滨海湿地转为海水养殖塘对生态系统固碳和温室气体排放的影响研究

Capture fisheries production has plateaued, unable to meet the growing demand for animal protein. This has led to an expansion of aquaculture, production from which has exceeded that of capture fisheries since 2014. China now dominates aquaculture production worldwide, with most of its mariculture systems having been converted from coastal wetlands. The importance of coastal wetlands in global carbon (C) balance is well known, however, the greenhouse gas (GHG) related cost of land-use change on such a vast scale has yet be effectively quantified. In this project, the exchange of CO2, CH4 and N2O between the atmosphere and the coastal wetland or wetland-converted mariculture ponds at different managing levels will be monitored by using the Eddy-covariance method and closed chamber technique. Meanwhile, the C burial rate in coastal wetlands will be measured and the C exchange in mariculture ponds during managing practices will be recorded. The C sequestration rates of coastal wetlands and mariculture ponds will be estimated by using the net ecosystem C balance (NCEB) method. To elucidate the mechanisms of underlying the effect of converting natural wetlands to mariculture ponds on CH4 and N2O emissions, the rates and pathways of CH4 production will be measured by adding trace 13C-labeled methanogenic substrates to anaerobically incubated soil samples, and the gross rates of nitrogen (N) transformation processes and the contributions of different processes to N2O production will be quantified in a 15N tracing study. The functional methanogens coupling to methanogenesis will be identified by 13C-DNA-SIP, and the abundance of and community structure of soil nitrifying and denitrifying microorganisms will be measured by qPCR and Illumina sequencing, respectively. The direct CH4 and N2O emissions from the aquacultured animals will be measured by incubating animals in enclosed bottles. After that, by compiling a worldwide database of CH4 and N2O emissions from different mariculture systems, the CH4 and N2O emission factor in different systems will be summarized. The objectives of this study are (1) to evaluate the impacts of converting coastal natural wetlands to mariculture on C sequestration and CH4 and N2O emissions; (2) to gain a mechanistic understanding of CH4 and N2O processes under such a land-use change event, and (3) to quantify the CH4 and N2O emissions from Chinese mariculture systems.

水产养殖业快速发展导致滨海湿地大量被围垦为海水养殖塘。那么，这一土地利用方式变更是否影响生态系统固碳和温室气体排放，机制是什么，我国海水养殖业对温室气体排放的贡献如何？本项目拟选取江苏滨海湿地和由湿地转化而来、不同管理水平的海水养殖塘为研究对象，利用涡度相关等技术，测定生态系统CO2净交换及CH4和N2O排放，明确湿地转为养殖塘对生态系统固碳和温室气体排放的影响；建立培养试验，利用13C示踪和15N双标法研究不同底物的产CH4速率、土壤氮素转化过程，利用DNA-SIP技术等，研究产甲烷功能菌和硝化反硝化微生物群落结构，明确湿地和养殖塘CH4和N2O产生途径及其差异的微生物机制；测定主要养殖品种的CH4和N2O排放速率，评估动物直接排放对养殖塘总排放的贡献；整合文献数据，提出不同管理水平海水养殖系统温室气体排放系数，估算我国海水养殖业CH4和N2O排放量，为国家温室气体排放清单编制提供依据。

水产养殖业快速发展导致滨海湿地大量被围垦为海水养殖湿地，近40年，我国损失的滨海自然湿地中，有超过50％被开垦为海水养殖塘等人工湿地。那么，这一土地利用方式的改变是否影响生态系统固碳和温室气体排放，机制是什么？本项目以江苏省盐城市滨海自然湿地和由自然湿地转变而来的海水养殖湿地为研究对象，利用涡度相关系统和静态箱法，研究滨海自然湿地转变为海水养殖湿地对生态系统固碳以及甲烷（CH4）、氧化亚氮（N2O）排放的影响；通过室内培养试验，利用同位素示踪结合分子生物学技术，研究滨海自然湿地转变为养殖湿地对CH4和N2O的产生途径和微生物机制的影响；利用室内微宇宙试验，研究养殖动物的CH4和N2O直接排放速率及其对养殖湿地总排放量的贡献。结果表明，滨海自然湿地转为养殖湿地导致净综合温室效应（nGWP）由-13.8 Mg CO2eq ha-1增加至2.16 Mg CO2eq ha-1，使得滨海湿地由净温室气体汇转变为温室气体排放源。自然湿地转为海水养殖湿地显著降低了滨海湿地的CH4排放，同时N2O排放量也处于较低水平。但是，这一过程导致滨海湿地失去固定大气二氧化碳（CO2）的能力，是滨海湿地nGWP增加的主要原因。自然湿地和海水养殖湿地优势产甲烷菌均为甲基营养型，CH4产生途径也以甲基化合物歧化途径为主，但自然湿地转为海水养殖湿地显著降低了滨海湿地非竞争性底物三甲胺和二甲硫醚的含量，显著降低了CH4产生和排放速率。动物培养试验表明，海水养殖系统中养殖动物直接排放CH4和N2O的速率很低，对养殖湿地CH4和N2O总排放量的贡献有限。通过本研究，明确了滨海湿地转变为海水养殖湿地对温室气体排放的影响效应与相关机制，为评估水产养殖业的环境影响和制定减排对策提供了科学基础。