Process factors driving dynamic exchange of elemental mercury vapor over soil in broadleaf forest ecosystems

Atmospheric Environment 219 (2019) 117047

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Atmospheric Environment journal homepage: http://www.elsevier.com/locate/atmosenv

Process factors driving dynamic exchange of elemental mercury vapor over soil in broadleaf forest ecosystems Wei Yuan a, d, Xun Wang a, Che-Jen Lin a, b, Jonas Sommar a, Zhiyun Lu c, Xinbin Feng a, d, e, * a

State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, 550081, China Center for Advances in Water and Air Quality, Lamar University, Beaumont, TX, 77710, United States c National Forest Ecosystem Research Station at Ailaoshan, Yunnan, 676209, China d University of Chinese Academy of Sciences, Beijing, 100049, China e Center for Excellence in Quaternary Science and Global Change, Chinese Academy of Sciences, Xian, 710061, China b

A R T I C L E I N F O

A B S T R A C T

Keywords: Air–soil mercury exchange Subtropical evergreen broadleaf forest Atmospheric Hg sink

A data gap in the evergreen broadleaf (EB) forest ecosystems has resulted in large uncertainties in estimating the quantity of global air–soil exchange of elemental mercury vapor (Hg0). In this study, we systematically measured the soil pore gas Hg0 concentration, air–soil Hg0 exchange flux and associated environmental parameters to elucidate the process factors driving the air–soil Hg0 exchange in the EB forest ecosystem. The observed air–soil Hg0 exchange flux shows evasion during summer and deposition during winter and indicates that the forest floor is a net atmospheric Hg0 source with an annual flux of þ6.7 � 20.5 μg m 2 yr 1. Structural equation modeling infers that temperature is the most important driver causing the air–soil Hg0 exchange, followed by atmospheric Hg0 concentration. Combined with air–foliage exchange data reported in Yuan et al. (2019) (https://doi.org/1 0.1021/acs.est.8b04865), the EB forest ecosystem emerges as an atmospheric Hg0 sink with a net flux of 20.1 � 24.1 μg m 2 yr 1. Using data documenting global air–soil Hg0 exchange flux in forest ecosystems, we estimate Hg0 emission from the EB forest floor to be 347 � 384 Mg yr 1, nearly 2 times greater than that from the boreal/temperate forest floor, highlighting the importance of EB forest ecosystems in the global Hg biogeo­ chemical cycle.

1. Introduction The Minamata Convention, aiming to protect the ecological environ­ ment and human health from mercury (Hg) pollution, occurred on August 16, 2017. This legally binding treaty decreases Hg release into the environment from anthropogenic sources that subsequently enters terrestrial and marine ecosystems through atmospheric Hg0 deposition through foliage uptake and oxidation to HgII that deposits via precipi­ tation or particulates (Obrist et al., 2018; Pirrone et al., 2008). Global terrestrial ecosystems remove gaseous elemental mercury vapor (Hg0) through atmospheric uptake by foliage (Jiskra et al., 2018; Yuan et al., 2019), and then accumulate Hg on the forest floor via litterfall, which captures 1000–1200 Mg yr 1 globally (Obrist, 2007; Wang et al., 2016b) and amounts to 50–60% of the anthropogenic Hg emission quantity of ~2000 Mg yr 1 (Lindberg et al., 2007; Pirrone et al., 2010; Streets et al., 2017). Given the large amount of anthropogenic Hg sequestered in forest soils through atmospheric transport, foliage accumulation, and

litterfall deposition, a better understanding of the fate of deposited Hg in forest ecosystems is critical to understand the global Hg biogeochemical cycle. Hg sequestered in forest soil has been regarded as the primary source of methylmercury found in surface runoff that enters aquatic ecosystems (Lindberg et al., 2007). Because Hg has a strong tendency to form a complex with organic matter in forest surface soil, only a small amount (<10%) of deposited Hg can be transported into aquatic ecosystems (Larssen et al., 2008; Scherbatskoy et al., 1998; Schwesig and Matzner, 2000; St Louis et al., 2001; Wang et al., 2009). Deposited Hg accumu­ lated in forest soils can be reduced to Hg0 and then re-emitted back to the atmosphere through photochemical, microbial, or abiotic reduction (Driscoll et al., 2013). Presently, significant knowledge gaps remain in the Hg0 re-emission processes from forest soil, making it difficult to confidently quantify the net Hg0 exchange flux between forest ecosys­ tems and the atmosphere (Agnan et al., 2016; Zhu et al., 2016). Air–soil Hg0 exchange is a dynamic bi-directional process (Bash

* Corresponding author. State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, 550081, China. E-mail address: [email protected] (X. Feng). https://doi.org/10.1016/j.atmosenv.2019.117047 Received 5 May 2019; Received in revised form 6 October 2019; Accepted 9 October 2019 Available online 10 October 2019 1352-2310/© 2019 Elsevier Ltd. All rights reserved.

