Climatic Variability, Hydrologic Anomaly, And Methane Emission Can Turn Productive Freshwater Marshes Into Net Carbon Sources

Freshwater marshes are well-known for their ecological functions in carbon sequestration, but complete carbon budgets that include both methane (CH₄) and lateral carbon fluxes for these ecosystems are rarely available. To the best of our knowledge, this is the first full carbon balance for a freshwater marsh where vertical gaseous [carbon dioxide (CO₂) and CH₄] and lateral hydrologic fluxes (dissolved and particulate organic carbon) have been simultaneously measured for multiple years (2011–2013). Carbon accumulation in the sediments suggested that the marsh was a long-term carbon sink and accumulated ~96.9 ± 10.3 (±95% CI) g C m⁻² yr⁻¹ during the last ~50 years. However, abnormal climate conditions in the last 3 years turned the marsh to a source of carbon (42.7 ± 23.4 g C m⁻² yr⁻¹). Gross ecosystem production and ecosystem respiration were the two largest fluxes in the annual carbon budget. Yet, these two fluxes compensated each other to a large extent and led to the marsh being a CO₂ sink in 2011 (−78.8 ± 33.6 g C m⁻² yr⁻¹), near CO₂-neutral in 2012 (29.7 ± 37.2 g C m⁻² yr⁻¹), and a CO₂ source in 2013 (92.9 ± 28.0 g C m⁻² yr⁻¹). The CH₄ emission was consistently high with a three-year average of 50.8 ± 1.0 g C m⁻² yr⁻¹. Considerable hydrologic carbon flowed laterally both into and out of the marsh (108.3 ± 5.4 and 86.2 ± 10.5 g C m⁻² yr⁻¹, respectively). In total, hydrologic carbon fluxes contributed ~23 ± 13 g C m⁻² yr⁻¹ to the three-year carbon budget. Our findings highlight the importance of lateral hydrologic inflows/outflows in wetland carbon budgets, especially in those characterized by a flow-through hydrologic regime. In addition, different carbon fluxes responded unequally to climate variability/anomalies and, thus, the total carbon budgets may vary drastically among years.