Investigation of factors affecting mercury emission from subtropical forest soil: A field controlled study in southwestern China

Abstract

Recent studies demonstrated that subtropical forest in China was considered as a large pool of atmospheric mercury and soils of forested watershed is a large reservoir of atmospherically deposited mercury. However, forest ecosystems not only act as sinks but also as sources of previously deposited mercury emitted back to the atmosphere. In this study a field controlled method was performed in Tieshanping National Forest Park (TNFP) to identify the effects of the most important parameters that controlled mercury emissions from soil surfaces, including chamber flushing flow turnover times (TOTs), soil water content and watering, total gaseous mercury (TGM) in air and understory. Flushing flow rates significantly affected the calculation of mercury flux and the optimal TOTs were 0.94 min in the forest. TGM in atmosphere was significantly inhibited mercury emission from soils, and the deposited mercury was not absorbed firmly by the soils in a short time and emitted back to atmosphere rapidly when TGM concentration decreased. Higher soil moisture reduced the emission of mercury and initial watering produces a spike in the mercury emissions due to the interstitial soil gas mercury displaced by infiltrating water physically. However, subsequent watering was reducing the fluxes, because surface soil was saturated and soil pores were blocked by water film and inhibited the soil mercury emission. Soils under the understory had a higher mercury concentrations and deep organic layers. However, the fluxes of soil under the understory significantly were inhibited in daytime because solar radiation was blocked by the understory and the higher litter layer.

Mercury in terrestrial forested systems with highly elevated mercury deposition in southwestern China: The risk to insects and potential release from wildfires

“…4) and mineral layers are hence substantially lower than in the overlying litter horizons. Deeper soil layers showed lower mercury concentrations as compared to upper soil horizons which is also evident by our previous study in this area (Zhou et al, 2016), and mercury concentrations drop strongly from A to B horizon. Declining mercury concentrations with soil depth are commonly observed: Zhou et al (2013) reported declining mercury concentrations from top horizons to deep horizons in a broad-leaf primary forest in Southwestern China; soil concentrations decreased sharply with depth in mineral horizons in Jeffrey pines forest in USA (Obrist et al, 2009);and Liu et al (2003) reported decreasing mercury concentrations in the peat blog of Xiaoxing'an Mountain, northeastern China with highest mercury concentrations in the top horizons.…”

“…Via atmospheric long-distance transport, large amounts of mercury deposited to the surrounding region of the city. Based on our previous studies here, the annual total input fluxes of mercury were 291.2 mg m À2 yr À1 ), which was several or even dozens of times higher than those in North America and Europe (Zhou et al, 2016). About over two-thirds of the deposited mercury was resident in the forest, which resulted in elevated mercury concentration in organic horizons (191 ± 65 ng g À1 ) and mercury pool (8.2 ± 2.6 mg m…”

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

“…Previous studies suggested that soil moisture contributed to TGM flux had optimum interval and should be under intermediate conditions, neither under fairly dry nor very wet (Gustin and Stamenkovic, 2005;Lin et al, 2010;Pannu et al, 2014;Obrist et al, 2014;Zhou et al, 2017b), which can elucidate the different correlations at different forest ecosystems. Fig.…”

“…The relationship is mainly attributed to photochemical reduction of soilbound Hg, which converts soil Hg 2+ to volatile Hg 0 (Amyot et al, 1994(Amyot et al, , 1997Carpi and Lindberg, 1997;Moore and Carpi, 2005;Xin et al, 2007;Zhou et al, 2017b). Photo-reduction is a major driver of Hg 0 generation and evasion from soils (Choi and Holsen, 2009;Engle et al, 2001;Zhou et al, 2015aZhou et al, , 2017b, although other abiotic and biotic processes also resulted in translation of Hg 2+ to Hg 0 production, including reduction by humic acids (Alberts et al, 1974;Allard and Arsenie, 1991) and iron oxides under anoxic conditions (Lin and Pehkonen, 1997) as well as reduction by microorganisms (Siciliano et al, 2002;Agnan et al, 2016) and/or microbial exudates (Poulain et al, 2007(Poulain et al, , 2004Fritsche et al, 2008). Additionally, other important correlation was identified with soil or air temperature, which is also significantly correlated to the Hg 0 production and observed with soil-atmosphere Hg flux in all the forests in China (n=30).…”

“…According to the two-resistance exchange interface model, the exchange fluxes are caused by the gradient of Hg 0 concentrations on both interfaces (Zhang et al, 2002); therefore, high Hg 0 concentrations in the atmosphere will reduce the potential of Hg 0 produced in the soil and diffusion to atmosphere. Laboratory and filed simulation studies showed that elevated atmospheric Hg concentrations significantly inhibited soil Hg volatilizations (Zhou et al, 2017b;Ericksen and Gustin, 2004;Hanson et al, 1995;Poulain et al, 2004). Atmospheric compensation point for Hg 0 flux was firstly presented by Hanson et al (1995), which is the atmospheric Hg concentration at which no net flux occurs between soil and air (flux to be 0).…”