Effects of Humic acid fertilizers on arsenic and mercury speciation and enzyme activities in soil

Abstract Arsenic (As) and mercury (Hg) pollution in soils pose a serious threat to the ecological environment and human health. Humic acid is often used for soil improvement and remediation due to its large number of active groups, but how humic acid fertilizers affect the behavior of As and Hg in soils remains to be clarified. Therefore, the effects of ammonium fulvic acid and sodium humate on the speciation of As and Hg in soil and their migration to plants, as well as the enzyme activity in As- and Hg-contaminated soil were studied. The results showed that all concentrations of ammonium fulvic acid decreased the content of the residual fraction As, but increased the the content of specifically sorbed fraction and oxide bound fraction As in the soil. So the passivation effect of ammonium fulvic acid on As in soil was not obvious. Sodium humate promoted the transformation of As in the soil to a more stable residual fraction state. Both ammonium fulvic acid and sodium humate increased the content of the organic bound fraction and the residual fraction Hg in the soil, that is, both humic acid fertilizers promoted the transformation of Hg in the soil to a more stable form. In the As and Hg polluted soil, the application of ammonium fulvic acid had no significant effect on the content of As and Hg in the shoot of pakchoi, but the application of sodium humate could reduce the content of As and Hg in the shoot of pakchoi. The activities of urease, alkaline phosphatase and sucrase in As and Hg polluted soil was increased by ammonium fulvic acid and sodium humate, and the enzyme activities were higher than those in the control group under different fertilizer additions.