Potential mechanisms for Hg isotope signatures in montane soils

Vertical variations of Hg isotopic composition in topsoils recorded in this study can be explained by isotope fractionation during Hg cycling in the forest ecosystem and/or mixing of Hg from different sources (e.g., atmospheric and geogenic origins). In mountain forest areas such as Mt. Leigong, the steep environmental gradients (e.g., temperature, precipitation and solar radiation) very likely influence the biogeochemical behaviour of Hg and lead to Hg isotope fractionation. The isotope fractionation of Hg in the Mt. Leigong elevation gradient may be a function of multiple physico-chemical processes, such as the evasion of Hg0 from soils, deposition of atmospheric Hg (e.g., precipitation, dry deposition and litterfall) and re-emission of wet-deposition Hg. To the best of our knowledge, the evasion of Hg from soils mainly involves processes such as photo-reduction22, volatilisation24,43 and the microbial reduction of soil Hg37. Generally, all these processes induce typical kinetic MDF values of Hg isotopes and produce Hg0 with significantly lower δ202Hg values than the original Hg2+. The photo-reduction of Hg may lead to the MIF of odd Hg isotopes20,22,44, whereas no significant MIF is recorded to be induced during volatilisation and microbial reduction processes24,37,43. A recent study by Demers et al34. also indicated that photo-reduction, volatilisation and microbial reduction could not be the major processes for the evaded Hg pool in forest areas. In soil humus such as that on Mt. Leigong, Hg binds strongly with thiols and other reduced sulphur groups associated with organic matter9. Soil evasion fluxes in pristine forest areas are generally extremely low because of the high organic matter content, suppression by leaf litter cover and canopy shading34.

On Mt. Leigong, the surface soils have received Hg from geological sources (e.g., weathering) and atmospheric sources (dry and wet deposition). In this study, the Hg levels in two rock samples (0.10 ± 0.02 mg·kg−1, 2 s.d., n = 2) were lower than those in the soil samples. The δ202Hg (−0.89‰ ± 0.04, 2 s.d., n = 2) and the Δ199Hg (−0.02‰ ± 0.04, 2 s.d., n = 2) in the rock samples are consistent with the data from previous studies, as Hg in geogenic material (e.g., mineral deposits45, hydrothermal emissions45 and volcanoes46) generally have δ202Hg values of approximately −0.60‰, with no evidence of a significant MIF (Δ199Hg < 0.2‰)19.

Despite the proposed geogenic sources of Hg, atmospheric sources of Hg could also have been incorporated into the organic soils through wet (e.g., precipitation) and dry (e.g., particulate Hg and litterfall) atmospheric Hg deposition. Few studies have focused on the Hg isotope composition of precipitation and direct atmospheric Hg species25,32,33. It is worth noting that all of the described studies have demonstrated a significant MIF of even isotopes (e.g., 200Hg and 204Hg) in precipitation and direct atmospheric Hg samples25,32,33,34. Generally, Hg0 is characterised by negative Δ200Hg values and precipitation (which contains mainly Hg2+ and Hg p ) displays positive Δ200Hg values. In the present study, the absence of any MIF of even Hg isotopes in surface soils could be explained by the mixing of different atmospheric Hg species and precipitation. Alternatively, Chen et al33. suggested that the MIF of even Hg isotopes is likely linked to photo-initiated Hg0 oxidation, being controlled by stratosphere incursion, the presence of aerosols, oxidant intensity, solar irradiation and air mass movement. Several studies have reported near-zero Δ200Hg values in ambient gaseous Hg in the Great Lakes region32, the Arctic25 and near the Wanshan Mercury Mine31, indicating that photo-initiated Hg0 oxidation occurring in certain areas may not induce a significant MIF of even Hg isotopes. The mechanism for the MIF of even Hg isotopes is still unclear33 and further studies are needed.

