Root exudates represent a critical belowground carbon flux; however, direct field-based quantification of these rates on intact mangrove roots remains limited due to methodological challenges. Here, we present, to our knowledge, the first in situ evaluation of root exudation rates in a subtropical Bruguiera gymnorhiza forest in Japan, employing a modified cuvette method specifically designed for field measurements on intact root systems. The net root exudation rates measured in artificial seawater at depths of 0–60 cm ranged from 0.01 to 0.97 mg C g−1 h−1, with a mean of 0.22 mg C g−1 h−1. Although this mean rate was comparable to values reported for tropical terrestrial forests, the spatiotemporal variation exhibited variable site-specific patterns. At the midstream site, exudation rates were closely coupled with fine root biomass under nitrogen-limited conditions and peaked during summer. In contrast, the upstream site exhibited unusually high exudation rates during winter, even in deep soil layers. Furthermore, contrary to patterns typically observed in terrestrial forests, exudation rates showed positive correlations with root C:N ratios and proton efflux. These findings suggest that root exudation in mangroves is regulated by complex interactions among site-specific hydrological regimes and stress-adaptation mechanisms, particularly salinity tolerance and nutrient acquisition, rather than by simple growth trade-offs. When integrated over a depth of 0–60 cm, the estimated annual root exudate carbon flux was approximately 0.4 kg C m−2 yr−1. This likely represents a conservative lower-bound estimate because fine root systems extend well below this depth in mangrove forests. Our results strongly suggest that root exudates constitute an important, previously under-recognized component of the “missing carbon” in mangrove ecosystems and underscore the need to explicitly incorporate this flux into blue carbon models to more accurately evaluate mangrove carbon sequestration capacity.
Kato et al. (Sat,) studied this question.