研究生知识库

森林生态系统植被和土壤的汞储量巨大，并且这部分汞在全球汞的生物地球化学循环中尤为活跃。清楚地认识森林生态系统中汞的生物地球化学过程对于完善全球汞循环模型至关重要。然而，相比于其他陆地生态系统，森林生态系统与大气之间的汞交换通量存在最大的不确定性，全球森林系统汞年际通量范围-727 Mg year-1 ~ 703 Mg year-1。因此，对于森林生态系统到底是大气汞的“源”还是“汇”这一论题一直存在争议。目前森林生态系统与大气Hg0交换过程不确定性主要来自如下两个方面。首先是关于森林大气-植被叶片间Hg0通量交换的直接监测数据非常匮乏，难以确定叶片到底是吸收大气Hg0还是向大气释放Hg0；其次是森林大气-土壤间Hg0通量的交换过程化学动力学机制不明确，难以厘清多过程在地气交换中的贡献。本博士论文将从以上两个科学问题出发，选取哀牢山亚热带常绿阔叶林为研究对象，利用动力学通量袋/箱法结合汞通量监测和汞同位素手段来量化大气-植被叶片界面和大气-土壤界面间的汞的迁移转化规律，旨在阐明森林生态系统与大气Hg0间的源汇关系。本博士论文主要得出如下认识：(1)森林大气与植被叶片间Hg0交换是一种双向的过程，即叶片从大气中吸收汞的同时，也能够向大气中释放汞。研究发现光照有效辐射对于大气-叶片Hg0交换通量存在最重要影响，主要是影响了再释放的过程。研究表明叶片表面汞还原和气孔内部汞还原占再释放过程中的比例分别约为30%和70%，明确了叶片再释放的汞主要来自于叶片内部的还原过程。哀牢山森林大气-叶片间汞的年际通量为-26.79±12.70 μg m-2 year-1，意味着叶片主要表现为大气Hg0的汇。(2)植物叶片吸收大气汞的过程发生了显著的汞同位素质量分馏。由于新生树叶同化大气汞的速率快，暂时保留了大气Hg0同位素信号；但随着叶片成熟，叶片吸收过程产生明显的汞同位素分馏，导致叶片中富集较轻的汞同位素。叶片成熟后，叶片HgII与大气Hg0的汞同位素质量分馏可以达到-3‰。植物叶片与大气Hg0交换过程中，还存在明显汞同位素的奇数非质量分馏。植物叶片在吸收大气汞过程中，保留了大气Hg0奇数汞同位素非质量分馏信号。但叶片中汞被光致还原后使得偏正的奇数非质量分馏特征的Hg0被释放，导致叶片中HgII的奇数非质量分馏偏负。因此相比于大气Hg0奇数非质量分馏，叶片中奇数汞同位素的非质量分馏值向更负的方向偏移(-0.15‰)。结合叶片内汞含量的变化，建立了大气-叶片Hg0交换的汞同位素质量平衡模型，模拟获得了植物叶片不同生长阶段吸收大气汞的真实通量及叶片向大气再释放汞的初始通量。综合而言，叶片生长周期内约有30%先前叶片吸收的大气汞能够经过再释放过程回到大气。 (3)在云南哀牢山亚热带常绿阔叶林森林利用汞、碳同位素相结合示踪两年的凋落物降解实验和0-15cm高分辨率的土壤剖面中汞的地球化学行为。实验结果表明汞的行为受限于土壤中碳的丢失过程。土壤剖面底部(~10cm)时，δ202Hg/δ13C、Δ199Hg/δ13C和Δ199Hg/Δ201Hg比值趋于稳定，证明汞已经达到“老化”阶段。在0-15cm有机层土壤剖面中Δ199Hg/Δ201Hg斜率为1.35±0.13，直接证明了有机质暗还原反应是导致土壤汞被还原而参与大气-森林土壤汞交换的重要因素。本研究的重要意义在于，高度腐殖化的森林有机土壤中的汞存在明显的“老化”固定的特征，而全球汞循环模型中关于土壤有机层活跃态汞的储库存在明显高估，需进一步优化。(4) 哀牢山森林大气-土壤间Hg0通量交换过程，存在明显的季节性变化特征，夏季时候土壤表现为大气汞的源，而冬季时土壤表现为大气汞的汇。大气-土壤间Hg0年际净通量为+6.7±20.5 μg m-2 year-1。进一步利用结构方程模型解析影响地气交换过程的主导环境因素，发现温度是最重要的促进地气汞交换通量的因素，其次为大气汞浓度。结合大气-叶片间Hg0交换通量，整体而言哀牢山森林系统表现为大气汞的汇，年际沉降通量为-20.08±24.11 μg m-2 year-1。进一步估算全球森林生态系统中大气-土壤Hg0交换通量发现，热带/亚热带森林系统汞释放通量为300 ± 450 Mg year-1，是温带/北方森林的土壤释放汞通量的2倍左右。因此，本研究强调热带/亚热带森林中汞的地球化学行为在全球汞循环中占据重要地位。(5)本次研究同时监测了大气暴露下和零汞空气暴露下，森林大气-土壤间Hg0交换过程的汞同位素变化。其中，土壤壤中气的汞同位素组成为δ202Hg=-0.94±0.32‰, Δ199Hg=-0.49±0.07‰，比土壤汞同位素相比，壤中气的MDF更加偏正而odd-MIF基本一致。与大气-土壤间Hg0通量的季节性变化相关，大气-土壤交换过程汞同位素变化规律也表现出季节性差异。相比于通量箱的进气口汞同位素(夏天时δ202Hg=0.43±0.13‰和Δ199Hg=-0.20±0.05‰；冬天时δ202Hg=0.66±0.22‰ 和Δ199Hg=-0.08±0.05‰)，夏天时出气口汞同位素MDF和odd-MIF特征都显著偏负(δ202Hg=-0.15±0.23‰ and Δ199Hg=-0.33±0.05‰, p<0.01, pair t-test)，而冬天时候出气口汞同位素则保持不变(δ202Hg=0.52±0.55‰ and Δ199Hg=-0.10±0.04‰, p>0.05, pair t-test)。这意味着相比于大气，土壤再释放Hg0存着(-)MDF和(-)odd-MIF信号，而大气Hg0沉降过程则不会造成汞同位素的变化。土壤再释放汞的同位素组成利用零汞条件来进行进一步探索。在零汞空气暴露实验中，夏季再释放Hg0同位素组成为δ202Hg = -2.20±0.63‰ 和Δ199Hg = -0.36±0.04‰，而冬季再释放Hg0同位素组成为δ202Hg = -3.45±0.16‰ and Δ199Hg = 0.19±0.08‰。结合土壤壤中气和环境空气中汞同位素组成，零汞空气下夏季土壤再释放Hg0的主要原因是孔隙气体的扩散，冬季则是表层土壤中HgII的光还原作用。定量解析大气汞沉降，表层土壤HgII的光致还原过程以及深层壤中气的Hg0扩散作用在森林大气-土壤Hg0交换过程中的贡献通量。夏天时，大气汞沉降通量为-11.2±10.6 ng m-2 h-1，表层土壤中HgII的光致还原过程通量为+3.0±5.7 ng m-2 h-1以及深层壤中气的Hg0扩散作用通量为+13.6±8.9 ng m-2 h-1。冬季时，大气汞沉降通量为-1.9±2.8 ng m-2 h-1，表层土壤中HgII的光致还原过程通量为+0.4±2.8 ng m-2 h-1以及深层壤中气的Hg0扩散作用通量为+0.1±0.5 ng m-2 h-1。总之，先前遗留的汞再排放过程夏季时主要是深层土壤的暗反应生产Hg0进而向地表扩散过程，而在冬季主要是由于表层土壤中的HgII经光还原作用生产Hg0的过程。