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Atmospheric Environment 219 (2019) 117047

et al., 2004; Wang et al., 2014; Zhang et al., 2001). Environmental factors such as irradiance, soil temperature, soil Hg content, soil hu­ midity, etc. have been demonstrated to influence Hg0 re-emission from forest soil (Gustin et al., 2006; Obrist et al., 2014; Sigler and Lee, 2006). In particular, air–soil Hg0 exchange flux data in tropical/subtropical forest ecosystems are relatively limited (Carpi et al., 2014; Ericksen et al., 2006; Fu et al., 2008, 2012; Kuiken et al., 2008b). Data collected in temperate/boreal forest ecosystems have suggested that physico­ chemical properties and near-surface atmospheric characteristics are the main factors controlling the Hg0 exchange processes (Ericksen et al., 2006; Lindberg et al., 1998; Zhang et al., 2001). In tropical/subtropical evergreen broadleaf (EB) forest ecosystems, the physical and chemical properties of soil are distinct from those found in temperate/boreal forests and therefore can influence the Hg0 exchange processes. For example, the reported Hg0 concentration in soil pore gas ranges from 0.3 to 0.6 ng m 3 in temperate forests but can be as high as 69.6 � 5.8 ng m 3 in subtropical EB forests (Fu et al., 2012; Obrist et al., 2014). The processes responsible for the observed pore gas Hg0 con­ centrations are poorly understood and deserve research attention. In this study, we systematically analyzed the process factors driving the air–soil Hg0 exchange on the forest floor based on comprehensive measurements of Hg0 fluxes, environmental factors, and Hg0 concen­ tration in pore gas in a sub-tropical EB forest ecosystem. Based on ob­ servations, implications regarding how future global climate warming could influence the air–soil Hg0 flux exchange are also discussed.

2. Materials and methods 2.1. Site description The study was conducted in the experimental area of the Ailaoshan Station for Subtropical Forest Ecosystem Research Studies (ASSFERS; 24� 320 N, 101� 010 E, 2476 m in elevation), Yunnan Province, China. The site has a sub-tropical climate with an annual mean temperature and a rainfall of 11.3 � C and 1840 mm, respectively (Tan et al., 2011). The Indian and East Asian monsoons result in abundant rainfall during the period June–October, while the Westerly induces a pronounced dry season during the period November–May. The canopy level is domi­ nated by old-growth (stand age > 300-year) and evergreen beech species (canopy coverage � 85%). The dominant tree species are Lithocarpus xylocarpus (LX), Castanopsis wattii (CW) and Schima noronhae (SN), ac­ counting for >70% of forest trees in the study area. The height of the forest canopy is 22–25 m. The site has few shrubs but a dense herbaceous vegetation cover beneath the forest canopy, with a grass coverage of 60 � 23% in the 50 sample plots of 2 � 2 m. As a site within the Global Mercury Observation System (GMOS) network, atmospheric Hg0 has been continuously measured at ASSFERS since 2011 (Sprovieri et al., 2016), with an annual mean of 1.4 � 0.2 ng m 3 measured in 2014. In this study, some atmospheric Hg0 concentration data during experiment periods in 2017 was also used in discussion. The annual atmospheric Hg0 concentration under the canopy during experiment periods is 1.10 � 0.11 ng m 3 for dry season

Fig. 1. Site location at Mt. Ailao. Experimental platform is for placing instruments, such as the Tekran 2537X etc. 2

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2.3. Measurement of the Hg0 concentration in soil pore gas and air–soil Hg0 exchange flux

(7th-16th, January, 2017) and 1.60 � 0.40 for rainy season (4th-18th, August, 2017), respectively. In this study, a 30 m � 30 m experimental area was prepared and divided into two sub-plots (A and B) at ASSFERS (Fig. 1) for measuring the Hg0 concentration in soil pore gas (A) and air–soil Hg0 exchange flux (B).

The Hg0 concentration in the soil pore gas was measured at sub-site A (Fig. 1) during January, March in 2018 for dry season and July in 2019 for rainy season. Continuous measurement was made for 72 h during January (n ¼ 3) and for 96 h during March and July (n ¼ 6, Fig. 2A). A Teflon tube with a 1/4-in outer diameter was inserted into a centrifuge tube, which was modified with 50 1.5-mm diameter holes on one side. The modified centrifuge tubes were horizontally inserted into the un­ disturbed 10-cm-deep soil profile. The horizontal distance between the two centrifuge tubes was 50–60 cm. Five centrifuge tubes were divided into one group and connected through two Teflon tees. The front end of the Teflon tube was connected by a silicone hose to a 0.22-μm pore size and 13-mm-diameter polytetrafluoroethylene filter, then to a Tekran Model 1115, and finally to a Tekran 2537X. During each 10-min sam­ pling cycle of the Tekran 2337X, 5 L of gas was pumped into a Tekran 2537X at a flow rate 0.5 L min 1. Thus, on average, 1 L of pore gas was collected from each centrifuge tube per cycle. Overall, three replicate measurements during January led to a 30-min manifold switching in­ terval and a 60-min interval for six replicates during March. A sketch of the setup and gas flow parameters is shown in Fig. S1A. Because the Hg0 concentration soil pore gas showed somewhat large data variability using 30-min manifold switching interval during January experiments in 2018 (Fig. 3A), a 60-min interval with six replicates was applied in later experiments during dry and rainy seasons to improve data

2.2. Soil sampling and basic properties measurement The soil in the Mt. Ailao forest is dominated by alfisols, with organic carbon content of 15.0–43.2% in 0–10 cm soil using MACRO cube analyzer (Elementar, Germany, detection limit: 10 ppm), and a pH value of 3.6–4.0, consistent with the reported pH of 3.5–4.8 (Yang and Yang, 2011) in the 0–10-cm soil profile. Field meteorological parameters including air temperature, air humidity, photosynthetically active ra­ diation (PAR), temperature, and water content at 5 and 10 cm depth were measured using a HOBO U30 automatic probe. Six surface soil samples were collected in 0–10 cm at the end of dry season (May) and rainy season (November) during 2017 using methods described else­ where (Lu et al., 2016; Zhou et al., 2013). The collected soil samples were dried in a ventilated oven at 45 � C for 96 h and then ground with agate mortar and sieved through a 200-mesh (74-μm) nylon screen. Total Hg concentration was measured using a Lumex RA-915þ (Lumex Ltd, Russia, detection limit: 1 ng g 1). Standard reference materials were measured in every 10 samples, yielding recoveries of 95–103%. GBW07405 (GSS-5, 290 � 40 ng g 1) was used as the soil Hg standard.

