Effect of Alfalfa (Astragalus sinicus) and Rice Straw Additions on Cd Fractions and Activity in Acid Soil Under Flooded Condition.

A soil incubation experiment was carried out to investigate the effect of China widespread organic materials practice such as alfalfa (Astragalus sinicus) and/ or rice straw on Cd-fractions in yellow-red acid soil under flooded condition. The results show that the Cd entered into yellow-red acid soil mainly transform into residue bound Cd (Res-Cd) regardless of organic materials additions, and followed by strongly bound organic Cd (SBO-Cd) or exchangeable Cd (Exch-Cd) according to the incubation time, there are only a little of sodium acetate (NaAc) extractable Cd and weakly bound organic Cd (WBO-Cd). Cd fractions in yellow-red acid soil change with incubation time. The SBO-Cd and Res-Cd increase and then decrease with incubation time, the Exch-Cd and WBO-Cd increases remarkably but the iron and manganese oxide bound Cd (Ox-Cd) decreases significantly at 120d of incubation compared with 40d and 80d of incubation, and the sodium acetate (NaAc) extractable Cd has little change within 120d of incubation. All the treatments show similar variation trend, but organic materials especially A1 with 22.5t/hm² of Astragalus sinicus addition lowers the change extent of Cd fractions significantly. Compared with non-addition CK, addition of Astragalus sinicus and rice straw alone or together all can reduce the ratio of high activity water soluble Cd and Exch-Cd to total Cd by 22.7%~60.4%, but increase the ratio of low activity Res-Cd and SBO-Cd to total Cd by 13.9%~43.2% within 120d of incubation. However, A1 always has the lowest ratio of high activity Cd regardless of incubation time and the highest ratio of low activity Cd especially at 120d of incubation, and shows obvious difference with other treatments. The results demonstrate that alfalfa and rice straw can be employed as cheap substances for the immobilization of Cd in yellow-red acid soil, but proper dose of Astragalus sinicus is more effective than rice straw and their mixture.