Origin, composition, and transformation of dissolved organic matter in tropical peatlands

Solid and dissolved organic matter (DOM) compositions were investigated in a pristine and a deforested tropical peat forest in Brunei Darussalam. A combination of elemental (%C, %N, C/N), isotopic (δ15N, δ13C, Δ14C), molecular (lignin phenol biomarkers) and optical (Specific UV Absorbance at 280 nm (SUVA280), fluorescence index (FI)) analyses were performed to characterize DOM in porewater and river water. The DOM composition was compared to vegetation and peat to assess DOM origin and transformations in the ecosystem. Significant relationships were observed between optical properties (SUVA280, FI) and bulk (Δ14C) and molecular (ratio of cinnamyl to vanillyl phenols, C/V) composition of DOM. The radiocarbon content of DOM shows that it is modern at both sites for both surface and deep (to 4.5 m) samples (Δ14C = 74.58 ± 8.53‰ at the pristine site and Δ14C = 87.84 ± 4.5‰ at the deforested site for the first 2 meters), indicating transport of young DOM to deeper layers. Stable carbon-13 content of vegetation, peat and DOM showed only slight fractionation and ranged from −32.2‰ to −28.3‰. Nitrogen showed a greater shift between ecosystem pools. Nitrogen-15 content was higher in the solid peat (δ15N = −0.92 ± 0.49‰ in the pristine site, δ15N = −1.22 ± 1.37‰ in the deforested site) than in DOM (δ15N = −4.2 ± 2.5‰ in the pristine site, δ15N = −4.1 ± 2.3‰ in the deforested site). This difference in δ15N is significantly correlated with the C/N values of peat and DOM and attributable to higher microbial degradation in the peat compared to DOM. Porewater DOM contains less lignin (λ8 = 1.10 ± 0.27 mg 100 mg dw−1 in the pristine site and λ8 = 1.38 ± 0.46 mg 100 mg dw−1 in the deforested site) than does the solid peat (λ8 = 5.66 ± 2.14 mg 100 mg dw−1 in the pristine site and λ8 = 10.29 ± 4.69 mg 100 mg dw−1 in the deforested site). All indicators of lignin degradation (P/(V + S), 3,5Bd/V, (Ad/Al)V, (Ad/Al)S) are significantly higher in DOM than in the solid peat. This shows that lignin can be processed rapidly in the porewater of upper layers of tropical peatlands. Logging activity affected the composition of organic matter at the deforested site. Higher lignin content (λ8) was observed in the solid peat, and the composition of DOM differed greatly in the deforested site: Δ14C, FI, 3,5Bd/V, (Ad/Al)V are significantly different from the pristine site. Thus it appears that the composition of DOM is more sensitive than the solid phase to the effects of land use change on organic matter dynamic in tropical peatlands.