Quantifying the effects of soil temperature, moisture and sterilization on elemental mercury formation in boreal soils Ravinder Pannu a, b , Steven D. Siciliano a , Nelson J. O'Driscoll b, * a Department of Soil Science, University of Saskatchewan, 51 Campus Drive, Saskatoon, SK S7N 5A8, Canada b Department of Earth and Environmental Science, Acadia University, K. C. Irving Environmental Science Center, Wolfville, NS B4P 2R6, Canada article info Article history: Received 29 January 2014 Received in revised form 19 June 2014 Accepted 24 June 2014 Available online xxx Keywords: Soil temperature Soil moisture Soil sterilization Mercury reduction Reaction rates Kinetics abstract Soils are a source of elemental mercury (Hg(0)) to the atmosphere, however the effects of soil temper- ature and moisture on Hg(0) formation is not well defined. This research quantifies the effect of varying soil temperature (278e303 K), moisture (15e80% water filled pore space (WFPS)) and sterilization on the kinetics of Hg(0) formation in forested soils of Nova Scotia, Canada. Both, the logarithm of cumulative mass of Hg(0) formed in soils and the reduction rate constants (k values) increased with temperature and moisture respectively. Sterilizing soils significantly (p < 0.05, n ¼ 10) decreased the percent of total Hg reduced to Hg(0). We describe the fundamentals of Hg(0) formation in soils and our results highlight two key processes: (i) a fast abiotic process that peaks at 45% WFPS and depletes a small pool of Hg(0) and; (ii) a slower, rate limiting biotic process that generates a large pool of reducible Hg(II). © 2014 Elsevier Ltd. All rights reserved. 1. Introduction Mercury is ubiquitous in the environment and cycles between terrestrial systems, the atmosphere, oceans, and living organisms. It is a global pollutant, and once released as Hg(0), it remains in the atmosphere for up to 1 year where it is transported globally (Lindberg et al., 2002). Soils are a reservoir in the mercury cycle (Kim and Lindberg, 1995). Research during the past decade has established the importance of natural soils in Hg cycling, showing that emission from soils may contribute substantially (700e1000 Mg Yr 1 ) to the global atmospheric load of Hg (Engle et al., 2001; Coolbaugh et al., 2002; Engle and Gustin, 2002; Gustin et al., 2003, 2006; Zhang et al., 2003). In order to better understand the mobilization of mercury from soil reservoirs we need to know more about the processes controlling the formation of Hg(0) in soils. Elemental mercury in soil can be produced by abiotic or biotic processes. It is generally thought that most of the elemental mer- cury produced in soil originates from the A horizon and is produced by the high microbial activity and abundance of reductants present in this soil horizon (Carpi and Lindberg, 1997, 1998; Schluter, 2000). Experimental studies performed on natural and contaminated soils have demonstrated the strong dependence of Hg(0) on climatic factors (Gustin et al., 1997a; Gustin et al., 1997b; Poissant and Casimir, 1998; Gustin et al., 2002; Scholtz et al., 2003; Gustin and Stamenkovic, 2005; Lindberg et al., 2007). For example, Gillis and Miller (2000) found that Hg(0) emission rates in low-mercury, fine sandy loam soil can be largely explained by variations in sur- face soil temperature (r 2 ¼ 0.88) and the Hg concentration gradient between the soil air and ambient air. This temperature dependence has been observed both in diurnal (Gustin et al., 2006) and seasonal studies (Sigler and Lee, 2006). Gustin and Stamenkovic (2005) have demonstrated that small additions of water significantly enhanced Hg(0) release from desert soils. Precipitation may result in Hg(0) release from natural soils (Lindberg et al., 1999; Wallschlager et al., 2000; Song and Van Heyst, 2005). Lin et al. (2010) showed that the synergistic effect from air temperature and soil moisture was 30% greater than the additive Hg flux for the two individual effects. They proposed this effect as a result of enhanced water evaporation at higher tem- perature, which promotes additional Hg(0) mobilization from soil; however no mechanism was suggested. Sigler and Lee (2006) suggest that Hg(0) bound to upper soil layers may be desorbed by an increase in soil temperature, thereby increasing the pool of gaseous Hg(0) in soil air spaces. However, little information is available on the controlled effect of soil temperature and moisture on Hg(0) formation in soils. * Corresponding author. E-mail address: nelson.odriscoll@acadiau.ca (N.J. O'Driscoll). Contents lists available at ScienceDirect Environmental Pollution journal homepage: www.elsevier.com/locate/envpol http://dx.doi.org/10.1016/j.envpol.2014.06.023 0269-7491/© 2014 Elsevier Ltd. All rights reserved. Environmental Pollution 193 (2014) 138e146