ④ Two consecutive straw removal measures (T1-T4) reduced the accumulation of Cd in different rice tissues, among which, under the treatment of all straw and root removal (T4), the Cd contents of brown rice in late rice planting in 2020 and early rice planting in 2021 decreased by 18.52% and 39.69%, respectively. Therefore, full or partial removal of straw in Cd-contaminated rice fields is a powerful measure to reduce the risk of exceeding Cd levels in brown rice.

[Distribution and Accumulation of Cadmium in Paddy Soil and Rice Affected by Pollutant Sources Control and Improvement Measures].

The objective of this study was to determine the effect of five scenarios on the accumulation of Cd in the soil-rice system, including the return of straw to the field and the lack of the return, atmospheric deposition control, use of clean water for irrigation, and the use of lime. For the field experiments, three typical paddies were selected and divided into five plots (5 m×6 m) in Xiangtan, Zhuzhou, and Liling in the Hunan province from April to October 2016. The results showed that the application of lime can increase pH by 0.87, while the available Cd concentration in the soil was decreased by 33.7%. The accumulations of Cd in roots, stems, and brown rice were decreased by 47.9%, 46.7%, and 54.8%, respectively, with a decrease in the corresponding bioconcentration factors. Irrigating with clean water and liming tended to increase the soil pH by 0.44 and 0.49, respectively, while the available Cd concentration in the soil was decreased by 18.2% and 14.5%, respectively. The Cd concentrations in roots, stems, and brown rice were decreased by 32.6%, 24.2%, and 18.0%, and 17.6%, 11.3%, and 25.4% with decreased bioconcentration factors under both treatments (irrigating with clean water and liming). The available Cd concentration in the soil was increased by 6.1% and the Cd accumulation in the rice plants also increased with the return of straw to the soil. The bioconcentration factors of the rice plants were also increased when the paddy straw was returned to the fields. The results showed that the measures, such as the use of lime, atmospheric deposition control, use of clean water for irrigation, and lack of the return of straw to the paddy soil, should be helpful for the safe production of brown rice. The possible long-term risks associated with returning straw to the paddy field should be evaluated scientifically.