Allometric equations may underestimate the contribution of fine roots to mangrove carbon sequestration

The biomass and production of fine roots (diameter < 2 mm) are often omitted or derived simply from allometric equations when calculating mangrove carbon sinks. The purpose of this study was to assess the significance of fine roots by measuring the biomass and production of the fine roots in two mangrove species (Kandelia obovata and Avicennia marina) with distinct root structures. The aboveground portion of K. obovata and A. marina contributed 44% and 32% of the total carbon stock, respectively. The nonfine roots accounted for 17% of the total carbon stock of both mangrove species. The fine roots and dead fine roots of K. obovata contributed 5% of the total carbon stock, whereas the contribution of fine roots and dead fine roots of A. marina, which possessed pneumatophores, was higher (12%). Comparison of the field measurements with the estimates of belowground net production derived from frequently used allometric equations showed that equation-derived estimates were notably underestimated, particularly for A. marina. The aboveground net production of K. obovata and A. marina averaged 17.04 and 7.46 Mg C ha^-1 yr^-1, respectively, but 84% and 92% of the litterfall was lost after a year. Subtracting only 4% of the fine root production of K. obovata and 17% of the fine root production of A. marina to account for further decomposition in the soils within a year, an additional 13.67 Mg C ha^-1 yr^-1 for K. obovata and 11.05 Mg C ha^-1 yr^-1 for A. marina were calculated to be buried in the soils, which can increase the carbon sequestration capacity estimated from aboveground litterfall only by 5 and 19 times, respectively, for each mangrove species. This suggests that the contribution of fine roots should be accounted for when estimating mangrove carbon sinks.