Variations in the rate of accumulation and chemical structure of soil organic matter in a coastal peatland in Sarawak, Malaysia

To determine the variation in the rate of accumulation and chemical structure of soil organic matter (SOM) in a tropical coastal peatland, series of soil cores samples were collected in the Maludam National Park, Sarawak, Malaysia and analyzed. Duplicate soil core samples were collected from three phasic communities in the peat swamp forests, Mixed Peat Swamp (MPS; depth of 50–450 cm), Alan Batu (ABt; depth of 200–700 cm), and Alan Bunga (ABg; depth of 200–800 cm), which were located at outer, middle, and inner sites on the peat dome. The 14C age of the SOM was determined at depths of every 50 or 100 cm and 4–6 samples from each profile were subjected to ramp cross polarization/magic angle spinning 13C nuclear magnetic resonance (NMR) analysis. The 14C age of SOM ranged from 1602 to 5162 years before the present (yBP), 1134–4043 yBP, and 928–3376 yBP in the cases of the MPS, ABt, and ABg forests, respectively. Those after calibration were in the ranges of 1482–5958 (MPS), 1322–4603 (ABt), and 837–3629 (ABg) yBP. The relationship between calibrated 14C age and soil depth, when regressed to a linear function with the constraint that the age of surface sediment is 0 year, showed that the rate of peat accumulation was 0.60 ± 0.01 (MPS), 1.5 ± 0.0 (ABt), and 1.9 ± 0.1 (ABg) mm y−1. However, in the case of the MPS forest, this rate was not constant but decreased toward the present. The 13C NMR results showed the decrease in the relative abundance of alkyl C and an increase in that of carbohydrate C with soil age/depth in the MPS forest soil profiles, suggesting an acceleration in the decomposition of SOM as the cause of the decrease in the rate of accumulation of the peat. Variations in the C composition of the other two forest soils with soil age were small. The relative content of aromatic C remained relatively constant, indicating that aromatic C is consistently a major component of the C pool throughout the peat soil layers.