In the present study, the significant negative MIF of odd Hg isotopes is established as an important feature of our investigated samples (i.e., lichens and surface soils). Two plausible mechanisms that might explain the odd-N MIF in this Hg isotope system include (1) the magnetic isotope effect (MIE)47 and (2) the nuclear volume effect (NVE)48. The Δ199Hg/Δ201Hg ratios of MIF produced by different mechanisms may be diagnostic. According to several recent studies, MIF occurring due to the NVE (e.g., Hg0 evaporation, abiotic dark reduction of Hg2+ and equilibrium Hg2+-thiol complexation) was estimated to result in a Δ199Hg/Δ201Hg ratio of 1.5 to 2.020,23,24. The MIE has been documented during photochemical reactions of aqueous Hg species (e.g., MeHg and Hg2+). When Δ199Hg and Δ201Hg values are plotted for each of these photochemical processes, CH 3 Hg+ and Hg2+ photo-reduction have slopes of 1.36 and 1.00, respectively22. The sign of MIF produced by MIE is dependent upon the type of organic ligand involved20,22,44. As shown in Fig. 3, the negative Δ199Hg values of the surface soils from Mt. Leigong indicate a deficit of odd isotopes, which is in close agreement with the direct/indirect air samples and surface soils from other regions in the world23. The slope of approximately 0.98 that was obtained here for Δ201Hg/Δ199Hg (which is not compatible with the NVE) indicates that a portion of Hg in the soil samples in this region may have undergone photo-reduction processes before being stored in continental pristine soils.

Potential mechanisms for Hg magnification in montane soils

Whether the levels of Hg, a typical volatile pollutant, are increased in the montane soils at higher elevations in colder zones depends primarily on the distinction of the relationship between the ‘source’ and the ‘sink’ of Hg and the corresponding geochemical processes that are influenced by the elevation difference. A suite of controls that may cause the preferential accumulation of Hg at higher altitudes in the investigated montane area are depicted in Fig. 5 and are addressed in the following discussion.

Figure 5 Illustration of potential mechanisms for mercury magnification in montane soils (by Hua Zhang and Jonas Sommar). Full size image

(1). Litterfall. Litterfall is a critical Hg input to mountain forest ecosystems in autumn, when deciduous trees enter dormancy and their leaves senesce17. Hg0 flux measurements over deciduous forest ecosystems have indicated growing seasonal patterns from significant net deposition following leafing to net emission towards the end of the foliar season49,50. This finding primarily reflects the assimilation of Hg0 by the foliage via the stomata and cuticle over time. Furthermore, Hg tends to become enriched in forest litter compared with aboveground fresh foliage (50%–800%)17,18. Dry deposition (litterfall) can account for 40% and 80% of the total Hg mass entrainment in the forest soil in winter and spring, respectively17,18. In a previous communication10, we reported that Hg depositional fluxes in the summit zone of Mt. Leigong were, on a yearly basis, substantially dominated by litterfall with minor contributions from precipitation and throughfall (39.5, 6.1 and 10.5 μg.m−2.yr−1, respectively).

Moreover, the foliage/air partition coefficient increases with lower temperatures and higher elevations51 (i.e., plant leaves may retain more atmospheric Hg at higher elevations). In a broad survey of background US forests, Obrist et al. identified latitude, in addition to factors such as precipitation, as a suitable predictor of the Hg burden in litter17. The foliar uptake of atmospheric Hg mediated by intercepting cloud/fog water may also constitute a viable pathway for Hg accumulation. Although a firm conclusion is inhibited by the scarcity of observations, the Hg concentration in clouds and fogs has been observed to be elevated compared with that in precipitation8,52, especially in persisting and stationary fog52. The positive correlation (r2 = 0.33, p < 0.01; Fig. S3) between the Hg concentrations in leaf litter samples and elevation may reflect increased foliage uptake promoted by lower temperatures and extensive cloud water contact at higher elevations.

In the canopy flora of Mt. Leigong, there is a transition (1400–1800 m, evergreen coniferous species mixed with broad-leaved deciduous) from a domination of evergreen coniferous species in the foothills (<1400 m) to broad-leaved deciduous forests in the higher elevation zones (>1800 m)11,12 (Fig. 5), which suggests an increase in areal litterfall mass at higher elevations. Therefore, the higher litterfall mass combined with the higher Hg concentrations in the leaf litter at high elevations described above further suggest that litterfall decomposition may play an important role in the amplification of soil Hg on Mt. Leigong.

(2). Temperature. A certain proportion of the Hg deposited into the forests is in labile forms that can be re-emitted (as Hg0 after reduction) in direct competition to the process of incorporation with the soil matrix through complexation18. Hg0 emission fluxes from terrestrial surfaces are influenced by the substrate (e.g., soil) temperature7,17,18,53, with high temperatures facilitating Hg0 volatilisation53,54. In addition, the aqueous (photo-)reduction of Hg2+ to Hg0 is facilitated in warmer vegetation zones exposed to more abundant sunlight55. Hence, decreasing temperature at high elevations16 supports a suppression of the Hg0 air–surface exchange, thereby indirectly enhancing the retention of Hg in the soil compartment.

