The speciation and activity of heavy metals in farmland were changed with the different soil properties and flooded environment, especially in the complex and rainy environment in soil of Guizhou Province. The objective of this study was to explore the concentrations of a variety of heavy metal activity and the speciation of those heavy metals in rhizosphere soil at different growth stages of Brassica campestris L. in a Karst mountainous area. Tessier's five-stage sequential extraction procedure, the potential ecological risk index, a Bayesian network, accumulation factors, translocation factors and a laboratory simulation experiment were applied in this study. The results showed that (1) no heavy metal concentrations (except the Cd concentration) exceeded the limits of the soil environmental quality risk control standards for soil contamination of agricultural land in China (GB15618-2018). (2) The orders of the accumulation factor and translocation factor values were Zn > Cd > Cu > Pb > Cr and Cd > Cu > Zn > Pb > Cr, respectively. The order of the heavy metal contents of different tissues during the whole growth period was roots > leaves > stems. (3) The indoor simulation test exhibited that the dry-wet alternation and flooding can reduce Cd activity in soil. (4) Redox potential (Eh), rather than pH or organic matter, was the main factor impacting the total content and chemical speciation of heavy metals in the soil, based on a dynamic Bayesian network. Based on the results, we suggest that the activity of heavy metals should be improved by using dry-wet alternation, whereas the proportions of ion-exchangeable forms of heavy metals are relatively low in the study area (except for Cd). Several measures may be taken to enhance soil acidity and reduce the Cd activity during Brassica campestris L. cultivation.

Karst regions are relatively short of arable land, but heavy metals are at high levels in arable land. Under this circumstance, different tillage methods have different effects on the activity of heavy metals in soil. In this study, through field experiments and lab analysis, the effects of different rotation systems on the activity of heavy metals and the transport and enrichment of heavy metals in soil when rape and rice were in different growth stages were analyzed. The results show that in rice-rape rotation, the distribution of the heavy metal content in soil was different. The contents of Cr, Pb and Cu in soil were at the level of “clean” and the content of Cd was at the level of middle pollution. The sequence of the enrichment factors in the growth of the rape was: Cd>Cu>Pb>Cr and its ability to accumulate the same heavy metal varied from different growth stages; leaf, root and stem were main enrichment organs, among which stem had the weakest enrichment ability for Cu, root was the main enrichment organ for Pb and rape had relatively weaker enrichment and transport ability for Cr. Rice mainly accumulated Cd and Cu; it had relatively weaker enrichment ability for heavy meals in the seedling and tillering stages; its enrichment ability for Cd was gradually strengthened; and heavy metals were mainly enriched in rice’s root system in various growth stages.