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

摘要

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 (Hg-0). In this study, we systematically measured the soil pore gas Hg-0 concentration, air-soil Hg-0 exchange flux and associated environmental parameters to elucidate the process factors driving the air-soil Hg-0 exchange in the EB forest ecosystem. The observed air-soil Hg-0 exchange flux shows evasion during summer and deposition during winter and indicates that the forest floor is a net atmospheric Hg-0 source with an annual flux of +6.7 +/- 20.5 mu g m(-2) yr(-1). Structural equation modeling infers that temperature is the most important driver causing the air-soil Hg-0 exchange, followed by atmospheric Hg-0 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 Hg-0 sink with a net flux of -20.1 +/- 24.1 mu g m(-2) yr(-1). Using data documenting global air-soil Hg-0 exchange flux in forest ecosystems, we estimate Hg-0 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 biogeo-chemical cycle.