Estimation of total fine root production using continuous inflow methods in tropical mangrove forest on Pohnpei Island, Micronesia: Fine root necromass accumulation is a substantial contributor to blue carbon stocks

Abstract

Fine root decomposition, mortality, and production are fundamental components of belowground carbon dynamics in mangrove ecosystems. Mangroves are highly productive ecosystems with huge carbon stocks; however, information on the dynamics of submerged fine roots in mangrove soils is limited. To estimate fine root production in several sites of mangrove communities on Pohnpei Island, Micronesia, we estimated continuous inflows using a simple mass balance model to combine ingrowth core and litterbag experimental results. According to our findings, previously reported mangrove fine root production values were severely underestimated because the fine root decomposition process was disregarded during the experimental period. We estimated that annual fine root production up to 0.3 m in depth ranged from 13.8 (Bruguiera gymnorrhiza in a landward plot) to 56.3 Mg ha À1 year À1 (B. gymnorrhiza in a seaward plot). However, comparing the fine root production with respect to the unit of stem cross-section area increase in the sample tree, the fine root production of Rhizophora spp. was greater than that of other species. The annual fine root mortality and decomposition to annual fine root production was 70%-105% and 11%-52% of annual production, respectively.The annual fine root production estimated here was much higher than that previously reported. These findings begin to quantify the contribution and importance of highly productive mangrove fine roots for carbon sequestration in blue carbon ecosystems. Moreover, this study provides new insights into the organic carbon dynamics in mangrove soils with mangrove vegetation succession.

Functional organic matter components in mangrove soils revealed by density fractionation

“…Although data on in-situ fine root production in deep mangrove soils (>30 cm) is scarce, Arnaud et al (2021) recently showed that a major fraction of fine root production occurs deeper than 30 cm. Other recent papers have also reported fine root production down to a depth of 50-60 cm below ground in mangrove forests (Fujimoto et al, 2021;Ono et al, 2022). Our visual inspection also revealed abundant live and dead fine roots in deep (> 50cm) soils.…”

Findings from long‐term monitoring studies of Micronesian mangrove forests with special reference to carbon sequestration and sea‐level rise

“…In the process of this succession, the tree density of Rhizophora species decrease gradually, while the B. gymnorrhiza trees increase gradually in the III(2)a stage, and the X. granatum trees are also mixed in the III(2)b stage. Ono et al (2021) revealed that Rhizophora species allocated more productivity to fine root production compared with other species on Pohnpei. Fujimoto et al (2021) showed that the fine root production of R. stylosa was higher than that of B. gymnorrhiza , which had approximately twice the trunk diameter.…”

“…We also investigated fine root production, which has a large effect on mangrove peat accumulation, using ingrowth core and litterbag methods (Ono et al, 2021). The study found that Rhizophora species produced more fine roots than other mangrove species when comparing the ratio of annual fine root production to the annual increase in aboveground biomass.…”

Difference in Water CO₂ Dynamics between Riverine and Fringe Mangroves–A Case Study in Subtropical Mangroves

“…The Yaeyama Islands in Okinawa, in particular, are the northern limit of colonization by Rhizophora species. Rhizophora species are renowned for their remarkable capacity to sequester carbon due to their abundant production of very fine roots [47]. Nevertheless, there is a dearth of research concerning the variability of carbonate chemistry parameters (pCO2, TA, DIC) in mangrove waters within this region [40,48].…”