洞庭湖湿地土壤碳排放及其对水文情势变化的响应机理

Wetland has been known as an important carbon sink in terrestrial system. The hydrological conditions (i.e. water table, submergence time and frequency) are key factors that influence wetland C emission, and wetlands may shift to C source due to the significant fluctuation of hydrological conditions. In recent years, the studies on wetlands C emission mainly focused on peatland or marsh-land distributed in high latitude district. However, the studies in freshwater lakes were relatively few, which might increase the uncertainty on C sink assessment of global wetlands. Dongting Lake, which has a significant change in water table regime (3.50 - 10.01 m per year), was selected as our object in this project. Firstly, the temporal and spatial variation of soil C emission and their relations to hydrological conditions were studied by field experiment in the fixed plots. Secondly, the detail influences of hydrological conditions on soil C emission were studied by controlled experiments, and furthermore, an indoor experiment was conducted to clarify the effectiveness of soil nutrient (C, N, P) or microbial activity stress regulate mechanisms on soil organic mater (SOC) mineralization. The objective of this project is to clarify the characteristics of soil C emission in Dongting Lake wetlands and to illustrate its response mechanisms to hydrological conditions’ change, which would have significances on improving the national greenhouse gas inventories, comprehensively understanding the soil C emission response mechanisms to global change and enhancing scientific management on natural wetlands.

湿地是陆地重要的碳汇之一，水文情势（水位、水淹时间及频率）是影响湿地碳排放的关键因子，其剧烈波动导致湿地碳源/汇功能的转变。当前湿地碳排放的研究多集中于高纬度泥炭地和沼泽湿地，淡水湖泊湿地研究相对薄弱，从而增加了湿地碳汇功能评估的不确定性。本项目拟以我国水情变化最大(年水位变幅3.50~10.01m)的淡水湖泊——洞庭湖湿地为对象，通过野外定点观测揭示洞庭湖湿地土壤碳排放（CO2和CH4）的时空变化特征及与水文情势的耦合关系；通过控制实验揭示水文情势变化对湿地土壤碳排放的影响；并通过室内有机碳矿化培养实验揭示土壤有机碳矿化的生源要素（C、N、P）有效性/微生物活性调控机制。最后整合各项研究结果，系统阐明洞庭湖湿地土壤碳排放特征及其对水文情势变化的响应机理。本研究对于完善我国湿地温室气体排放清单、全面理解湿地碳汇功能对全球变化的响应以及加强湿地管理等方面具有重要意义。

湿地是陆地重要的碳汇之一，水文情势（水位、水淹时间及频率）是影响湿地碳排放的关键因子，其剧烈波动导致湿地碳源/汇功能的转变。当前湿地碳排放的研究多集中于高纬度泥炭地和沼泽湿地，淡水湖泊湿地研究相对薄弱，从而增加了湿地碳汇功能评估的不确定性。本项目以我国水情变化最大(年水位变幅3.50~10.01m)的淡水湖泊——洞庭湖湿地为对象，通过野外定点观测及室内控制实验揭示了洞庭湖湿地土壤碳排放（CO2和CH4）的时空变化特征及水文驱动机理。取得的主要结果如下：洪水显著抑制土壤CO2的排放，但促进甲烷排放。在苔草地，非洪水期土壤CO2排放速率明显高于洪水期，分别为：1126.98 ± 201.83 mg C m-2 d-1 VS 478.99 ± 223.69 mg C m-2 d-1；然而，在光滩地中，CO2排放速率在洪水季与非洪水季无显著差异，分别为725.99 ± 103.25 mg C m-2 d-1 VS 680.43 ± 176.69 mg C m-2 d-1。在苔草和光滩地，洪水期CH4通量要显著高于非洪水期。该结果表明当前人类活动干扰下的水淹天数降低可能导致湿地土壤碳流失。通过非洪水期和洪水期土壤碳通量与水位、土壤水分和土壤温度间的偏相关分析表明：在非洪水期，土壤碳排放主要受土壤温度的影响，而在洪水季，土壤甲烷排放主要受水位的影响，而二氧化碳排放受温度影响较大。利用涡相关法（Eddy Covariance）研究了洪水来临前期、洪水期和退水期芦苇湿地生态系统CO2净交换(Net Ecosystem Exchange, NEE)。结果表明：洪水前期芦苇湿地NEE为-43 mg C m-2 h-1，洪水期NEE显著降低为-204 mg C m-2 h-1，退水期NEE快速升至19 mg C m-2 h-1。该结果进一步说明洞庭湖退水期提前导致湿地碳汇功能的减弱。室内水位控制实验表明，随着水位的降低，土壤呼吸对温度变化的响应更加敏感，土壤矿化速率显著增加。通过检测土壤生源要素的有效性和微生物活性，结果初步表明淹水条件下微生物活性的降低可能是限制土壤有机碳矿化的根本原因。