Arsenic (As) is one of the hazardous elements that is ubiquitous in soils. Since soil As dissolves into soil solution under reduced conditions, paddy rice plants are susceptible to uptake As from soil. Application of organic amendments (OAMs) potentially accelerate As solubilization through the following mechanisms: reductive dissolution of As(V) and As-bearing Fe oxides, inhibition of As sorption, and formation of soluble complexes with As. On the contrary, some studies demonstrated that application of OAMs inhibited As dissolution from soils. To find the cause of these discrepancy, the objectives of the present study were (1) to compare As solubility in flooded soils amended with different types of OAMs, and (2) to clarify the important factor to determine As solubility in soils amended with OAMs.
A surface paddy soil was collected and passed through a 2-mm mesh sieve. The soil had 7.42 mg kg-1 of HCl-extractable As. We used 15 OAMs from three different sources, namely (1) plant-based composts (PBCs), (2) cattle manures (CMs), and (3) fermented livestock (swine and poultry) droppings (FLDs). They were milled mechanically and were sieved through 1-mm mesh sieves with keeping moisture in them. Their carbon to nitrogen ratio (C/N) ranged from 5.1 to 45.7, and the average of C/N of each source of OAM decreased in the order of PBCs>CMs>FLDs. Moist soil (10 g as oven-dried basis) and OAM (0.05 g as oven-dried basis) were mixed with ultra-pure water to bring the total water up to 30 mL in a glass vial. Corresponding vials without OAM (control soils) were also prepared. Vials were tightly capped after N2 gas purging, then were manually shaken well. They were incubated at 30 °C for a specific period with shaking every 2 or 3 days. After incubation, the soil solution was collected and immediately acidified. The acidified soil solution was diluted and used for As measurement by ICP-MS. Residual soil suspension was used for measuring pH and Eh.
After incubation, all soils with FLD contained significantly greater dissolved As than those with control soils at 10 and 39 days after incubation. The dissolved As in soils with two CMs were also significantly greater than control soils at day 39, while the rest of the OAMs did not promote As solubilization. The amounts of dissolved As in all soils eventually became comparable at 80 days. Averaged relative ratio of dissolved As in soils with FLBs to control soils was significantly greater than those with the PBCs and CMs at days 10 and 39 (P < 0.05, Turkey test). Amendment C/N and soil Eh were positively correlated at days 10 and 39 (Spearman’s rank correlation), indicating amendments with low C/N have high bio-decomposability. Conversely, the dissolved As was negatively correlated with C/N, reflecting a negative relationship between dissolved As and soil Eh. Furthermore, the addition of OAMs did not substantially change the relationship between dissolved As and soil Eh. These results indicated that acceleration of soil reduction was the major cause of increased As solubilization by OAMs. Therefore, we concluded that FLDs have a greater potential of solubilizing As than PBCs and CMs during short time after flooding soil, because of their greater ability for accelerating soil reduction associated with their higher bio-decomposability.