Process factors driving dynamic exchange of elemental mercury vapor over soil in broadleaf forest ecosystems

Abstract

A data gap in the evergreen broadleaf (EB) forest ecosystems has resulted in large uncertainties in estimating the quantity of global air–soil exchange of elemental mercury vapor (Hg0). In this study, we systematically measured the soil pore gas Hg0 concentration, air–soil Hg0 exchange flux and associated environmental parameters to elucidate the process factors driving the air–soil Hg0 exchange in the EB forest ecosystem. The observed air–soil Hg0 exchange flux shows evasion during summer and deposition during winter and indicates that the forest floor is a net atmospheric Hg0 source with an annual flux of +6.7 ± 20.5 μg m−2 yr−1. Structural equation modeling infers that temperature is the most important driver causing the air–soil Hg0 exchange, followed by atmospheric Hg0 concentration. Combined with air–foliage exchange data reported in Yuan et al. (2019) (https://doi.org/10.1021/acs.est.8b04865), the EB forest ecosystem emerges as an atmospheric Hg0 sink with a net flux of −20.1 ± 24.1 μg m−2 yr−1. Using data documenting global air–soil Hg0 exchange flux in forest ecosystems, we estimate Hg0 emission from the EB forest floor to be 347 ± 384 Mg yr−1, nearly 2 times greater than that from the boreal/temperate forest floor, highlighting the importance of EB forest ecosystems in the global Hg biogeochemical cycle.