Controls on Interannual Variability of Carbon Dioxide (CO2) in a Coastal Wetland Ecosystem: Insights from Two-year Measurements in a Temperate Salt Marsh

Globally, salt marshes play a pivotal role as efficient carbon (C) sinks, absorbing and storing substantial C in the soil. Despite their high C sequestration potential, understanding C exchange mechanisms in salt marshes along the Pacific coast in western Canada remains limited, resulting in uncertainties in estimating regional C budgets. This study addresses this gap by analyzing two years of eddy covariance (EC) measurements in a temperate salt marsh, revealing a shift from being a C sink (-185.3 gC m^-2 yr^-1) in the first year to a weak C source (10.4 gC m^-2 yr^-1) in the second year.

Exploring the annual patterns of Gross Primary Productivity (GPP) and ecosystem respiration (R_eco) sheds light on the dynamics of C exchange within the ecosystem. GPP of the first year is significantly higher than that of the second year during the growing season, from April to October. The light response curve indicated that the second year had lower light use efficiency and light-saturated net photosynthesis rate than the first-year data. Moreover, we found similar values in temperature sensitivity of soil respiration (Q₁₀) for both years using R_eco and soil temperature, with the first year slightly higher. Notably, the annual estimates for 2021 reveal a GPP of 1488.9 gC m^-2 yr^-1 and R_eco of 1303.6 gC m^-2 yr^-1. Conversely, for 2022, GPP was 1147.8 gC m^-2 yr^-1, and R_eco was 1158.2 gC m^-2 yr^-1. The contrasting GPP values between the two years suggest a significant influence of GPP over R_eco on the overall C balance of the ecosystem, which predominantly controls the variations in NEE.

In delving into the relationship between environmental factors and C exchange patterns, it becomes evident that water availability emerges as a key determinant at this site. The rainfall during the growing season of the first year closely matched 30-year averages from nearby meteorological stations, approximately 15% above the Climate Normals. In contrast, the second growing season precipitation was below average, only 52% of the long-term average. Additionally, it is noteworthy that in 2022, the growing season had a significantly higher Vapor Pressure Deficit (VPD), which led to lower GPP. This salt marsh demonstrates enhanced C uptake capabilities in a specific range of VPD, with peak efficiency observed at VPD levels ranging from 3 to 10 kPa. Conversely, CO₂ absorption capacity diminishes when VPD falls outside this range.

Water scarcity negatively impacts plant life, potentially leading to ecosystem instability and reduced C uptake capacity under climate change. A focused exploration of influencing factors is warranted to enhance our understanding of the observed transition from a C sink to a weak C source in the second year. Considering the broader implications of water scarcity on plant and ecosystem health could inform strategies to mitigate climate-induced stress. Addressing these areas will advance our knowledge of C dynamics in salt marsh ecosystems, guiding conservation and management efforts.