Effect of Humic Acid-based Amendments on Exchangeable Cadmium and its Accumulation by Rice Seedlings

Cadmium (Cd) in the food chain is toxic to humans and mainly attributed to the accumulation by plants. Organic amendments are considered effective to reduce the bioaccessiblity of heavy metals through complexation, only if insoluble complexes are formed. A 42-day soil incubation followed by a 40-day rice pot experiment was conducted. The result of the incubation showed that all seven amendments (humic-ammonium, humic-calcium, humic-nitro-calcium, humic-urea, humic-potassium, humic-nitro-potassium, and woody peat) significantly increased soil pH and reduced MgCl2-exchangeable Cd contents by over 32% on day 12. But soil pH decreased and exchangeable Cd contents increased with time and at the end of the incubation, no significant difference in either soil pH or exchangeable Cd contents between CK and most amendments was found. The contents of Cd in rice seedlings were reduced by amendments both under the aged and unaged groups (unaged: soil with freshly added amendments; aged: rice was planted in the soil used in the incubation without further addition of amendments). Compared with the corresponding unaged amendments, the aged amendments significantly increased the contents of Cd in rice shoots, except humic-potassium. Therefore, the humic acid-based amendments could reduce exchangeable Cd in soil but the effect was not persistent and accordingly, the unaged amendments would lead to much less Cd accumulation in rice seedlings than the aged ones. (C) 2017 American Institute of Chemical Engineers