Divergent response of heavy metal bioavailability in soil rhizosphere to agricultural land use change from paddy fields to various drylands.

The heavy metal pollution induced by agricultural land use change has attracted great attention. In this study, the divergent response of the bioavailability of heavy metals in rhizosphere soil to different agricultural land uses was analyzed using sequential extraction, and possible influence paths were constructed. The results show that land use change can affect the heavy metal bioavailability by influencing the soil organic matter and redox potential (Eh). The average concentrations of N, P, K, Ca, Mg, S, and Fe in the soil showed no significant differences. However, the conversion direction and extent of chemical speciation of heavy metals were different across land use changes from paddy fields to various drylands. After conversion from paddy to wheat field, the bioavailability of heavy metals decreased due to an increase in permanganate oxidizable carbon (KMnO-C) and a decrease in Eh. The transformation from paddy to celery soil is accompanied by a change in the soil's KMnO-C content, increasing the proportion of the bioavailable states of heavy metals. However, the response of bioavailability to changes in the soil KMnO-C varied among heavy metals. In contrast, when land use changed to grapevine culture, the bioavailability of heavy metals increased due to a change in the KMnO-C content. Moreover, the dissolved organic carbon (DOC) content increased, which positively affected the Eh and, in turn, increased the bioavailability of heavy metals. This research is of great significance for understanding the impact of land use change on the heavy metal migration and activity in the rhizosphere microenvironment of soil.