Mercury fluxes, budgets and pools in forest ecosystems of China: A critical review

Abstract. Mercury (Hg) accumulation and retention in forest ecosystems play a key role in global biogeochemical cycling of Hg. Especially in China, forests are suffering highly elevated Hg loads. Numerous studies have been conducted to characterize the fluxes and pools of Hg in the terrestrial forests in China during the past decade, which provide insights into spatial distributions and estimate the Hg mass balance in forests through observations at widely diverse subtropical and temperate locations. In this paper, we present a comprehensive review of the research status of forest Hg in China to characterize the Hg budgets and pools. Averaged total Hg (THg) inputs at remote forests and rural & suburban forests in China are about 2 to 4-fold and 2.5 to 5-fold higher than the observed values in Europe and North America, respectively. The highly elevated THg inputs are mainly derived from the elevated atmospheric Hg concentrations. Additionally, production of litterfall biomass is showed to be an important influential factor raising the high Hg inputs at subtropical forests. Compared to the input, THg outputs from the forest ecosystems are relative small, which results in large amount of Hg resided in the forest soils. The annual THg retentions range from 26.1 to 60.4 µg m⁻² at subtropical forests and from 12.4 to 26.2 µg m⁻² at temperate forests of China, which are about 3.8- to 7.9-fold and 1.2 to 2.8-fold higher compared to those in North America. Given the large areal coverage, THg retention in forest is appropriately 69 t yr⁻¹ in China and is much high than that in global scale estimated by models. The much higher THg retention has elevated the THg pools in Chinese subtropical forests, which poses a serious threat for large Hg pulses remitted back to the atmosphere and additional ecological risks in the forest. The current study has implication for the role of China forests in the global Hg biogeochemical cycle and the optimization of atmospheric Hg transport and deposition models.