Organic matter dynamics of coastal peat deposits in Sumatra, Indonesia

Organic matter dynamics were investigated in the surface peat layer (acrotelm) in study sites traversing three raised ombrotrophic peat deposits, containing up to 3, 6 and 12 m, respectively, of peat located on the east coast of Sumatra, Indonesia. The three deposits were uniform in climate, topography, surficial geology and were under continuous forest cover. Increased peat depth and distance from the edge of the deposit was associated with important changes in species composition, structure and morphology. To account for the differences in peat depth, I hypothesized that the relative importance of: 1) peat age, 2) organic matter decomposition and 3) litter additions, in controlling peat accumulation varies among the three deposits. Age differences, using 1 4C dating, did not account for variable peat accumulation. Peat at the clay-peat interface was approximately 4000 to 4500 years old in each study site, while acrotelm peat ranged from 45 to 660 years old. The relatively recent age of the acrotelm layer suggested that peat accumulation in the study sites was either at steady state or expanding. Older peat in this layer would have indicated surface degradation of the raised peat deposits. Small roots and root fragments in the acrotelm of the deep peat deposit were considerably younger than the matrix of amorphous peat. Samples of acrotelm peat were incubated under aerobic conditions for 30-day periods in glass jars. There were no significant differences in peat respiration rates between samples at different moisture levels. Significantly higher respiration rates, however, were measured in acrotelm peat from the 12 m deposit compared with the same layer in the 9 and 6 m sites in the same deposit and in the sites on the 3 and 6 m deposits. Buried cotton strips disappeared at the same rate at all study sites. However, the disappearance of leaf litter from mesh bags was most rapid in the 3 m site and slowest in the 12 m site. Decay rates were mainly controlled by varying organic matter quality due to species composition differences across the gradient of increasing peat depth. Several chemical parameters were significantly correlated with indices of litter and peat decay in the following order of importance: soluble C fraction > lignin:N > C:N > P. Litter quality in the study sites was generally low compared to other tropical forests on nutrient poor soils. Organic matter additions varied between the three peat deposits. Rates of small and fine litterfall declined significantly while small and fine root mass was increased across the gradient of increasing peat depth. Preliminary measurements of root growth into mesh bags of root-free peat indicated higher production of small roots in the acrotelm of the 9 and 12 m peat sites. A continuous 20-40 cm thick mat of fine and small roots present in the 12 m site restricted aboveground litter fall from being preserved in the peat matrix below the root mat. The presence of the root mat suggested that aboveground organic additions contribute less to peat accumulation with increasing peat depth. High water table levels were important in controlling peat accumulation and decay in the 3 and 6 m peat sites, while resource quality appeared more important in the 9 and 12 m sites. The results suggested that the increases in peat mass among the study sites were attributed mainly to increased additions of fine and small roots at the base of the acrotelm, rather than slower rates of aboveground litter decay at the top of the acrotelm. The study concludes that several of the assumptions of the two-layer model for accumulation in Sphagnum peatlands do not apply directly to the deposits of woody peat in East Sumatra. The results were consistent, however, with a key assumption of the Sphagnum peat model that continued accumulation of peat is due to an increase in the mass entering the catotelm layer. The greater input of small roots of poor resource quality appeared to be the most important process contributing to peat accumulation among the study sites. The high root inputs, however, also appeared to promote the cessation of peat accumulation in the 12 m peat deposit. This and further studies should provide a better basis for more selective management of vegetation (species composition, stand structure, tree morphology, etc.) and environmental (moisture, temperature) variables in Sumatran peat deposits.
  • Authors: Brady, M.A.
  • Subjects: soil organic carbon, peat, peat soils, coastal areas, soil depth
  • Publication type: Thesis
  • Year: 1997
  • DOI:
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