Carbon exchange in a freshwater marsh in the Sanjiang Plain, northeastern China

Northern wetlands are critically important to global change because of their role in modulating atmospheric concentrations of greenhouse gases, especially CO₂ and CH₄. At present, continuous observations for CO₂ and CH₄ fluxes from northern wetlands in Asia are still very limited. In this paper, two growing season measurements for CO₂ flux by eddy covariance technique and CH₄ flux by static chamber technique were conducted in 2004 and 2005, at a permanently inundated marsh in the Sanjiang Plain, northeastern China. The seasonal variations of CO₂ exchange and CH₄ flux and the environmental controls on them were investigated. During the growing seasons, large variations in net ecosystem CO₂ exchange (NEE) and gross ecosystem productivity (GEP) were observed with the range of −4.0 to 2.2 (where negative exchange is a gain of carbon from the atmosphere) and 0–7.6gCm⁻²d⁻¹, respectively. Ecosystem respiration (RE) displayed relatively smooth seasonal pattern with the range of 0.8–4.2gCm⁻²d⁻¹. More than 70% of the total GEP was consumed by respiration, which resulted in a net CO₂ uptake of 143±9.8 and 100±9.2gCm⁻² for the marsh over the growing seasons of 2004 and 2005, respectively. A significant portion of the accumulated NEE-C was lost by CH₄ emission during the growing seasons, indicating the great potential of CH₄ emission from the inundated marsh. Air temperature and leaf area index jointly affected the seasonal variation of GEP and the seasonal dynamic of RE was mainly controlled by soil temperature and leaf area index. Soil temperature also exerted the dominant influence over variation of CH₄ flux while no significant relationship was found between CH₄ emission and water table level. The close relationships between carbon fluxes and temperature can provide insights into the response of marsh carbon exchange to a changing climate. Future long term flux measurements over the freshwater marsh ecosystems are undoubtedly necessary.