The accuracy in the assessment of arsenic toxicity using soil alkaline phosphatase depends on soil water contents

Abstract

Soil phosphatase is commonly used as an indicator to evaluate the arsenic (As) toxicity in soil. However, the As ageing process and the accuracy of using soil phosphatase activity to assess As contamination under various soil water contents are not clearly known. This study explored the effects of soil moisture condition on the As fractions (soil solid phase species) and soil alkaline phosphatase (ALP) over time. Results showed that elevated soil moisture would increase the content of F1-F4 (water soluble, exchangeable, carbonate and organic-bound) and decrease the ratio of F6 (residual). Whereas, the F5 fraction (bound to Fe and Mn/Al oxides), although did not show significant variation in the early period (up to 15 d), showed an increase at later period (30 d). The As bioavailability (K) increased with increasing soil moisture. Ageing process exhibited two stages. In the stage1, as the ageing progress the fractions, F1, F2 and F3 (former) gradually transformed into F4, F5 and F6 (later) under all tested As concentrations. The second stage is that under the low As concentrations, F1 and F4 transformed to F5 fraction. While, under the high As concentrations (200 mg kg−1 and 400 mg kg−1), F1 and F2 transformed to F4 and F5. Meanwhile, soil moisture had a significant influence on soil ALP activity. Soil ALP activity under 110% WHC was smaller than the activity of soil ALP under 35% WHC. The variation partitioning analysis (VPA) showed that soil moisture contributed 63.19% for this effect on soil ALP activity and was identified as the dominant factor. The value of ED10 indicated that the As toxicity under 35% WHC was greater than that under 110% WHC. This could be due to that the excessive moisture content depressed soil ALP activity. Therefore, it is important to consider soil moisture content while assessing the As toxicity to soil ALP.