Remediation effects of citric acid activation combined with Sedum plumbizincicola intercropped with maize on slightly Cd-contaminated soil

Adding citric acid to the soil can activate the heavy metals in the soil, further improving the bioavailability and the remediation efficiency of hyper accumulators. Intercropping between plants can also achieve the same effect. Sedum plumbizincicola intercropping with maize can be an in site, environmentally friendly, and effective way to remediate the slightly cadmium-contaminated soil, indicating a better combined remediation effect. In this study, the field micro-plot experiments were carried out to explore the mechanism and effect of different citric acid addition in the cooperation with S. plumbizincicola and maize intercropping mode on the remediation of slightly Cd-contaminated paddy soil. A total of 9 treatments were adopted using the randomized block design, where 3 kinds of citric acid addition (no, and 1 addition (40 L)), 5 additions (single addition of 8 L, once every other day, 5 additions, a total of 40 L)), and 3 planting modes: maize monocropping, S. plumbizincicola monocropping, S. plumbizincicola intercropping with maize. Each treatment area was 4 m2. Citric acid was added after the maize emerged, and the amount of citric acid was 0.2 mol/m2. The test area was uniformly applied for the citric acid as required, and the control area was uniformly added with the tap water. Samples were collected before the addition of citric acid and on the 10th, 50th, and 80th days after the addition of citric acid, respectively. The results showed that when the addition amount of citric acid was 0.2 mol/m2, the effect of different citric acid addition was different from the strengthening and restoration of S. plumbizincicola intercropping with the maize mode: Firstly, the addition of citric acid in the same planting pattern changed the form of heavy metal Cd in the soil for the high effectiveness of Cd in soil, and then significantly improve the remediation efficiency of S. plumbizincicola. The effect of adding 5 times was the most outstanding, where the restoration efficiency was improved in the single cropping at 2.09%, and 10.54% when intercropping. Secondly, with the addition of citric acid, the content of Cd in the soil of the three citric acid intercropping treatments decreased by 11.19, 12.44, and 13.66 μg/kg, respectively, compared with the single crop of S. plumbizincicola. The restoration efficiency of S. plumbizincicola increased by 3.42%, 3.82%, and 4.27%, respectively. At the same time, the dry matter weight of maize shoots increased, but the absorption of soil Cd by each part decreased, indicating a better effect of safe maize production. Thirdly, adding citric acid 5 times synergistically with S. plumbizincicola and maize intercropping, the Cd accumulation in stems, leaves, and roots of S. plumbizincicola reached 71.21 and 10.63 mg/kg, respectively, and the removal of Cd in S. plumbizincicola reached 18.59 mg/m2. This treatment performed the best remediation effect on the mildly Cd contaminated soil, and the restoration efficiency reached 10.54%. Fourthly, both citric acid and S. plumbizincicola intercropping with maize enhanced the restoration efficiency of S. plumbizincicola. A contrastive analysis showed that the remediation of S. plumbizincicola intercropping with maize was more effective than the restoration of adding citric acid on the restoration of mildly cadmium-contaminated soil by S. plumbizincicola, and the synergistic restoration effect. Therefore, the remediation mode of citric acid combined with S. plumbizincicola and maize intercropping can be expected to enhance the remediation efficiency of S. plumbizincicola. This mode can be applied to the remediation of paddy soils slightly Cd-contaminated for the high production at the same time.