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Underlying dynamics and effects of humic acid on selenium and cadmium uptake in rice seedlings

Zhang, Hongyu, Xie, Shuyun, Bao, Zhengyu, Tian, Huan, Carranza, Emmanuel John M., Xiang, Wu, Yao, Lingyang, Zhang, Hai
Journal of soils and sediments 2020 v.20 no.1 pp. 109-121
Oryza sativa, adsorption, agricultural soils, anions, biogeochemistry, cadmium, cations, fertilizers, humic acids, iron, planting, polluted soils, rice, seedlings, selenium, sowing, tissues, China
PURPOSE: Natural organic acids, such as humic acid (HA), play crucial roles in biogeochemistry of anions and cations in soil due to their numerous functional groups on their surfaces. Selenium (Se) and cadmium (Cd) could bind strongly to HA; nevertheless, it is still unclear as to the effects of HA on Se and Cd uptake in rice which will be focused on in this paper. MATERIALS AND METHODS: Pot experiments were carried out at Huazhong Agricultural University, Wuhan City, Hubei Province, China. Agricultural soils were treated with different concentrations of HA (0, 4, and 8 g kg⁻¹ soil) and Se (Seᴵⱽ or Seⱽᴵ) (0 and 2 mg kg⁻¹ soil) as well as with base fertilizer 3 days prior to planting. For Cd treatment, experimental soils were treated with Cd (0 and 2 mg kg⁻¹ soil) 1 month before sowing. For element determination, root (after DCB extraction) and shoot samples were digested with a mixed solution of HNO₃-HClO₄, and the Se and Cd in digest solution were measured by HG-AFS and ICP-MS, respectively. Fe, Se, and Cd in iron plaque were extracted by DCB extraction and measured by AAS, HG-AFS, and ICP-MS, respectively. RESULTS AND DISCUSSION: HA reduced Se (or Cd)-induced growth stimulation and Se and Cd uptake in rice seedlings, whereas iron plaque formation varied little with different treatments. HA inhibited Seᴵⱽ (or Seⱽᴵ) uptake in rice seedlings by reducing Se translocations from soil to iron plaque (or by increasing Se adsorption capacity of iron plaque and decreasing Se transport from iron plaque to root). HA reduced Cd uptake in rice seedlings by reducing Cd transport from soil to iron plaque and from iron plaque to root. Compared with single addition of Seᴵⱽ or Seⱽᴵ or HA, adding HA combined with Seᴵⱽ or Seⱽᴵ could further reduce Cd uptake in rice seedlings, whereas Se contents of aerial tissues did not change obviously. CONCLUSIONS: HA inhibited the accumulation of Se (Seᴵⱽ or Seⱽᴵ) and Cd in rice seedlings; nevertheless, the mechanism was different. Compared with adding Se (or HA) alone, application of Se mixed with HA might be a more effective way to produce Se-enriched and Cd-deficient crop in Cd-contaminated soil.