Fig. 2. A. Setup of soil pore gas Hg0 concentration measurements in sub-plot A, B. Setup of air-soil Hg0 exchange flux measurements carried out in sub-plot B. 3

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consistency. Three replicate measurements were randomly configured under the canopy of the three dominated tree species at sub-site B for quantifying the Hg0 exchange flux over the soil, termed nearby LX, nearby CW, and nearby SN (Fig. 2B). Each flux measurement occurred over a 4–5-day period during spring, summer, autumn, and winter using the novel dy­ namic flux chamber (NDFC) method described in our earlier work (Lin et al., 2012; Zhu et al., 2015, 2016). Briefly, the NDFC inlet and outlet were connected to the synchronized multiport sampling system (Tekran Model 1115) using the Teflon tube with the 1/4-in outer diameter and then connected to the automated ambient air analyzer (Tekran Model 2537X) for measuring the ambient air and efflux air Hg0 concentration, respectively. A sketch of the setup and gas flow parameters is shown in Fig. S1B. All flow rates were converted to rates under standard tem­ perature and pressure conditions (0 � C and 1013 hPa). Finally, the Hg0 flux was calculated as follows:

FHg0 ¼

Q ⋅ðCout Schamber

Cin Þ

(1)

where FHg0 is the Hg0 flux (ng m 2 h 1); Q is the NDFC internal flushing flow rate (m3 h 1); Schamber is the NDFC footprint area (m2); and Cout and Cin are the Hg0 concentration at the NDFC outlet and inlet, respectively. In this study, Schamber was equal to 0.09 m2. A positive FHg0 indicates Hg0 emission from the forest floor to atmosphere, while a negative FHg0 in­ dicates atmospheric Hg0 deposition onto the forest floor.

2.4. Quality assurance/quality control and data analysis The filter in the tubing outlet was changed every 5 days aiming to avoid cross-contamination. The filter and soda-lime scrubber in the inlet of the Tekran 2537X were changed every 12–15 days during the experimental periods. The Tekran 2537X was calibrated before each

Fig. 3. Variations of soil pore gas Hg0 concentration: (A.) in January, 2018, (B.) in March, 2018 and (C.) in July, 2019. 4

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atmospheric Hg0 deposition (Jiskra et al., 2017; Woerndle et al., 2018). The Hg0 concentrations in the soil pore gas during both seasons show no discernible diurnal patterns, consistent with results of a previous report (Obrist et al., 2014). The levels are 3–9 times higher than the concen­ tration in the ambient air (~1.4–1.5 ng m 3), potentially driving an evasion flux. The pore gas Hg0 concentration in this study is one order of magni­ tude smaller than the reported concentrations in the subtropical EB forest soil of Mt. Dinghu, Guangdong, China (up to 69.6 � 5.8 ng m 3) (Fu et al., 2012), but 2–10 times greater than values of the temperate forest (Moore and Castro, 2012; Obrist et al., 2014; Sigler and Lee, 2006) and Arctic tundra (Obrist et al., 2017) soils in the US (0.3–1.3 ng m 3). The higher atmospheric Hg0 concentration at Mt. Dinghu (5.5 � 0.9 ng m 3) is possibly the main reason for the distinctly higher soil pore gas Hg0.Given the similar atmospheric Hg0 concentration at Mt. Ailao and in the aforementioned temperate forests (1.0–2.0 ng m 3), atmospheric Hg0 is not likely an Hg source in the soil pore gas.

experiment to ensure that the bias between the two gold tubes in the Tekran 2537X was less than 3% and the peak area error of multiple automatic corrections was within 5% during the experimental periods. Because the measurement set-up disturbed the soil profile, the mea­ surements of soil gas Hg0 concentration were conducted after 2 months of soil backfill and stabilization. The blanks of NDFC were reasonably low with 0.6 � 4.1 ng m 2 h 1 and considered negligible (Lin et al., 2012). Meanwhile, the temperature inside the NDFC is typically 2–4 � C higher than the ambient temperature during the day and 1–2 � C during the night, leading to a 5–10% increase in flux (Lin et al., 2012). Correlation analysis, multiple linear regression, and structural equation modeling (SEM) of the collected data were performed using SPSS version 17 (IBM, US) and Amos software version 24. SEM, devel­ oped from a fully conceptual model using χ2 tests with maximum like­ lihood estimation, was conducted to infer the interplay of environmental factors on the air–soil Hg0 exchange flux. We used p-values (at least > 0.05), χ2 values, and Akaike information criterion (AIC) as the criteria for evaluation of the SEM fit. Principal component analysis (PCA) was applied for creating a multivariate functional index to represent temperature and soil water content. We used the first principal components of “air temperature,” “temperature at 5-cm soil depth,” and “temperature at 10-cm soil depth” to represent the factor “temperature.” Similarly, the first principal components of “air humidity,” “water content at 5-cm soil depth,” and “water content at 10-cm soil depth” were suggested as the factors for “soil water content.” All variables were standardized into Z scores to eliminate the influence of dimension as follows: X* ¼

