不同CO2浓度升高水平对稻田CH4产生和排放的影响

There is a large difference in atmospheric CO2 concentration in the future under different emission scenarios, and the increase of atmospheric CO2 concentration is a gradual process. Most experiments are conducted by a high elevated CO2 concentration to investigate the effects of the rising CO2 on CH4 emission from paddy field. However, the ratio relationship and potential mechanism between elevated levels of CO2 concentration and CH4 emission are largely unknown. This study will be conducted with the treatments of increase in CO2 concentration by using open-top chamber (OTC) over 4-year consecutive experiment of rice. A step increase of 40 ppm will be designated for the elevated levels of CO2 concentration. OTCs with ambient CO2 concentration will be used as check. CH4 and CO2 fluxes were measured by a transparent static chamber–gas analyzer method. We will investigate the characteristic of CH4 emission during important growth period of rice under different elevated levels of CO2 concentration, and explore mechanism of CH4 emission to CO2 increase according to root exudates, methanogenic and methane-oxidizing bacterial, and aerenchyma tissue. The relationship between CH4-C and NEE (Net Ecosystem CO2 Exchange). Our objectives are to understanding the characteristic of CH4 emission under different elevated levels of CO2 concentration, and to illuminate potential mechanism. It is expected that the characteristic of CH4 emission, and the relationship between CH4-C and NEE under different elevated levels of CO2 concentration will be identified by this study, which will advance objective estimation of the amount of greenhouse gases emission from farmland in the future under the scenario of elevated CO2.

在不同排放情景下，未来大气CO2浓度差异很大，且大气CO2浓度是一个渐增的过程。国内外主要通过CO2高浓度试验，研究其对稻田CH4排放的影响。但目前尚不清楚不同CO2浓度升高水平与CH4排放之间是否存在一定的比例关系，可能的机制是什么。本项目拟采用开顶箱，以背景大气CO2浓度为对照，设置不同的CO2浓度升高水平，开展连续4年的水稻试验。使用透明静态箱-气体分析仪法观测CH4和CO2通量。主要研究内容：不同CO2浓度升高水平下水稻关键生育期CH4排放的特征；影响CH4产生和排放的机制；不同CO2浓度升高水平下稻田排放CH4碳与NEE的关系。研究目标：揭示不同CO2浓度升高水平下水稻关键生育期CH4排放的特征，并初步阐明其影响机制。通过本项目的实施，有望在不同CO2浓度升高水平下CH4排放特征、CH4碳与NEE的关系方面取得一定的突破，有助于准确评估未来CO2浓度升高水平下农田温室气体排放量。

在不同排放情景下，未来大气CO2浓度差异很大，且大气CO2浓度是一个渐增的过程。但目前尚不清楚不同CO2浓度升高（e[CO2]）水平与CH4排放之间是否存在一定的比例关系，可能的机制是什么。本项目采用开顶箱，以背景大气CO2浓度为对照，设置了不同的CO2浓度升高水平，开展了田间水稻试验。使用透明静态箱-气体分析仪法观测CH4和CO2通量。主要研究内容：不同CO2浓度升高水平下水稻关键生育期CH4排放的特征；影响CH4产生和排放的机制；不同CO2浓度升高水平下稻田排放CH4碳与NEE的关系。研究结果表明：与CK相比，CO2浓度升高处理对稻田CH4累积排放量（CAC）的促进效应并不显著，但CO2浓度升高显著提高了水稻单位产量CH4排放量。CO2浓度升高对2018和2019年水稻生长季的土壤产甲烷菌丰度影响并不显著。但在2020年水稻季，CO2浓度升高显著增加了土壤产甲烷菌丰度，使各生育期土壤产甲烷潜势（MPP）均显著提高。分蘖期e[CO2]+200ppm处理的产甲烷菌丰度和MPP分别显著提高了59.3%和24.1%，拔节期的产甲烷菌丰度显著提高了72.2%，灌浆期的MPP显著提高了24.4%。此外，与CK处理相比，e[CO2]+200ppm处理显著提高了甲酸甲烷规则菌（Methanoregula）和亨氏甲烷螺菌（Methanospirillum）的相对丰度。CO2浓度升高促进了稻田甲烷氧化潜势和甲烷氧化菌丰度的增加，还使得稻田土壤中甲烷氧化菌的群落结构发生了显著变化，其优势菌从II型菌转变为I型菌。使甲烷氧化潜势提高了11.7%，甲烷氧化菌的丰度增加了53%。CO2浓度升高所致的土壤中甲烷、氧气浓度以及氮素水平等的改变很可能对稻田甲烷氧化过程产生了重要影响。基于土壤产甲烷潜势、mcrA基因丰度和DOC浓度的线性模型可以解释稻田CH4通量67.5%–81.6%的变异。在e[CO2]+200ppm处理下，CH4-C/NEE比例变化为1.28-11.32%。2018-2021水稻生长季，与CK处理相比，e[CO2]+200ppm处理下CH4-C/NEE的比值分别增加了10.54%、45.26%、63.19%和18.52%，表明CO2浓度升高在一定程度上使稻田CH4-C/NEE的比例增大。本研究有助于准确评估未来CO2浓度升高水平下农田温室气体排放量。