Enhancing Blue Carbon Potential: The Role of Northward Mangroves in Replacing Spartina alterniflora Across Stand Age Dynamics

Mangroves, with their remarkable carbon sequestration capacity, are presented with significant opportunities for northward expansion driven by climate change, which may offer a potential restoration strategy for regions in China where Spartina alterniflora has been removed. However, few studies have compared the carbon absorption and accumulation between northward-afforested mangroves of varying stand ages and S. alterniflora. This study investigates carbon exchange dynamics and soil carbon accumulation in Kandelia obovate-dominated wetlands (3, 9, and 20 years old) and S. alterniflora in a subtropical estuary in China, while also collecting data on soil sedimentation rates and surface organic carbon content across the four wetlands. We estimated annual carbon exchange using gross primary production (GPP) models based on light intensity, along with ecosystem respiration (R_eco) and methane (CH_4eco) models adjusted for temperature, combined with year-round monitoring data. Using a space-for-time substitution approach, we examined the carbon sequestration capacity and accumulation rates across different stand ages of northward mangroves replacing S. alterniflora. Our monitoring findings revealed that although net ecosystem exchange (NEE) increased significantly with stand age, it remained slightly lower than that of S. alterniflora. Both R_eco and GPP also increased with stand age, but a higher R_eco/GPP ratio could offset CO₂ uptake. It was estimated that the R_eco/GPP ratio of S. alterniflora (0.89) exceeded that of mangroves. Therefore, the annual carbon fixation through photosynthesis for the 3 yr, 9 yr, and 20 yr stands were 3.6, 5.8, and 9.9 t C m^-² ha^-¹, respectively. The mature stands exhibited significantly higher carbon fixation compared to S. alterniflora (-750 g C m^-² a^-¹), suggesting that although the carbon sequestration capacity of the northward mangroves is lower than that of S. alterniflora in the early stages, the carbon sink potential of the progressively maturing mangroves increases with stand age, ultimately surpassing that of S. alterniflora. Besides, CH_4eco emissions in mangroves were negligible, similar to S. alterniflora, and lower than those in natural mangroves, likely due to the absence of aerial roots that mediate CH_4eco release. Consequently, the net radiative cooling effect of mangroves increased with age, with the sustained-flux global warming potential metric with 100-year (SGWP₁₀₀) of 20 yr mangroves being double that of 3 yr mangroves, and it exceeds with 9 yr mangroves and S. alterniflora. This indicates that the warming mitigation potential of mature northward mangroves surpasses that of S. alterniflora. The calculation results show that, although the plant carbon pool of S. alterniflora (5.4 t C ha⁻¹ a⁻¹) is significantly higher than that of the 3 yr (3.2 t C ha^-1 a^-1), 9 yr (2.4 t C ha^-1 a^-1), and 20 yr (1.5 t C ha^-1 a^-1) mangrove stands, it lacks long-term stability. Moreover, the soil carbon accumulation rate in S. alterniflora (1.5 t C ha^-1 a^-1) was significantly lower than that in the 20 yr mangrove stand (2.6 t C ha^-1 a^-1). This suggests that replacing S. alterniflora with northward-afforested mangroves is an effective long-term strategy for future coasts to enhance blue carbon sequestration.