μÞ

ðX δ

3.2. Seasonal and daily patterns in air–soil Hg0 exchange flux “Hg0 flux” in Table 1 shows the daily average of air–soil Hg0 flux during measurement periods carried out at sub-plot B of Mt. Ailao (range: 3.87 to þ11.75 ng m 2 h 1), and the arithmetic mean value of air–soil Hg0 flux is 1.04 ng m 2 h 1. Such flux is consistent with the observations in subtropical forest soil of East Asia, such as at Mt. Dinghu, southeastern China ( 2.2 to 40 ng m 2 h 1) (Fu et al., 2012); Mt. Gongga, southwestern China (0.5–9.3 ng m 2 h 1) (Fu et al., 2008); and Mt. Simian, southwestern China (3.5–8.4 ng m 2 h 1) (Wang et al., 2006). The reported flux in the tropical forest soil of South America is substantially lower (0.33 � 0.09 ng m 2 h 1) (Carpi et al., 2014). The observed flux in this study is also comparable to that observed in tem­ perate/boreal forest ecosystems, such as the 1.1–6.0 ng m 2 h 1 in de­ ciduous forest soil of Mt. Changbai, northeastern China (Fu et al., 2016), and 2 to 4 ng m 2 h 1 in boreal forest soil of Sweden, northern Europe (Lindberg et al., 1998; Xiao et al., 1991). Air–soil Hg0 exchange fluxes under forest canopy are significantly lower than those of bare soil measurements ( 3.7 to 135 ng m 2 h 1, p < 0.05) (Carpi et al., 2014; Fu et al., 2008, 2012; Nacht and Gustin, 2004; Schroeder et al., 2005; Wang et al., 2006) because canopy shading can significantly reduce soil Hg0 evasion (<15 μE m 2 s 1 during winter, summer, and autumn and 30–45 μE m 2 s 1 during spring under the canopy, compared to 110–150 μE m 2 s 1 above canopy). The empirical equations between the air–soil Hg0 flux and environmental parameters via multiple linear regression (MLR) are shown in the Supplemental Information (SI) Sec­ tion 1, which demonstrate that the forest floor of Mt. Ailao is an atmo­ spheric Hg0 source with a 6.7 � 20.5 μg m 2 year 1 Hg0 emission. Fig. 4 shows the seasonal and diurnal patterns of the air–soil Hg0 exchange flux. To represent the flux characteristics at the subtropical forest floor site, replicate measurements were completed at three randomly selected locations. During winter (January), we observed negative air–soil Hg fluxes, i.e. 1.63 � 0.73 ng m 2 h 1, 0.46 � 0.70 ng m 2 h 1, and 1.82 � 0.44 ng m 2 h 1 for the loca­ tions nearby LX, SN, and CW, respectively. This suggests net Hg0 deposition during winter. During summer (August), the air–soil Hg0 flux is positive during all the experimental periods, suggesting net Hg0 emission from the soil. The mean air–soil Hg0 emission flux is þ1.99 � 0.93 ng m 2 h 1, þ2.52 � 0.98 ng m 2 h 1, and þ11.75 � 1.80 ng m 2 h 1 for the locations nearby LX, SN, and CW, respectively. During spring and autumn, the air–soil Hg0 flux is characterized by the transition from deposition to emission or emission to deposition. For example, during spring, the mean soil-air Hg0 exchange flux is 0.97 � 0.95 ng m 2 h 1 nearby LX, þ0.56 � 0.78 ng m 2 h 1 nearby SN, and þ4.84 � 1.26 ng m 2 h 1 nearby CW, showing significantly difference (t-test, p < 0.05). Such spatial heterogeneities in flux may be attributed to the variation in vegetative coverage or species, sunlight, and soil microorganism at each subplot. It is unlikely caused by the variation of soil water content as the

(2)

where X* is the normalization variable value, X is the experimental variable value, μ is the mean of the variable value data, and δ is the standard deviation of the variable value data. From the SEM path network, the standardized path coefficient (β) represents the direct ef­ fect of one variable on another. 3. Results 3.1. Soil pore gas Hg0 concentration The study site locates in subtropical evergreen forest regions, and has distinct dry (November to May) and rainy seasons (June to October) (Wang et al., 2016b). The total Hg content in soils is comparable be­ tween the dry season and rainy season (223.5 � 22.7 ng g 1, n ¼ 6 versus 201.6 � 27.9 ng g 1, n ¼ 6; Pair t-test, p ¼ 0.199). These values are consistent with the earlier reported values at Mt. Ailao (Lu et al., 2016; Zhou et al., 2013). The pore gas Hg0 concentration at 10-cm depth is 5.3 � 2.2 ng m 3 (range: 3.5–8.6 ng m 3, Mean � SD, n ¼ 3, and January) and 7.4 � 3.6 ng m 3 (range: 3.8–16.0 ng m 3, Mean � SD, n ¼ 6, and March) during dry season and 9.9 � 2.6 ng m 3 (range: 5.4–13.3 ng m 3, Mean � SD, n ¼ 6, and July) during rainy season, respectively (Fig. 3). The soil pore gas Hg0 concentration remains at a relatively stable level over 72–96 h with decrease below 15%, suggest­ ing the measurement setup was leak-proof without significant ambient air entering during the measurements. The ~30% elevated Hg0 con­ centration during rainy season can be caused in part or in combination by two reasons. One is the higher soil temperature during rainy season, which can induce a stronger microbial reduction to produce Hg0 (Obrist et al., 2014). The second is a higher soil water content that reduces the soil pore gas storage capacity (Gustin and Stamenkovic, 2005). HgII reduction in soil pore water at the higher temperature during rainy season produces greater amount of Hg0 in soil pore gas. Little evidence was found to support wet deposition as an important source responsible for Hg0 re-emission. Instead, stable Hg isotopes in environmental sam­ ples suggested that HgII in soil pore water and runoff is mainly from 5

