Mangrove forests are among the most carbon-rich ecosystems in the world, containing >1,000 Mg of carbon per hectare, which is 3–4 times greater than typical terrestrial forests. The vast majority (ca. 71–98%) of the carbon in mangroves is stored in the soil. Despite the global significance of mangrove forests in mitigating climate change, the dynamics root production in mangroves remain underestimated, due to the practical challenges of field measurements.
Here, we addressed this challenge by combining the ingrowth-core method and litter-bag method, which allows us to estimate the production and decomposition of roots. By installing and monitoring multiple ingrowth cores and litter bags in the field across seasons, we aimed to reveal the spatio-temporal dynamics in soil carbon.
We conducted this study in the Sundarbans mangrove forest in Bangladesh, the world’s largest mangrove forest. We established five 100 m² (10 m × 10 m) plots and measured the productivity of fine roots (< 2 mm), which comprise a large component of net primary productivity in mangrove forests, by using ingrowth cores (cylindrical mesh tube with an opening of 15 mm² allowing roots and microorganisms to enter) and litter bags (roots included in 120 μm nylon mesh with a size of 6 × 5 cm). We buried a total of 75 ingrowth cores and 75 litter bags in soil and collected in three consecutive seasons.
We found that the average fine root biomass productivity in the mangrove study site was be 213.48 g m⁻² year⁻¹. The rates of fine root production varied across space. Seasonal fine root production exhibited a cyclic trend, with rates of 7.1 kg ha⁻¹ day⁻¹ during the monsoon season, 5.7 kg ha⁻¹ day⁻¹ in summer, and 4.7 kg ha⁻¹ day⁻¹ in winter, potentially influenced by soil temperature and rainfall.
We observed remarkably higher below-ground fine root biomass productivity in the mangrove forests (4.7–7.1 kg ha⁻¹ day⁻¹) compared to that in terrestrial forest ecosystems (e.g., ~1 kg ha⁻¹ day⁻¹). This high production rate provides us with insights about the capacity of mangrove forests that can store substantial amounts of below-ground carbon. Our findings underscore the importance of seasonally repeated measurements of root dynamics to better understand below-ground biomass accumulation. Our results call for continued effort for conserving mangrove forests, which play a critical role as a global carbon sink and in climate change mitigation.