The distribution of arsenic fractions and alkaline phosphatase activities in different soil aggregates following four months As(V) ageing

Soil as a heterogeneous mass is composed of different size aggregates. The distribution of different arsenic (As) fractions in soil aggregates is vital to assess the potential risk of As pollution. In this study, soil samples were aged for 4 months with different arsenate [As(V)] concentrations. Dry sieving method was used to obtain five different size aggregates and the content of As in these fractions was determined. The results showed that P4 (0.1-0.25 mm) contained the highest organic matter (OM) than other size aggregates. After 4 months of ageing, available phosphorus (AP) content increased with the increase of As(V) concentration among 5 aggregates. The distribution of different arsenic fractions among 5 aggregates was similar. The relative contents of water-soluble (F1), exchangeable (F2) and carbonate (F3) fractions increased with the increase in As concentration, while the residual fraction (F7) decreased sharply. Humic-bound (F4), and Fe and Mn oxide bound fractions (F5) were about 35% and 20% respectively, after 4 months of As(V) ageing. Generally, the alkaline phosphatase (ALP) activities of P4 were lowest among five aggregates under each concentration of As(V). Moreover, F2 and F3 exhibited a strong inhibition of ALP activity. This study demonstrates that not only water-soluble and exchangeable arsenic but also humic-bound fraction should be considered when assessing As bioavailability and toxicity.