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Table 1 Summary of air-soil Hg0 fluxes and meteorological parameters. All results are displayed as Mean � SD. Location

Variable

winter

spring

summer

autumn

nearby Castanopsis watii

Date Soil Hg conc. (ng g 1)* Hg0 flux (ng m 2 h 1) Air Hg0 conc. (ng m 3) Air temp (� C) PAR (μE m 2 s 1)** RH (%)*** WC_10 cm (%)**** Soil temp_10 cm(� C)

7-10, Jan. 2017 – 1.82 � 0.44 0.86 � 0.08 5.47 � 1.22 9.25 � 2.04 87.47 � 4.92 61.01 � 0.55 13.23 � 0.26

30, Apr.-4, May. 2017 244.0 � 0.7 4.84 � 1.26 1.58 � 0.10 13.20 � 1.39 51.58 � 15.39 68.19 � 7.98 57.80 � 0.68 19.23 � 0.29

4-8, Aug.2017 – 11.75 � 1.80 2.23 � 0.51 15.20 � 0.50 9.22 � 4.18 97.16 � 0.94 64.25 � 1.32 23.65 � 0.13

2-7, Nov.2017 219.3 � 5.3 0.45 � 1.84 2.04 � 0.45 6.89 � 2.22 6.21 � 4.02 92.98 � 3.40 59.66 � 0.24 16.17 � 0.62

nearby Lithocarpus xylocarpus

Date Soil Hg conc. (ng g 1)* Hg0 flux (ng m 2 h 1) Air Hg0 conc. (ng m 3) Air temp(� C) PAR (μE m 2 s 1) RH (%) WC_10 cm (%) Soil temp_10 cm(� C)

10-13, Jan. 2017 – 1.63 � 0.73 0.83 � 0.12 6.59 � 0.83 9.92 � 2.35 86.55 � 3.12 59.71 � 0.28 13.87 � 0.14

5-9, May.2017 220.8 � 2.6 0.97 � 0.95 1.64 � 0.13 12.46 � 1.99 46.83 � 23.56 77.04 � 12.13 55.59 � 0.42 19.32 � 0.52

8-12, Aug.2017 – 1.99 � 0.93 1.57 � 0.18 14.20 � 0.45 6.47 � 2.42 97.83 � 0.95 64.53 � 1.76 22.96 � 0.19

7-11, Nov.2017 226.2 � 37.0 3.87 � 2.27 1.70 � 0.45 9.83 � 0.98 5.94 � 3.01 92.25 � 3.31 58.93 � 0.29 16.72 � 0.44

nearby Schima noronhae

Date Soil Hg conc. (ng g 1)* Hg0 flux (ng m 2 h 1) Air Hg0 conc. (ng m 3) Air temp(� C) PAR (μE m 2 s 1) RH (%) WC_10 cm (%) Soil temp_10 cm(� C)

13-16, Jan. 2017 – 0.46 � 0.70 0.88 � 0.09 5.46 � 0.68 11.66 � 0.47 76.36 � 5.06 58.87 � 0.27 13.37 � 0.22

9-13, May.2017 205.7 � 33.0 0.56 � 0.78 1.60 � 0.10 13.92 � 1.58 41.53 � 13.84 85.04 � 7.73 56.67 � 1.96 20.21 � 0.31

12-18, Aug.2017 – 2.52 � 0.98 1.64 � 0.34 14.62 � 0.89 8.78 � 4.31 96.91 � 2.21 62.97 � 1.19 23.08 � 0.23

11-16, Nov.2017 186.5 � 26.2 0.86 � 0.34 1.03 � 0.06 8.18 � 0.79 4.55 � 1.48 89.08 � 3.98 57.35 � 0.58 16.32 � 0.23

* The samples were collected on May 8, 2017 (spring) and November 9, 2017 (autumn). ** PAR: photosynthetically active radiation, unit: μE m 2 s 1. *** RH: relative humidity, unit: %. **** WC_10 cm is an abbreviation for water content in 10 cm soil, unit:%.

