## 3 Results and Discussion
### Carbon Exchange between Natural and Managed Mangroves  
In addition to natural biophysical drivers, anthropogenic drivers also have an impact on mangrove carbon exchanges. Previous studies have shown significant differences in carbon sequestration and greenhouse gas emissions between natural, disturbed, and restored mangroves based on meta-analyses [20; 63]. Table 2 shows the collected annual carbon fluxes based on the eddy covariance method and the summation method. The summation method involves the evaluation of the amount of carbon influx and efflux among various parts of a forest ecosystem [64]. The natural mangroves acted as CO\({}_{2}\) sinks, with NEE values ranging from \(-249\) g C m\({}^{-2}\) a\({}^{-1}\) to \(-1076\) g C m\({}^{-2}\) a\({}^{-1}\), while varying dramatically from site to site (Table 2). The GPP and Re values ranged from \(-1919\) g C m\({}^{-2}\) a\({}^{-1}\) to \(-3703\) g C m\({}^{-2}\) a\({}^{-1}\) and \(1022\) g C m\({}^{-2}\) a\({}^{-1}\) to \(2082\) g C m\({}^{-2}\) a\({}^{-1}\), respectively. The significant differences in GPP and Re between sites depend on the location, plant species, environmental conditions [8], and the evaluation method [64]. The NEE values of restored mangroves in Leizhou, China, and Sawi Bay, Thailand, are comparable to those of natural mangroves (Table 2). Compared with the disturbed mangroves in Matang, Malaysia, and Beihai, China, the above two restored mangroves showed higher NEE values. Previous studies also showed that relative to degraded conditions, restored costal wetland sites stored significantly greater amounts of carbon, suggesting that restoration is a useful management approach for offsetting greenhouse gas (GHG) emissions [63]. However, the observed annual NEE values found in this study were relatively low and even had positive values in the first 2 years (Table 1). This may be attributed to the restored age and human management [21]. A previous study from a subtropical mangrove Kandelia candel forest in Japan illustrated that the annual differences between GPP and canopy respiration had a maximum value at an optimum LAI of 4.54 m\({}^{2}\)[45]. The LAI values of the mangroves in Nansha fluctuated by approximately 0.9 (Figure 2f),  
Figure 5: **(a)** The relationships between GPP and Ra, the color scale represents the values of LAI; (**b**) the relationships between GPP and LAI, the color scale represents the values of PI; (**c**) the relationships between Re and Ta, the color scale represents the values of PI; and (**d**) the relationships between CH\({}_{4}\) and Ta, the color scale represents the values of PI. which corresponded to a relatively small difference between GPP and canopy respiration [45] and resulted in low NEE values (Tables 1 and 2). Based on global analysis, the maximum mangrove biomass carbon sequestration capacity was reached between 15 and 40 years after regeneration [20]. As of 2022, the recovery age of the mangroves in Nansha was 14 years, and it may be expected to have a stronger capability to sequester carbon in the future. In addition, compared with the mangroves with high planting densities in Leizhou, China [65], the mangroves in the Nansha Wetland Park have a low density and are surrounded by small ponds (Figure 2h). Respiration is the summation of the result from the vegetation and open water. Small ponds act as significant CO\({}_{2}\) sources, which may contribute to the lower NEE values in the Nansha Wetland Park [66]. CH\({}_{4}\) emissions partially offset \"blue carbon\" burial in mangroves. Based on global analysis, a previous study showed that high CH\({}_{4}\) emission rates have the potential to partially offset blue carbon burial rates in mangrove sediments on average by 20% based on the 20-year global warming potential [6]. The offsets differ greatly from site to site.