红树植物地上根系对表层沉积物碳库的影响机制

Mangroves are among the most carbon-rich forests in the tropics and are efficient carbon sinks in coastal areas. Mangrove soils store 50-90% of this total carbon. Accordingly, soil carbon sequestration and loss are greatly influenced by some biological processes associated with the forest floor. Aerial roots are typical structures in mangroves and assist trees in adapting to tidal inundation and fluctuation. As a conduit for the passage of oxygen, aerial roots may assist mangroves in ameliorating anaerobic soil conditions by alleviating oxygen deficiency in root tissues. However, the same conduits can also channel CO2 from soil respiration to the atmosphere, which has not been extensively studied as an important component of soil carbon loss from mangrove ecosystems.This process is also often excluded from global carbon budgets in mangroves. At the same time, aerial roots can potentially slow water velocities, trapping plant litter and detrital particles, and promote sedimentation, which would also promote the carbon gain to mangrove soil. Since there are large numbers of morphologically variable structures associated with mangrove aerial roots, soil carbon sink potential by these biological and physical processes would be underestimated by neglecting these functions. We will conduct a series of in situ studies in mangroves dominated by different types of aerial roots in Dongzhaigang Mangroves Nature Reserves, Hainan Province. Field investigations of forest structure, aerial root structure, root productivities, litterfall trapping, and decomposition will be conducted. We will install sets of Rod Surface Elevation Tables (RSETs), and deploy these with feldspar marker horizons (RSET-MH method), to discern the mechanisms of surface elevation change by aerial root type, density, and height that might affect soil surface carbon gain. RSET-MH approaches allow us to identify rates of vertical accretion, shallow subsidence, and/or root zone expansion and their relative influence on soil surface elevation change.We also plan to conduct comparative measurements of gas (CO2 and CH4) flux from both the soil surface and different types of aerial roots using static flux chamber techniques followed by gas chromatography. Furthermore, we will also determine carbon sources for trapped surface sediments by aerial root type, and from emitted greenhouse gases by tracing their carbon stable isotope signals. The aim of this research is to determine the influences of aerial roots on soil surface carbon gain through their effects on sedimentation, litterfall and root decomposition, and soil respiration. We will also quantify the contribution of aerial roots to total soil carbon gain in mangrove ecosystems. We hope that the results of these studies will provide useful information to improve our understanding of the mechanisms used in mangrove forests to store carbon so efficiently, and thus influence management decisions to affect greater carbon storage in the future.

红树林是热带碳储量最丰富的森林和滨海湿地的高效碳汇。沉积物是红树林的重要碳库。地表生物学过程包括沉积物和凋落物的捕获和分解、温室气体排放，都影响着沉积物的碳收支。地上根系(气生根)是红树植物适应潮间带厌氧和潮汐动力机械作用的特殊结构。气生根作为氧气进入地下根的门户已被广泛认识，然而它们在促进沉积、释放沉积物和地下根产生的温室气体等与沉积物碳库密切相关的作用还鲜为人知。由于红树林气生根数量巨大、结构多变，忽视它们对沉积物碳库收支的贡献，将极大影响红树林生态系统碳库的准确评估。 本研究采用野外原位测定、地面高程监测系统、气体通量测定技术和稳定同位素示踪法，探讨气生根的类型、密度、高度等对红树林沉积物碳固定、垂直沉积和地下碳释放的调控机理，评价它们对红树林蓝碳碳汇的重要贡献。在揭示气生根对沉积物碳收支影响机制的同时，还将为准确评估红树林地下碳库提供理论依据。

红树林是热带碳储量最丰富的森林和滨海湿地的高效碳汇。沉积物是红树林的重要碳库。地表生物学过程包括凋落物和沉积物的捕获和分解、温室气体的排放都影响着表层沉积物的碳累积。地上根系(气生根)是红树植物适应潮间带厌氧和潮汐动力机械作用的特殊结构。气生根作为氧气进入地下根的门户已被广泛认识，然而它们在促进沉积、释放土壤和地下根产生的温室气体等与沉积物碳库密切相关的作用还鲜为人知。本研究利用野外实验和野外长期定位研究方法，对不同红树林地上根系参与的沉积物表层沉积过程、凋落物捕获和分解过程、地下根区的碳积累和分解过程，以及地上根系温室气体排放和碳来源等沉积物碳收支各环节的探讨，研究沉积物界面的碳动态特性。.研究结果表明：1）红海榄群落的支柱根结构对沉积物表层沉积的影响不如秋茄的板状根、海莲的膝状根和无瓣海桑的呼吸根显著；2）秋茄的板状根、根区的增长速率最快，地上根系对沉积物的捕获和根区的快速生长共同导致了秋茄群落对沉积物表面高程增加的贡献最大；3）土壤呼吸受到温度的影响极其显著，温度的细微上升可能大幅度加速呼吸作用；但不同群落沉积物之间的土壤呼吸的差异不显著；4）呼吸根CO2通量与地下根生物量具有显著相关关系，说明地下根系也影响着呼吸根的呼吸能力；呼吸根的气体交换是红树林地下碳排放的贡献者之一。本研究在揭示地上根系对红树林沉积物碳库调控机理、为红树林高效固碳机理提供补充的同时，将为准确评估红树林生态系统碳库和红树林的管理提供理论依据。