Article pubs.acs.org/est
Boreal Forests Sequester Large Amounts of Mercury over Millennial Time Scales in the Absence of Wildfire Reiner Giesler,*,† Karina E. Clemmensen,‡ David A. Wardle,§,∥ Jonatan Klaminder,⊥ and Richard Bindler⊥ †
Climate Impacts Research Centre, Department of Ecology and Environmental Science, Umeå University, 981 07 Abisko, Sweden Department of Forest Mycology and Plant Pathology, Swedish University of Agricultural Sciences, 750 07 Uppsala, Sweden § Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, 901 83 Umeå, Sweden ∥ Asian School of the Environment, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 ⊥ Department of Ecology and Environmental Science, Umeå University, 901 87 Umeå, Sweden ‡
S Supporting Information *
ABSTRACT: Alterations in fire activity due to climate change and fire suppression may have profound effects on the balance between storage and release of carbon (C) and associated volatile elements. Stored soil mercury (Hg) is known to volatilize due to wildfires and this could substantially affect the landair exchange of Hg; conversely the absence of fires and human disturbance may increase the time period over which Hg is sequestered. Here we show for a wildfire chronosequence spanning over more than 5000 years in boreal forest in northern Sweden that belowground inventories of total Hg are strongly related to soil humus C accumulation (R2 = 0.94, p < 0.001). Our data clearly show that northern boreal forest soils have a strong sink capacity for Hg, and indicate that the sequestered Hg is bound in soil organic matter pools accumulating over millennia. Our results also suggest that more than half of the Hg stock in the sites with the longest time since fire originates from deposition predating the onset of large-scale anthropogenic emissions. This study emphasizes the importance of boreal forest humus soils for Hg storage and reveals that this pool is likely to persist over millennial time scales in the prolonged absence of fire.
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INTRODUCTION Large stocks of mercury (Hg) reside in soils as a result of current and past sequestration of Hg deposited from the atmosphere.1,2 Re-emission of this reservoir of Hg may have profound effects on the global Hg cycle, with potential consequences for future Hg deposition rates.1,3,4 Recent estimates suggest that a large part of these secondary soil Hg emissions has anthropogenic origin, and that fallout of this Hg may account for up to 60% of present-day atmospheric deposition.5 In northern forest ecosystems, Hg accumulation is specifically associated with the sequestration of carbon (C) in soils.6−12 This belowground Hg accounts for 91−97% of the total forest Hg stock, with only a small proportion residing in the vegetation.4,13 It is estimated that globally, boreal forest ecosystems have sequestered three to nine times as much as the current-day atmospheric Hg pool,4,9 but the long-term stability of this pool remains uncertain. The chronology of atmospheric Hg deposition appears to be preserved in the depth-wise profile of the organic horizon (Ohorizon or mor layer) of undisturbed boreal forest soils in northern Sweden11 (the average depth of which is about 10 cm, (http://www-markinfo.slu.se/eng/soildes/humus/humtjock. html). This horizon has been suggested to function as a “semiarchive” of atmospheric deposition from which limited losses occur over a time scale of decades.14 In boreal forests, a major © 2017 American Chemical Society
cause of Hg losses at decadal to centennial time scales is wildfire, through which the combustion or heating of the soil organic matter (SOM) in the O-horizon causes a pulse release of gaseous Hg from the SOM.15−19 The extent of such losses are highly dependent on fire frequency and intensity, both of which are expected to be enhanced in some regions of the boreal zone due to expected climate warming but reduced in other regions due to deliberate fire suppression.16,20−23 The long-term sink capacity for soil Hg storage in boreal forests is strongly dependent on their fire history and we anticipate that sites that burn only occasionally would accumulate more Hg than would sites with frequent burning.22 Secondary emissions of Hg may also result from mineralization of SOM, where Hg can evade as Hg (0) as a byproduct of heterotrophic soil respiration.3 Over the short-term this evasion is limited,24 but the role and time-scale for this release, as well as its effect on the long-term storage of Hg, remains uncertain.25 A large part of the soil Hg is bound to SOM, specifically to reduced sulfur groups.26−28 Only a small fraction of the organically bound Hg pool is assumed to be bound within the Received: Revised: Accepted: Published: 2621
December 16, 2016 February 2, 2017 February 3, 2017 February 3, 2017 DOI: 10.1021/acs.est.6b06369 Environ. Sci. Technol. 2017, 51, 2621−2627
Article
Environmental Science & Technology
reflected in the depth-wise distribution of Hg in the humus profiles across the time-since-fire gradient.
faster cycling pools of SOM, and most of it is instead present in more recalcitrant pools that have much longer mean lifetimes.1 The Hg in the recalcitrant pools is protected until it is released either due to reductive Hg emission, soil water runoff, or through wildfires. In Swedish boreal forests, fires have usually occurred with return intervals of between 50 and 400 yrs, at least in the absence of active fire suppression.29−32 A study from a boreal forest fire chronosequence in northern Sweden, spanning over a time range of 5300 yr, has shown that in the absence of fire, humus C accumulates at the rate of 0.005 kg m−2 yr−1.33 As such, the humus soil C stock can be ten times greater for sites that have not burned for millennia relative to those that burn more frequently.34 Deep humus layers that have accumulated over millennia can reveal novel insights about the long-term stability of atmospheric Hg accumulated in the Ohorizon. The Hg that is bound in SOM is assumed to be key to understanding the global Hg exchange between the land and the atmosphere.1 However, there are still large uncertainties regarding the coupling between Hg and soil carbon.25 It has been shown in forest landscape studies that retention of atmospheric derived Hg in soil is tightly linked to the soil’s capacity to store carbon7 and increasing time because fire is a key factor in promoting the sequestration of both C and Hg.4,23 However, a number of factors may confound the relationship between the sequestration of Hg and C over time; for example, the net accumulation of soil Hg may also be affected by temporal variation in atmospheric Hg deposition rates and reemission, and by the frequency and intensity of fire. In this study, we utilize the fire chronosequence in northern Sweden studied by Wardle et al.33−35 to assess the long-term (millennial) stability of atmospherically derived Hg in Ohorizons of boreal soils (Figure 1). The chronosequence
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MATERIAL AND METHODS Study Site. The study was conducted in a boreal forest wildfire chronosequence across a series of 30 forested islands located in two adjacent lakes, Hornavan and Uddjaure (65 ́ 55′N to 66 09′N; 17 43′E to 17 55′E) in northern Sweden.33−35 The close proximity of the islands to each other removes potential confounding variables such as climate (precipitation, temperature), land-use histories or proximity to emission sources. The main disturbance regime on these islands is wildfire by lightning strike; larger islands are struck by lightning more often than smaller islands and have therefore burnt more frequently.33,35 Consistent with previous work on this study system,30,36,37 we classify the 30 islands into three size classes with ten islands in each: large (>1 ha), medium (0.1−1 ha) and small (