infer the air–soil Hg0 exchange processes (Fig. 5). It is clear that the atmospheric Hg0 concentration is the only dominant factor driving air­ –soil Hg0 exchange flux during the winter period (Fig. 5) because the atmospheric Hg0 concentration has the highest direct negative feedback on the flux (direct effect: 0.90 to 0.47). The atmospheric Hg0 con­ centration is also significant during autumn (direct effect: 0.87 to 0.46). This suggests that a lower atmospheric Hg0 concentration re­ duces the gross Hg0 depositional flux by reducing the atmospheric Hg0 diffusion at the interfacial surfaces (Xin and Gustin, 2007). Temperature has a significant positive effect on soil Hg0 emission during autumn, summer, and spring (direct effect: 0.52–0.93, except one value of 0.15) (Fig. 5), which is consistent with the effect of temperature on air–soil Hg0 exchange flux diurnal patterns (Fig. 4). Temperature enhances soil Hg0 flux by increasing the HgII microbial reduction rate (Giovanella et al., 2016) and Hg0 diffusion from soil to the near-surface air (Sigler and Lee, 2006). Previous studies in forest ecosystems sug­ gested that temperature was among the predominant factors deter­ mining the air–soil Hg0 flux (Han et al., 2016; Lindberg et al., 1998; Zhang et al., 2001). During the experiment periods, the direct effect from PAR was relatively small (<0.53), suggesting sunlight does not strongly increase the Hg0 flux under the canopy because of the shading effect. The PAR intensity under the canopy of forest at the study site is typically less than 15 μE m 2 s 1 during winter, summer, and autumn and only 30–45 μE m 2 s 1 during spring; however, the average PAR without a canopy is 110–150 μE m 2 s 1. Because of the consistently high yet nonvarying soil humidity in evergreen forests, the effect of a soil water content from 55% to 65% does not significantly alter the flux (<0.4). In summary, atmospheric Hg0 concentration and temperature are the most important factors that modify air–soil Hg0 flux in the subtropical EB forests of Mt. Ailao. The soil pore Hg0 is an important link to understanding bi-directional air–soil Hg0 flux in forest ecosystems (Obrist et al., 2014). The Hg0 concentrations in soil pore gas are 3–10 times greater than those in the

three sites has a similar soil water content. Except during winter, we observed obvious diurnal patterns for all flux measurements. The lowest air–soil Hg0 flux was observed from midnight to sunrise. Air–soil Hg0 flux increases with soil temperature during the day and reaches its highest value at noon (Fig. 4). Such air–soil Hg flux diurnal patterns are consistent with previous observations in forest ecosystems (Choi and Holsen, 2009; Ericksen et al., 2006; Fu et al., 2012; Poissant and Casimir, 1998). 4. Discussion 4.1. Factors driving the air–soil Hg0 exchange flux Air–soil Hg0 exchange flux is the net sum of the (1) Hg0 release rate from the soil to atmosphere (including reduction, desorption, and diffusion processes at the soil–air interface) and (2) atmospheric Hg0 deposition rate at the soil surface (Nacht and Gustin, 2004; Wang et al., 2014; Zhou et al., 2017). The aforementioned processes can be influ­ enced by meteorological parameters (soil/air temperature, air humidity, PAR, atmospheric turbulence, etc.), soil physical-chemical characteris­ tics (moisture, porosity, organic matter content, Hg content, microbial activity, pH, pore Hg0, etc.), and surface atmospheric characteristics (Hg0 concentration and oxidizing gas concentration, such as that of O3) (Zhu et al., 2016). Significant correlations were found among environmental factors using principal component analysis (PCA), such as air temperature, temperature at 5-cm soil depth versus temperature at 10-cm soil depth, and similar to air humidity, water content at 5-cm soil depth versus water content at 10-cm soil depth. Moreover, there are significant cor­ relations among temperature, humidity, and PAR (e.g., r ¼ 0.44–0.75 between temperature and PAR in this study as shown in Table S1). The effects of these environmental factors on Hg0 flux are confounded (Lin et al., 2012). Considering the synergistic effect from multiple factors, we applied SEM using the comprehensive datasets collected in this work to 6

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Fig. 4. Diurnal variations of air-soil Hg0 exchange flux and environmental parameters. The column represents the mean Hg0 exchange flux, and the pink line is the soil temperature, and the yellow line is the air Hg0 concentration. The error bar is �1standard deviation of multiple day observations. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

air under forest canopy (~1.4–1.5 ng m 3) in both dry and rainy seasons during our experiment periods. Such soil pore Hg0 concentration is significantly higher than those in two temperate forests in U.S.(Obrist et al., 2014). The observed soil pore Hg0 concentration difference be­ tween the US and Mt. Ailao sites can be attributed to two other factors. One is that the soil Hg concentrations at the Chinese forest sites are significantly higher than those in the two temperate forests (120–260 ng g 1 versus 20–50 ng g 1). The other is that the higher Hg2þ reduction rate in the soil of subtropical EB forest ecosystems (Wang et al., 2016b). Coincidently, as a subtropical evergreen broadleaf forest, the soil in the Mt. Ailao forest site is dominated by alfisols, with a higher organic carbon content in the 0–10 cm soil of 15–43% than the reported carbon content in temperate forest of ~3%, dominated by sand (Obrist et al., 2014). It has been shown that both microbial and organic re­ ductions in Hg2þ in soil and Hg0 absorption are highly linked to soil C mineralization; a higher C content in soil can induce greater Hg2þ re­ ductions (Obrist et al., 2014; Wang et al., 2016b; Zheng et al., 2019). The warmer and wetter conditions in subtropical soil further enhance the C

and Hg mineralization processes (Wang et al., 2016b). The higher soil pore Hg0 concentration during rainy season, which is ~30% higher than the values found in dry season, caused the elevated emission flux during air–soil Hg0 exchange. Given the porous soil structure, the concentration gradient of gaseous Hg0 drives the diffusion process between air–soil surfaces. Interestingly, although Hg0 concen­ trations in soil pore gas were 3–10 times greater than those in ambient air under forest canopy (~1.4–1.5 ng m 3), atmospheric Hg0 deposition on soil clearly occurred despite the concentration gradient favoring Hg0 evasion from soil to air, especially in winter. It is likely that the large specific surface area and strong Hg affinity of organic soil constrains Hg0 diffusion in the soil layer. Briggs and Gustin (2013) suggested that Hg0 flux is controlled by meteorological parameters (i.e. temperature and light) and soil water content. Because of negligible effects from PAR and soil water content at this site, temperature is most likely the dominant factor on the EB forest floor with weak irradiance and saturated air moisture. Other than temperature, spatial heterogeneity of soil proper­ ties, including microorganism, litterfall coverage and foliage root 7

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Atmospheric Environment 219 (2019) 117047

Fig. 5. Interplays of environmental factors on air-soil Hg0 exchange flux obtained by structural equation model (SEM).

