Organic matter compositions and loadings in soils and sediments along the Fly River, Papua New Guinea

Abstract

The compositions and loadings of organic matter in soils and sediments from a diverse range of environments along the Fly River system were determined to investigate carbon transport and sequestration in this region. Soil horizons from highland sites representative of upland sources have organic carbon contents (%OC) that range from 0.3 to 25wt%, carbon:nitrogen ratios (OC/N) that range from 7 to 25mol/mol, highly negative stable carbon isotopic compositions (δ13Corg<−26‰) and variable concentrations of lignin phenols (1<LP<5mg/100mgOC). These compositions reflect inputs from local vegetation, with contributions from bedrock carbon in the deeper mineral horizons. Soils developed on the levees of active floodplains receive inputs of allochthonous materials by overbank deposition as well as autochthonous inputs from local vegetation. In the forested upper floodplain reaches, %OC contents are lower than upland soils (0.8–1.5wt%) as are OC/N ratios (9–15mol/mol) while δ13Corg (−25 to −28‰) and LP (2–6mg/100mgOC) values are comparable to upland soils. These results indicate that organic matter present in these active floodplain soils reflect local (primarily C3) vegetation inputs mixed with allochthonous organic matter derived from eroded bedrock. In the lower reaches of the floodplain, which are dominated by swamp grass vegetation, isotopic compositions were less negative (δ13Corg>−25‰) and non-woody vegetation biomarkers (cinnamyl phenols and cutin acids) more abundant relative to upper floodplain sites. Soils developed on relict Pleistocene floodplain terraces, which are typically not flooded and receive little sediment from the river, were characterized by low %OC contents (<0.6wt%), low OC/N ratios (<9mol/mol), more positive δ13Corg signatures (>−21‰) and low LP concentrations (∼3mg/100mgOC). These relict floodplain soils contain modern carbon that reflects primarily local (C3 or C4) vegetation sources. Total suspended solids collected along the river varied widely in overall concentrations (1<TSS<9000mg/L), %OC contents (0.1–60wt%), OC/N ratios (7–17mol/mol) and δ13Corg signatures (−26 to −32‰). These compositions reflect a mixture of C3 vascular plants and freshwater algae. However, little of this algal production appears to be preserved in floodplain soils. A comparison of organic carbon loadings of active floodplain soils (0.2 and 0.5mgC/m2) with previous studies of actively depositing sediments in the adjacent delta–clinoform system (0.4–0.7mgC/m2) indicates that Fly River floodplain sediments are less effective at sequestering organic carbon than deltaic sediments. Furthermore, relict Pleistocene floodplain sites with low or negligible modern sediment accumulation rates display significantly lower loadings (0.1–0.2mgC/m2). This deficit in organic carbon likely reflects mineralization of sedimentary organic carbon during long term oxidative weathering, further reducing floodplain carbon storage.