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, β-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 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.

A field experiment was conducted in moderately and severely Cd contaminated paddy fields in Beishan Town, Changsha City, Hunan Province. This study examined the effects of LS amendment (limestone+sepiolite), in combination with soil application and foliar spraying of Zn fertilizer, on Cd uptake in early and late rice plants. The results showed that: ① the application of LS (2250 kg·hm-2 and 4500 kg·hm-2) significantly increased pH and CEC values in paddy soil during the early and late rice seasons, but the addition of Zn fertilizer (90 kg/hm2) to soil and through foliar spraying (0.2 g·L-1 and 0.4 g·L-1) had no significant effects on the pH or CEC of the soil. ② LS application decreased concentrations of TCLP-Cd and CaCl2-Cd in the soils, by 11.5%-38.8% and 24.0%-81.0%, respectively, while neither of the treatments involving the addition of Zn fertilizer to soil or through foliar spraying had any significant effects on the concentrations of TCLP-Cd and CaCl2-Cd. ③Single treatments involving only LS amendment, Zn fertilizer in soil, or foliar spraying of Zn fertilizer also reduced Cd concentrations in brown rice, but to a lesser degree than the combined treatments. The combined treatments (L1Z1F1, L1Z1F2, L2Z1F1, and L2Z1F2) reduced Cd concentrations in brown rice by 64.9%-67.5% and 56.1%-80.6%, for early and late rice, respectively, while L2Z1F1 (4500 kg·hm-2 LS+90 kg·hm-2 Zn fertilizer+foliar spraying 0.2 g·L-1 Zn fertilizer) resulted in the largest reduction in Cd concentration in brown rice. ④ The Cd/Zn ratio in brown rice was significantly positively correlated with Cd concentrations, indicating that increased Zn concentration in different rice tissues was one of the key reasons for decreased Cd concentration in brown rice. Clearly, as a remediation technology, combining LS amendments with zinc fertilizer is an effective method for achieving the safe utilization of moderately and severely Cd contaminated paddy fields, by effectively inhibiting the uptake, accumulation, and transportation of Cd in rice plants and decreasing Cd concentrations in brown rice.