Hg0 exchange are temperature, soil water evaporative stage, followed by atmospheric Hg0 concentration. Air–soil Hg0 exchange can result in feedback to the atmospheric Hg0 concentration. During winter, the observed near-surface atmospheric Hg0 concentration was 0.86 � 0.13 ng m 3, significantly less than the 1.10 � 0.11 ng m 3 Hg0 concentration under the forest canopy (p < 0.05, t-test), indicating active removal of atmospheric Hg0 via deposition. During summer, photoreduction in soil and diffusion of soil pore gas have an effect on the emission process, leading to a significantly higher Hg0 concentration near the soil surface (1.81 � 0.46 ng m 3 compared to 1.60 � 0.40 ng m 3 under the forest canopy, p < 0.05, ttest). Considered that HgII deposition through precipitation on forest floor can be easily reduced, the elevated near-surface atmospheric Hg0 concentration during summer periods may also be partially contributed by the reduction of wet HgII deposition.

distribution (Grigal, 2003; Zhu et al., 2016) is the main cause of for the flux data variability observed at different sites. Fig. 6 shows a conceptual model of the Hg0 air–soil exchange process on the EB forest floor. During winter, the lower temperature leads to a soil under the energy-limiting stage, thus soil pore Hg0 is not conducive for evading from the soil given the strong affinity to Hg0 of soil materials (e.g., soil organic carbon and iron manganese minerals). Therefore, at­ mospheric Hg0 concentration controls the magnitude of the depositional flux. This also explains why the air–soil Hg0 exchange flux does not show diurnal variations because diurnal cycles of atmospheric Hg0 concen­ tration are absent. During the warmer season, the higher temperature increases Hg0 diffusion and Hg2þ reduction rates in the soil, thus resulting in a Hg0 emission that offsets the atmospheric Hg0 deposition. This is supported by the regression equation showing the Hg0 flux versus the inverse of temperature based on an Arrhenius equation (Fig. 7). The evaluated temperature nonlinearly promotes Hg0 evasion. We applied the equation to estimate air–soil Hg0 flux in EB forests under a scenario of a warming climate. For an increase in temperature of 1.5 � C–2.0 � C, the air–soil net Hg0 flux increased to 2.00–2.32 ng m 2 h 1 compared to the field result (1.04 ng m 2 h 1) annually. We further verified this predominance of temperature in driving the air–soil Hg0 exchange in forest ecosystems using reported data observed at global forest sites (Table 2). It was found that, for forest ecosystems, the factors driving

4.2. Hg0 exchange flux between the forest and atmosphere in the subtropical forest at Mt. Ailao The air–foliage Hg0 exchange flux at the Mt. Ailao EB forest was assessed in our previous work (Yuan et al., 2019). We estimated the foliar Hg0 exchange flux as 3.7 � 0.3 ng m 2 h 1 during the cold (winter–spring) season and 7.7 � 1.0 ng m 2 h 1 during the warm 8

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Atmospheric Environment 219 (2019) 117047

Fig. 6. Conceptual model of air-soil Hg0 exchange flux in subtropical EB forests. The foliage-air Hg0 flux result is from Yuan et al. (2019). A. and B. represent the cold and warm season model result, respectively. The arrow with blue color represents the gross atmospheric Hg deposition process, while the arrow with brown color represents the soil Hg reemission process. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

forest at Mt. Changbai, the total Hg0 flux is 4.9 ng m 2 h 1 from foliage and þ2.8 ng m 2 h 1 from forest soil during the growing season, resulting in a net sink of 2.1 ng m 2 h 1 (Fu et al., 2016). Vegetation is a known sink of atmospheric Hg0 with 0.12 ng m 2 h 1 ( 0.47 ng m 2 h 1 – þ0.26 ng m 2 h 1) deposition in global forest ecosystems, controlling on seasonal variation in global atmospheric mercury concentrations (Agnan et al., 2016; Ericksen and Gustin, 2004; Jiskra et al., 2018; Ms et al., 2003). In contrast, forest soil is a source of atmospheric Hg0 with þ0.48 ng m 2 h 1 evasion (0.31–0.79 ng m 2 h 1) (Agnan et al., 2016; Gustin et al., 2004). The net Hg0 exchange flux between forest and atmosphere is 59 Mg year 1( 727 to 703 Mg year 1) across all forest ecosystems in worldwide estimates (Agnan et al., 2016), though great uncertainties remain. These un­ certainties include differences in measurement methods (Zhu et al., 2016), variation caused by vegetation types (Fu et al., 2016; Millhollen et al., 2006), and a lack of a consistent methodology to estimate the exchange flux over all the forest ecosystems. There is limited data availability of Hg0 exchange flux in tropical and subtropical forest ecosystems; therefore, more Hg0 flux data and mass balance assessments are needed at sites in tropical and subtropical EB forest ecosystems. Forest soil is an atmospheric Hg0 source during the warm season and a sink during the cold season. The annual Hg0 flux is estimated to be þ6.71 � 20.49 μg m 2 yr 1 at our study site. However, the whole forest ecosystem is an atmospheric Hg0 sink with a net deposition of 20.1 � 24.1 μg m 2 yr 1. Temperature and atmospheric Hg0 concen­ trations are the predominant factors governing the air–soil Hg0 ex­ change flux in subtropical forests. Given temperature is a governing factor shaping the variation in the air–soil Hg0 flux globally, the ongoing global warming trend may enhance the air–soil Hg0 exchange flux in forest ecosystems. Based on the estimate in this work, a small temper­ ature increase may double the Hg0 evasion flux from soil. Considered the increasing temperature under global warming and decreasing atmo­ spheric Hg0 concentration through the observation results in Asia, Europe and North American (Tang et al., 2018; Zhang et al., 2016), both tendencies may enhance the legacy Hg emission from forest soil globally under the changing climate. The climate-induced feedback on atmo­ spheric Hg0 concentration can possibly offset the positive effects of reducing anthropogenic Hg emissions mentioned by the Minamata

