[M-IS03] Exploring the role of soil in earth science: ecological/biogeochemical linkage and beyond

[MIS03-P05] Accumulation processes of organic carbon in peat deposit in a tropical mangrove forest on Pohnpei Island, FSM.

*Kenji ONO¹, Syuntaro HIRADATE², Kiyoshi FUJIMOTO³, Yasumasa HIRATA¹, Masakazu HIRAIDE¹, Ryuuichi TABUCHI¹, Saimon LIHPAI⁴ (1.Forestry and Forest Products Research Institute, 2.National Institute for Agro-Environmental Sciences, 3.Nanzan University, 4.Department of Land and Natural Resources, Pohnpei State Government)

Keywords:peat deposit, organic carbon, 14C dating, coral reef-type mangrove forest, solid-state 13C CPMAS NMR

To quantitatively evaluate organic carbon accumulation processes in peat deposits on a tropical mangrove ecosystems, solid-state ¹³C cross-polarization and magic angle spinning nuclear magnetic resonance (CPMAS NMR) signals were monitored to determine the organic carbon composition of humified leaf and root, which account for a large majority of mangrove-produced litters and mangrove peat in a coral reef-type Rhizophora forest on Pohnpei Island, Federated States of Micronesia. Radiocarbon dating was also used to estimate the average turnover times of peat deposits at each depth. The mass loss rate of mangrove leaves during humification was much higher than that of roots. Mass loss rates of mangrove leaves and roots are expected to be affected by their varying chemical characteristics and the different aerobic/hydrological conditions present in the two litter types during humification processes. The decomposability of individual organic carbon components also varied markedly between leaf and root litters. Significant increases in aryl-C/O-alkyl-C and aliphatic-C/O-alkyl-C ratios and minor increases in aryl-C/aliphatic-C ratio during the humification of leaves implied that only the O-alkyl-C component was relatively labile compared with aryl- and aliphatic-C, and that the decomposability of aliphatic-C was also slightly higher than that of aryl-C, while the difference was not significant in leaf litters on the forest floor. Regarding roots, a stable aryl-C/O-alkyl-C ratio during humification suggests that the decomposability of aryl- and O-alkyl-C components did not differ greatly in the peat deposit, while the concomitant minor increase in the aliphatic-/O-alkyl-C ratio and the substantial decrease in the aryl-/aliphatic-C ratio with humification imply that aliphatic-C was more recalcitrant than aryl- and O-alkyl-C in the peat. Conversely, the compositional properties of organic carbon and the ages of ¹⁴C of the peat deposit were quite homogenous and relatively modern throughout the peat profile, suggesting that large amount of mangrove roots penetrate up to at least 80-cm depth. These findings provide quantitative and qualitative insights into the potential importance of very high production of mangrove fine roots for organic carbon accumulation in peat in tropical mangrove ecosystems.