互花米草生物量变化对盐沼沉积物有机碳的影响

The response of organic carbon content to biomass dynamics in Spartina alterniflora marsh

以江苏王港典型互花米草(Spartina alterniflora)盐沼湿地为研究对象,分析光滩及互花米草滩沉积物中有机碳的水平和垂向分布特征,了解互花米草生物量的季节动态变化,探讨二者之间的相互关系,在此基础上研究互花米草生物量分布和季节变化对沉积物中有机碳(TOC)含量的影响。结果表明,互花米草枯落物中的有机碳数量在两个月内衰减了40%,而表层沉积物中TOC含量及其中互花米草来源TOC所占比例的变化,均与互花米草地表枯落物量的季节变化存在两个月的"相位差",这与枯落物快速分解时间大致吻合,说明枯落物是表层沉积物中TOC的重要来源。高达60%的互花米草地下生物量分布在0—20cm深度内,该深度范围内沉积物中TOC含量较高,且TOC主要来源于互花米草。此外,不同深度TOC含量与地下生物量之间存在良好的正相关关系,说明地下生物量是影响沉积物TOC含量的重要因子。研究区互花米草年固碳能力为2274g m-2a-1,盐沼沉积物中TOC埋藏速率达到了470 g m-2a-1,是地表一个重要的碳汇;同时研究区每年向近岸水域输出大量的TOC,是近岸海域生态系统的一个重要碳源。

Spartina alterniflora （C4） was introduced to the coastal region of Jiangsu Province as an exotic species in 1982. This perennial grass then gradually invaded regions previously occupied by native C3 plants. Much research has focused on variation of the ecosystem carbon cycle and has demonstrated that this S. alterniflora invasion has increased the carbon accumulation rate. However, the impact of plant biomass （ both aboveground and belowground ） on the organic carbon content of salt marsh sediments requires further study. To explore the response of organic carbon content to the biomass dynamics in an S. alterniflora marsh, the contents of total organic carbon （TOC） , total nitrogen （TN） , and stable carbon isotope composition （δ3C） of sediments, in an S. alterniflora marsh as well as the bare fiat, were investigated by analysis of coral and surface sediments, together with the biomass variations of S. alterniflora. The sediment water content, grain size, and δ13C of coastal water and S. alterniflora were also determined. The Litterbag method was used to measure the S. alterniflora litter decomposition rate. The results indicate that the organic carbon of newer litter rapidly decomposed in the first 2 months, and then remained stable. The seasonal variation of the TOC content of surface sediments in the S. alterniflora marsh was significant, and a 2-month phase shift was not only observed in the TOC content variations and surface litter mass but was also recorded for the proportion of organic carbon derived from S. alterniflora and surface litter mass. However, this phase-shift phenomenon did not exist on the bare flat, indicating that the litter is a dominant source of the organic carbon in surface sediments. The results show that 60% and 33% of belowground biomass was observed in the top 20 cm and at 20--40 cm depths, respectively. Compared with the bare fiat, the vertical changes of TOC content in the S. alterniflora marsh showed a different distribution pattern. That is to say, the TOC content of sediments at depths 〈 20 cm was significantly higher than those at depths 〉 20 cm, and had an increasing trend from depth 20 cm to the surface. A similar variation was observed for the proportion of organic carbon derived from S. alterniflora in the top 20 cm of sediments. Furthermore, there was a remarkably positive relationship between the belowground biomass of S. alterniflora and TOC content. This suggests that belowground biomass plays an important role in TOC variation along the profiles. The greatest and least plant organic carbon storages in the S. alterniflora marsh were observed in October and April, respectively. The average carbon fixation of S. alterniflora, with quantity 2274 g · m-2· a-1, was 460% that of terrestrial vegetation in China. Similarly, the burial rate of organic carbon of S. alterniflora marsh sediments, with quantity 470 g · m-2· a-1, was significantly greater than the average value of coastal salt marshes in China. Thus, the S. alterniflora salt marsh fixed large amounts of carbon dioxide from the air, and enhanced the organic carbon accumulation of the sediment.