Fig. 7. Scatter plot of air-soil Hg0 exchange flux versus reciprocal of Kelvin temperature in subtropical EB forests. The regression equation is built on all the result regardless of locations and seasons. In this regression equation, y0 equals to 21.80 � 25.80, a equals to 54886.37 � 424208.00 and b equals to 2209.47 � 2515.84.

(summer–autumn) season (Yuan et al., 2019) (Fig. 6). Combined with the soil–air Hg0 flux in this study, we estimate the Hg0 exchange flux between the atmosphere and the whole EB forest ecosystem to be 5.0 � 0.6 ng m 2 h 1 and 2.3 � 5.6 ng m 2 h 1 during winter and summer, respectively. This amounts to 20.1 � 24.1 μg m 2 year 1 of Hg0 deposition in the forest ecosystem. Subtropical coniferous forest plays as a strong atmospheric Hg0 source at Qianyanzhou, Southeast China (Yu et al., 2018). The Hg0 ex­ change flux between the forest and atmosphere was measured as 5.5 ng m 2 h 1, 5.3 ng m 2 h 1, 8.1 ng m 2 h 1, and 7.9 ng m 2 h 1 during winter, spring, summer, and autumn, respectively, using the aerodynamic gradient method. The main reason for the difference is that the reemission Hg0 flux of 10.9 μg m 2 year 1 from the canopy at Qia­ nyanzhou forest is greater than that at Mt. Ailao (26.8 � 12.7 μg m 2 year 1 deposition) (Luo et al., 2016; Yuan et al., 2019). In the coniferous 9

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Atmospheric Environment 219 (2019) 117047

Table 2 The global documented air-soil Hg0 exchange flux (ng m erning environmental factors.

2

flux measurement data covering a longer time period using the same method should be collected to reduce the uncertainties of the worldwide estimates.

h 1) and the gov­

Forest types

Location

Mean

SD

Key factors *

Reference **

Subtropical forest

Mt. Ailao, China Tieshanping, China Tieshanping, China Mt. Jinyun, China Mt. Siman, China Mt. Siman, China Mt. Dinghu, China Mt. Gongga, China Amazon, Brazil Amazon, Brazil Amazon, Brazil Asia North America

0.77

2.34

This study

0.31

0.79

Tair, Tsoil and AHg

2.73

–

(b)

14.23

6.46

AHg and Hsoil Tair and SR

(c)

Acknowledgements

9.85

3.35

Tair

(d)

11.23

11.13

Tair

(e)

6.6

7.1

Tsoil

(f)

5.45

3.23

(g)

4.6

1.6

0.33

0.09

AHg and Tsoil Hsoil and Tair –

This work was funded by the National Natural Science Foundation of China (41430754, 41829701), Strategic Priority Research Programs of the Chinese Academy of Sciences, the Pan-Third Pole Environment Study for a Green Silk Road (Pan-TPE, XDA2004050201) and K.C. Wong Education Foundation. The data used in this study can be found in SI.

0.02

0.36

(j)

1.8 1.05

1.42 1.3

Europe

0.27

0.49

Tsoil and Hair Tsoil or Tair Tsoil, Tair, Hsoil and Hair Tsoil, Tair, Hsoil and Hair

Tropical forest

Temperate/ boreal forest

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

(a)

Appendix A. Supplementary data

(h)

Supplementary data to this article can be found online at https://doi. org/10.1016/j.atmosenv.2019.117047.

(i)

References

(k) (l)

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(m)

* where AHg is atmosphere Hg0 concentration, Tair is air temperature, Tsoil is soil temperature, Hair is air humidity, Hsoil is soil water content/humidity and SR is solar radiation. ** References. (a) Du et al. (2014). (b) Zhou et al. (2017). (c) Ma et al. (2013). (d) Ma et al. (2016). (e) Ma et al. (2018). (f) Fu et al. (2012). (g) Fu et al. (2008). (h) Almeida et al. (2009). (i) Carpi et al. (2014). (j) Magarelli and Fostier (2005). (k) Fu et al. (2016); Han et al. (2016). (l) Carpi and Lindberg (1998); Poissant and Casimir (1998); Zhang et al. (2001); Nacht and Gustin (2004); Schroeder et al. (2005); Kuiken et al. (2008a); Kuiken et al. (2008b); Choi and Holsen (2009). (m) Schroeder et al. (1989); Xiao et al. (1991); Ferrara et al. (1997); Lindberg et al. (1998).

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