Microbial mercury methylation in the cryosphere: Progress and prospects

Soil aggregate-associated mercury (Hg) and organic carbon distribution and microbial community characteristics under typical farmland-use types

In order to get insight into the distribution characteristics of mercury (Hg) and organic carbon in soil aggregates, and the diversity and composition of soil microbial community under different farmland-use types (soil form three adjacent cultivation systems, i.e., corn, vegetable, and rice fields, named as CFS, VFS, and RFS), a field investigation close to Wanshan Hg mining area was conducted. Results indicated that soil total Hg (0-20 cm) presented in decreasing order of RFS (5.27 mg kg^-1) > VFS (4.32 mg kg^-1) > CFS (2.21 mg kg^-1), implying soils from rice field with higher ability of Hg accumulation. Soil aggregate-associated Hg and organic carbon enriched with the decrease of particle size under all farmland-use types, with the maximum at microaggregates (<0.053 mm). Due to the mass ratio of soil aggregates fraction, soil aggregate-associated Hg and organic carbon mainly distributed in >2 mm particles for RFS, whereas 0.25-2 mm particles for CFS and VFS. Furthermore, 16S rRNA results revealed the obvious differences in RFS and dry land soils (CFS and VFS), such as the observed species and unique OUTs, Shannon index, relative abundance at phylum and genus, which implied the diversity and composition of soil microbial community were greatly affected by farmland-use types. Spearman correlation and RDA results suggested farmland-use types, pH and total Hg were main drives for differences in soil microbial community. These findings provide evidence that farmland-use type is an important factor that affects soil total Hg accumulation, soil aggregate-associated Hg and organic carbon distribution, as well as the indigenous microbial community profiles.