Net exchanges of CO ₂, CH ₄ and N ₂O between marshland and the atmosphere in Northeast China as influenced by multiple global environmental changes

Natural wetland ecosystem plays an important role in global climate change due to its large amounts of stored carbon and nitrogen. The Sanjiang Plain, Northeast China, encompasses large area of natural freshwater marshy wetlands. However, the magnitude and temporal patterns of major greenhouse gases (GHGs: CO ₂, CH ₄ and N ₂O) in this region remain far from certain. Here we used a process-based ecosystem model to examine GHGs fluxes and their underlying mechanisms in the marshland across the Sanjiang Plain over the period 1949-2008. Simulation results indicated that during the past 60 years, the Sanjiang Plain's marshland acted as a net CO ₂ sink of 4.20 ± 0.44 Tg C yr ^-1, while approximately 0.46 ± 0.02 Tg C yr ^-1 for CH ₄ and 0.02 ± 0.00 Tg N yr ^-1 for N ₂O were released to the atmosphere. Land cover and land use change (LCLUC) was the primary driver for GHGs changes. Climate change and tropospheric ozone (O ₃) pollution decreased CO ₂ uptakes, yet elevated CO ₂ concentration and nitrogen deposition increased CO ₂ uptake. Tropospheric O ₃ pollution and nitrogen deposition decreased CH ₄ emission by 7.94 Gg C and 0.41 Gg C, respectively, while elevated CO ₂ concentration increased the CH ₄ emission by 133.81 Gg C. Accumulatively, tropospheric O ₃ pollution and climate change contributed approximately 5.37% and 4.89% to the increased N ₂O emission, respectively, while elevated CO ₂ concentration reduced 2.81% of the N ₂O emission. The global warming potential (GWP) ranged from -1.81 Tg CO ₂eq yr ^-1 in the 1970s to 29.15 Tg CO ₂eq yr ^-1 in the 2000s. The decadal GWP by CO ₂ fluxes shifted from negative values between the 1950s and the 1990s to positive values in the 2000s, while CH ₄ and N ₂O emissions enhanced GWP from the 1950s to the 2000s. The total GWP decreased from the 1950s to the 1970s, but increased from the 1980s to the 2000s. This suggested that the reduced GWP by decreased CH ₄ emissions was gradually offset by the increased GWP by increased N ₂O emissions and decreased CO ₂ sink from the 1980s to the 2000s, implying that a full accounting of the greenhouse gas balance is essential in assessing global change impacts.