Forest biomass and soils comprise a large active mercury (Hg) reservoir in terrestrial ecosystem. Understanding of the Hg biogeochemical processes in forest ecosystems is critical for estimating the global Hg cycle. However, compared to other terrestrial ecosystems, there exists the largest uncertainty in Hg0 exchange flux between atmosphere and forest ecosystem, ranges from -727 Mg year-1 to 703 Mg year-1. So there have been conflicting reports regarding the role of forest ecosystems as a Hg0 sink or source. Currently, the uncertainties of Hg cycling in forest ecosystens are mainly attributed to two parts. Firstly, the atmosphere-foliage Hg0 exchange flux data is scarce, leading to the difficulty to determine whether the foliage uptakes atmospheric Hg0 or emits Hg0. In addition, the limited understanding of the kinetics and mechanism of HgII reducition in soil, resulting in unable to quantify the flux of multiple processes in atmosphere-soil Hg0 exchange. Based on the above two scientific issue, we quantify the migration and transformation of mercury between the atmosphere-foliage and the atmosphere-soil combined with mercury isotope approach and mercury flux monitoring, aiming to clarify the relationship between forest ecosystems and atmospheric Hg0, source or sink. Get the following understanding:(1) The atmosphere-foliage Hg0 exchange is a bi-direction exchange process. This means the foliage can uptake the atmospheric Hg0, meanwhile the foliage can reemit the Hg0 to atmosphere. This study displays the PAR has a great influence on the atmosphere-foliage Hg0 exchange, especially the reemission process. This study further investigates the Hg source in the foliage reemission process, suggesting the foliage Hg0 reemission is mainly occurred in leaf interior, about 30% from the surface and 70% from stomatal cavities. The estimated annual Hg0 flux between atmosphere and foliage is -26.79±12.70 μg m-2 year-1, means the foliage as a sink of atmospheric Hg0.(2) Atmospheric Hg0 is the dominant source of foliar Hg. Upon Hg0 uptake, the sprout leaves temporarily retaines the atmospheric Hg0 isotope signature, but the maturing foliage becomes progressively enriched in lighter Hg isotopes. Finally the δ202Hg shift between the maturing foliage and atmosphere can up to -3‰. Meanwhile, the maturing foliage also becomes depleted in odd-MIF. Considered the uptake process can’t cause the odd-MIF shift, we demonstrate that re-emitted Hg0 exists (+)odd-MIF, leading to the maturing foliage exists negative odd-MIF shift. Compared to the atmospheric Hg0 odd-MIF, the odd-MIF shift is about -0.15‰ in maturing foliage. An isotopic differential mass balance model indicates that the proportion of foliar Hg0 efflux to uptake gradually increase from emergence to senescence with an average flux ratio of 30%.(3) This study investigated mercury stable isotope and carbon stable isotope in 2-year litterfall decomposition experiments and 0-15cm high-resolution soil profiles in evergreen broad-leaf forest at Ailao Mountain. The results show Hg behavior is closely linked to C loss, which is consistent in litterfall decomposition experiments and undisturbed soil profiles. All the isotopic features and ratios tend to be constant in lower part of the organic soil profile, indicating that complete immobilization Hg is present in the deeper organic soil. In addition, the slope of Δ199Hg/Δ201Hg in the 0-15cm organic soil profile is 1.35±0.13, as a directly evidence that the abiotic dark reaction driven by NVE plays a dominant role in forest soil HgII reduction. A major implication of these findings is that in the global Hg cycling models the storage of active cycling Hg is overestimated due to immobilized Hg in deeper organic soils, thus those models need to be further optimized.