山东省东平湖菹草(Potamogeton crispus)腐烂过程中水体温室气体溶存浓度及扩散通量变化

The effect of Potamogeton crispus decomposition on the dissolved concentrations and fluxes of greenhouse gases in Lake Dongping,Shandong Province

为了解东平湖菹草(Potamogeton crispus)腐烂分解对水体温室气体溶存浓度和界面扩散通量的影响,于2016年5-7月在东平湖菹草腐烂期采集上覆水和沉积物柱样,测定上覆水和孔隙水中温室气体(N2O、CH4和CO2)的溶存浓度,采用Fick第一定律和双层模型计算沉积物-水-气界面扩散通量,同时分析上覆水和沉积物的理化性质,并采用网袋分解法于现场进行菹草腐烂分解试验,以探究东平湖菹草腐烂过程中温室气体溶存和扩散的主要影响因子及其主要来源.结果表明,菹草腐烂符合二次指数模型,分为快速衰减和慢速分解两个阶段;菹草腐烂过程中上覆水pH和亚硝态氮浓度表现为先降低后升高,而溶解氧、氨氮、硝态氮和可溶性正磷酸盐浓度则为先升高后下降,沉积物中铵态氮含量表现为先升高后降低,硝态氮为先降低后显著升高,有机质和p H呈降低-升高-降低的波动变化;上覆水中各温室气体浓度和水气界面扩散通量均表现为CO2>CH4>N2O,其扩散通量分别为5862.9±5441.4、31.15±41.3和0.15±0.57μmol/(m2·h),整体表现为大气温室气体的"源",并以碳排放为主;上覆水中N2O浓度和水-气界面扩散通量均先降低后升高,孔隙水中N2O浓度在快速和慢速分解阶段分别出现极大值(22.7和55.6 nmol/L),而其沉积物-水界面通量前期持续增加至腐烂结束后迅速降低;上覆水和孔隙水中CH4浓度及其各界面通量均表现为前期略有降低后持续升高;上覆水中CO2浓度和水-气界面通量表现为持续升高后降低并趋于稳定,而孔隙水中CO2呈波动变化,在菹草腐烂初期向孔隙水扩散,后期向上覆水扩散.水温是影响上覆水中温室气体浓度和水-气界面通量的主要因素;沉积物是水体N2O和CH4的主要来源,孔隙水中浓度是控制其沉积物-水界面扩散的重要因素;而上覆水中CO2呈现多源性,但以上覆水中有机物质的矿化为主.

In order to study the effect of Potamogeton crispus decomposition on the dissolved concentrations of greenhouse gases( GHGs) and their diffusion fluxes at the sediment-surface water-air interface in the Lake Dongping,surface water and sediment core samples were collected in situ seven times from May to July in 2016,and the decomposition experiment was also carried out by using the litterbag method to explore the dynamic of the dry mass loss of P. crispus. Dissolved concentrations of carbon dioxide( CO2),methane( CH4) and nitrous oxide( N2 O) in the surface water and pore water were concurrently measured,and the diffusion fluxes at the sediment-surface water interface and the surface water-air interface were calculated by using the Fick’s first law and the two-layered model respectively. In addition,the physicochemical properties of surface water and sediment were also measured to explore the main factors that might affect the concentrations and diffusion fluxes of GHGs as well as their sources during the decomposition of P. crispus. The results showed that the dry mass loss of P. crispus could be described accurately by the double exponential model which suggested two stages( rapid and slow) of P. crispus decomposition. The pH and nitrite concentrations in the surface water decreased first and then increased,while the change of the dissolved oxygen,ammonia,nitrate and dissolved inorganic phosphorus concentrations was reversed. In sediment,the ammonium content first increased and then decreased,while the nitrate content first decreased and then significantly increased,the organic matter content and p H of the sediment fluctuated. Both the concentrations and diffusion fluxes of GHGs at the water-air interface were in the order of CO2> CH4> N2 O,and the average fluxes were 5862.9±5441. 4,31. 15 ± 41. 3 and 0. 15 ± 0. 57 μmol/( m2·h),respectively. Generally,the waterbody acted as the source of the GHGs to the air,dominated by the carbon emission. The concentrations of N2 O in surface water and its diffusion fluxes at the surface water-air interface first decreased and then increased,and N2 O concentrations in pore water presented maximum values of 22.7 and 55.6 nmol/L in the rapid and slow decomposition stage respectively,while its fluxes at the sediment-water interface increased slowly in the early stage and then decreased rapidly at the end of the decomposition. For CH4,its concentrations in the surface water and pore water and interface diffusion fluxes all dropped slightly in the initial stage and then continued to rise. The concentrations of CO2 in surface water and its diffusion fluxes at the surface water-air interface increased continually and then decreased significantly to lower levels at the end of the rapid decomposition and then remain stable;while the concentrations of CO2 in pore water showed large fluctuations,which diffused to pore water at the initial stage of the decomposition and to surface water at the later stage. The correlation analysis between the concentrations and diffusion fluxes of GHGs as well as the physicochemical properties of surface water and sediment suggested that water temperature was the main factor that influenced the concentrations of GHGs in surface water and their fluxes at the water-air interface. N2 O and CH4 in the water body mainly originated from sediment,and their concentrations in the pore water was the important factor that could control the sediment-water interface diffusion;while CO2 in the surface water had multi sources,which were dominated by the mineralization of the organic matter in surface water.