Phytoavailability of roxarsone and its metabolites for turnip as affected by soil pH

Roxarsone (ROX) is widely used as a feed additive in intensive animal production. It has been reported that most ROX fed to an animal is excreted unchanged, and its metabolites in the fresh manure include low contents of As(V), As(III), monomethylarsonic acid (MMA), dimethylarsinic acid (DMA) and other unknown As compounds. Some metabolites of ROX might be more potent to humans than inorganic As, and the potential risk of As-bearing manure application needs to be evaluated. Soil pH greatly affects the geochemistry of As species. A pot experiment was conducted to investigate the uptake, translocation and distribution of ROX, As(V), As(III), MMA and DMA in turnip (Brassica rapa var. rapa) plants grown in soils, with the pH values of 4.5, 5.5, 6.5, 7.5 and 8.5, respectively. The results show that this species of turnip cannot accumulate ROX and MMA at a detectable level. The tuber DMA insignificantly (P < 0.05) increased and the shoot As(III) and As(V) decreased with the increasing soil pH. The transfer factors of As(V), As(III) and DMA from turnip tubers to shoots were the highest in the soil with pH 4.5, however, no close relationship between transport efficiencies of As species and soil pH value was observed. The percentages of DMA in both turnip tubers and shoots were increased by elevated soil pH, in contrast to reduced percentages of As(V). Though soil pH did affect the accumulation, translocation and distribution of As(V), As(III) and DMA, no clear pattern was observed under varied soil pH values. It indicates that the effect of soil pH on phytoavailability of ROX and its metabolites might be influenced by other major components from chicken manure.