One process affecting certain persistent organic pollutants, termed the ‘grasshopper effect’, may also be engaged in the Hg enrichment process in montane soils. In this process, Hg evaporates from warmer zones in neighbouring lowlands (especially where pollution sources exist), travels through the atmosphere and is deposited in cooler, higher montane zones when the temperature drops. This process can be repeated in ‘hops’ (Fig. 5). However, the repeated mountain ‘hops’ (local transport) may be relatively limited and might insignificantly contribute to Hg enrichment in remote montane soils compared with long-range transport processes1,53. Additionally, a diurnal variation of wind patterns controlled by temperature has previously been proposed51 as an important driver of volatile pollutants in mountain regions. Specifically, more volatile contaminants are carried by upslope winds in warmer daytime temperatures than by downslope winds in cooler night-time temperatures.

(3) Precipitation. Orographic effects driven by temperature at higher elevations would result in greater Hg wet deposition due to higher precipitation relative to neighbouring lowlands8. Atmospheric hydrometeors (rain drops and snow) are very efficient scavengers of aerosol particles and ionic Hg species8. In the context of quantifying atmospheric wet deposition processes, the ratio of a chemical's concentration in precipitation to its concentration in the air is known as the scavenging ratio, W. The W values reported in the literature for Hg-p span a large range from 300 to 150056. Compared with Hg0 and Hg-p, GOM exhibits greater dry deposition velocities57. Even without experimental evidence, the W value of GOM applied in models is usually treated as that of an acidic gas (e.g., HNO 3 ). Simple theoretical considerations have indicated that W is a function of inverse temperature. Drevnick et al. (2010)13 reported a significant increase in W with increasing altitude in the western U.S., which is consistent with our recent observations in southwestern China58. A recent study by Huang and Gustin (2012)7 also indicated higher levels of Hg wet deposition at sites with higher elevations.

On Mt. Leigong, precipitation increases with the altitudinal decrease of temperature (detailed in the Supporting Information)11,12. Significantly positive correlations between the soil Hg levels and precipitation or the inverse of temperature (r2 = 0.67–0.69, p < 0.01 for both) (Fig. S7) were observed in the present study and these relationships may be indicative of the enhanced retention/deposition of Hg in high-elevation soils due to the temperature- and precipitation-related mechanisms described above. However, other mechanisms may also contribute to the altitudinal enrichment processes. For example, solar radiation has been suggested as a significant factor controlling the Hg flux between the soil and atmosphere59. Solar radiation decays with rising elevation on Mt. Leigong26 and in most alpine regions (due to increased cloud cover and an increased number of rainy days)16, thereby limiting direct photolytic degradation1, which, in turn, reduces the Hg emissions from the land surface to the atmosphere.

On Mt. Leigong, the temperature and solar radiation decrease and the precipitation, fog/cloud and air humidity increase with increasing elevation11,12,26. Hence, re-emission (the ‘grasshopper effect’) produces a negative influence on the sequestration of atmospheric Hg and decreases the Hg concentration, especially at low elevations. In contrast, scavenging of atmospheric Hg by precipitation and enhanced litterfall provoke increases in the soil Hg concentration at higher altitudes. These processes may be the main reasons that explain the increased Hg concentrations in soil samples with rising elevation.

Implications for regional or global Hg cycling

A negative elevational dependence was observed in the MDF and MIF signatures of Hg isotopes. The application of a MIF (Δ199Hg) binary mixing approach and the traditional inert element method unanimously indicated that the fraction of Hg derived from the atmosphere distinctly increased with altitude. Our study, for the first time, demonstrates that a ‘mountain trapping effect’ of semi-volatile Hg can occurs in montane environments and provides a systematic discussion of the possible mechanisms. Mercury magnification in high-elevation montane soils is likely driven by the altitudinal dependence of temperature, precipitation, litterfall and other factors (e.g., solar radiation), Of these factors, litterfall may be the most critical. Our observations infer that previous studies on regional or global Hg cycles/distribution may have significantly underestimated the Hg mass trapped by mountainous regions, as mountains account for a significant proportion of the global terrestrial area. Our study shows that Hg stable isotope ratios can be used to track atmospheric Hg deposition in upland forest systems. This technique may be useful in future studies that assess environmental changes in montane forest ecosystems.