(4) This study shows that the atmosphere-soil Hg0 exchange flux has distinct seasonal variations at Mt. Ailao, as an atmospheric Hg0 source in summer and a sink in winter. The annual net flux over forest floor is +6.7±20.5 μg m-2 year-1. Structural equation modeling (SEM) infers that the temperature is most important driver causing the atmosphere-soil Hg0 exchange, followed by atmospheric Hg0 concentration. Combined with the atmosphere-foliage exchange result, the EB forest ecosystem emerges as the atmospheric Hg0 sink with a net flux of -20.08±24.11 μg m-2 year-1. Finally, using the data documenting global documented air-soil Hg0 exchange flux in forest ecosystems, we estimate Hg0 emission from tropical/subtropical forest floor to be 300 ± 450 Mg year-1, ~2 times greater than the release from global boreal/temperate forest floor, highlighting the importance of tropical/subtropical forest ecosystems in global Hg biogeochemical cycling.(5) In this study, we investigated the soil-air Hg0 exchange in ambient air and under Hg0-free gas exposures, as well as its isotopic shift caused by the exchange. It has been found that soil pore gas has δ202Hg of -0.94±0.32‰ and Δ199Hg of -0.49±0.07‰, more positive δ202Hg and no significant Δ199Hg compared to Hg in soil (δ202Hg = -2.08±0.26‰; Δ199Hg = -0.57±0.10‰). Soil-air Hg0 exchange flux exhibit a seasonal pattern, suggesting soil an atmospheric Hg source in summer and a sink in winter. Compared to inlet isotopic composition (δ202Hg=0.43±0.13‰ and Δ199Hg=-0.20±0.05‰ in summer; δ202Hg=0.66±0.22‰ and Δ199Hg=-0.08±0.05‰ in winter), the isotopic compositions of outlet display a significantly more negative δ202Hg and Δ199Hg in summer (δ202Hg=-0.15±0.23‰ and Δ199Hg=-0.33±0.05‰, p<0.01, pair t-test), but comparable values in winter (δ202Hg=0.52±0.55‰ and Δ199Hg=-0.10±0.04‰, p>0.05, pair t-test). This suggests Hg0 re-emission from soils with much more negative δ202Hg and Δ199Hg than values in air, and Hg0 deposition with no distinct MDF and MIF. The Hg0 re-emission isotopic composition is further determined by soil Hg0 evasion under Hg-free gas exposure. Soil Hg0 evasion under Hg-free gas exposure in summer has δ202Hg of -2.20±0.63‰ and Δ199Hg of -0.36±0.04‰, while in winter δ202Hg of -3.45±0.16‰ and Δ199Hg of 0.19±0.08‰. Given the isotopic compositions found in soil pore gas and ambient air, the promoted soil Hg0 evasion under Hg-free air in summer is mainly attributed to the diffusion of pore gas, and in winter to photo-reduction of divalent Hg in surface soil. Hence, the apparent soil-air Hg0 exchange represents a result of Hg0 deposition, Hg0 evasion from surface soil induced by photo-reduction and Hg0 diffusion from soil pore. Using an isotopic mass balance model, the estimated flux in summer is -11.2±10.6 ng m-2 h-1 by Hg0 deposition, +3.0±5.7 ng m-2 h-1 by Hg0 evasion from surface photo-reduction, and +13.6±8.9 ng m-2 h-1 by Hg0 diffusion from soil pores. In winter, atmospheric Hg0 deposition and Hg0 evasion from photo-reduction and Hg0 diffusion are much weaker, with the value of -1.9±2.8 ng m-2 h-1, +0.4±2.8 ng m-2 h-1, and +0.1±0.5 ng m-2 h-1, respectively. Overall, legacy Hg re-emission is largely caused by dark reaction processes in deep soil in summer, and by photo-reduction processes in surface soil in winter.