报告一：汞同位素示踪热带强风化土壤成土过程中汞的来源和转运
报告二：环境因素对陆地净生态系统CO2交换量的影响
时 间：5月25日（周四）中午12:30
地 点：16教学楼221报告厅
报告一：汞同位素示踪热带强风化土壤成土过程中汞的来源和转运
主讲人：2020级博士 高鑫
摘 要：
土壤是地球表层最大的汞(Hg)储库和再排放源之一。成土过程中Hg的累积和再排放直接影响Hg从陆地环境向水生环境和生物区系的转运，但目前这些过程仍缺乏相关研究。本文通过对湛江新生代玄武岩风化形成的砖红壤剖面土壤和基岩样品Hg浓度和同位素比值的研究，探讨了土壤成土过程中汞的来源、迁移和转化过程，同时评估了热带土壤对全球Hg循环的潜在影响。整个土壤剖面表现出明显负的质量分馏 (MDF, δ202Hg = −2.97‰ to −2.54‰) 和非质量分馏 (MIF, Δ199Hg = −0.68‰ to −0.43‰)，δ202Hg从基岩向部分风化层(C层)明显正漂，Δ199Hg整体沿剖面正漂。这些同位素特征表明，Hg主要来自大气Hg(0)，以凋落物（HgLitter）的形式输入，而来自基岩的地质源Hg（HgGeo）贡献有限(<15%)。HgLitter在土壤表面发生显著的光还原作用，同时被土壤有机质结合沿剖面向下淋溶转化，导致整个砖红壤剖面Δ199Hg为负值。δ202Hg从基岩向弱风化母岩的正漂指示了基岩风化过程中HgGeo的大量损失，玄武岩中原生含硫矿物的溶解作用可能是形成原因。此外，δ202Hg在某些层位上与土壤pH值有较强的相关性，这可能是由于土壤表面的吸附作用造成Hg形态变化的过程中触发了同位素分馏，而吸附作用受土壤表面电荷性质和pH值的影响。综上，我们的研究结果为热带砖红壤剖面成土过程中Hg转运、转化和再活化的来源和机制过程提供了新的认识，并为基岩风化过程中HgGeo的损失提供了直接证据，这可能是热带地区重要的Hg源之一，并对河流/湖泊和海洋环境中水体和生物群落的Hg同位素特征产生重大影响。
Soil is one of the largest reservoirs and re-emission sources of mercury (Hg) on Earth’s surface. The accumulation and remobilization of Hg during pedogenesis significantly affect how Hg is transported from terrestrial to aquatic environment and biota, but these processes are poorly understood. Here we present Hg concentration and isotope ratios of soil and bedrock samples from a latosol profile formed through intense weathering of Cenozoic basalts in Zhanjiang, Guangdong Province, China, to trace the sources, transport and transformation processes of Hg during pedogenesis, and evaluate the potential impact of tropical soils on global Hg cycling. The entire soil profile shows obviously negative δ202Hg values (−2.97‰ to −2.54‰) and Δ199Hg values (−0.68‰ to −0.43‰), with a distinct positive shift of δ202Hg from the bedrock to the partially weathered horizon (Horizon C) and a general downward positive shift of Δ199Hg below 35 cm. These isotopic signatures suggest a dominant Hg input from atmospheric Hg(0) in the form of litterfall (HgLitter) and limited contribution (<15%) of geogenic Hg (HgGeo) from the bedrock. The HgLitter is subject to significant photoreduction on soil surfaces and the subsequent downward migration in association with soil organic matter, which are likely responsible for the strongly negative Δ199Hg value throughout the latosol profile. Moreover, the positive shift of δ202Hg value from bedrock to the weakly weathered rock indicates a substantial loss of HgGeo during bedrock weathering, likely due to the dissolution of primary minerals in basalt. In addition, the δ202Hg value shows a strong correlation with soil pH at some horizons, attributable to the isotopic fractionation during the change of Hg speciation likely triggered the adsorption on soil surfaces, which are affected by surface charge properties and soil pH values. Overall, our results provide new insights into the source and mechanisms/processes driving Hg transport, transformation and remobilization during pedogenesis of a tropical latosol profile, and offer direct evidence for the loss of HgGeo during the weathering of bedrock, which may serve as important sources of Hg in tropical regions and have a significant impact on the Hg isotope signatures of water and biota in river/lake and marine environments.
报告二：环境因素对陆地净生态系统CO2交换量的影响
主讲人：2020级博士 孔喆
摘 要：
净生态系统CO2交换量（Net Ecosystem Exchange of CO2，NEE）是调节陆地生态系统和大气之间碳交换的关键过程。然而，阐明NEE与环境变量的相互作用如何在不同的时间尺度以及不同生态系统和气候类型间的变化仍然是一个巨大挑战。本研究基于FLUXNET网络长期观测站点，发现NEE与环境变量间的耦合程度在日和月尺度上比在年尺度更强。另外，本研究量化了环境因素对日尺度NEE的相对贡献率。结果表明叶面积指数和短波辐射是大多数站点日尺度NEE的主要驱动变量，平均值分别为35.5%和27.8%。特别地，叶面积指数是落叶阔叶林和非森林的主要控制因素；辐射是常绿森林地区的主要控制因素。同时，气温、土壤含水量和饱和水汽压差的相对贡献较小。另外，日尺度NEE对干旱区植被动态的响应更为敏感。本研究有助于进一步理解环境因素如何在不同的地表和气候条件下调节NEE动态。
Net ecosystem exchange (NEE) of CO2 is a key process modulating carbon exchanges between terrestrial ecosystems and the atmosphere. However, it remains a grand challenge to elucidate how the interactions of NEE with environmental variables vary among ecosystems and climate regimes across timescales. Based on the FLUXNET sites with long-term observations, the results showed that the couplings between NEE and surrounding environments were stronger at daily and monthly scales than at annual scales. Moreover, we quantify the relative impacts of environmental controls on daily NEE variations. It revealed that leaf area index (LAI) and shortwave radiation (Rs) were the major drivers of daily NEE variations at most sites with the average contribution of 35.5% and 27.8%, respectively. Particularly, LAI was the principal control in deciduous broadleaf forested and non-forested sites, while Rs was the leading factor in evergreen forested sites. Meanwhile, air temperature, soil water content, and vapor pressure deficit exerted smaller influences.Furthermore, NEE was more sensitive to vegetation dynamics in drier climate regions. This study provides additional understanding of how environmental factors regulate NEE dynamics across diverse land surface and climatic conditions.
