Tropical rivers deliver ∼60% of particulate organic carbon to the world ocean. However, compositions and loadings of sedimentary organic matter (OM) from tropical small mountainous rivers are largely unknown. Here, we provide an initial constrain on sources of sedimentary OM from 28 fluvial systems across Luzon in the Philippines by measuring total organic carbon (TOC), total nitrogen (TN), stable carbon isotope (δ¹³C) and radiocarbon activity of TOC (expressed as fraction modern—F_m), as well as grain size and mineral surface area (SA) of sediments. Results indicate that sediments in Luzon rivers contain both contemporary and ¹⁴C-depleted OM (F_m: 0.71–1.06, mean 0.97 ± 0.07) with a wide range of δ¹³C values (−28.3‰ to −17.7‰, −24.9 ± 2.2‰). This is attributed to the OM sources originated from modern surface soil and ¹⁴C-depleted subsoil and deep soil vegetated by C3 and C4 plants, with mean fraction of C3 plant at 80% ± 11%. Minor input from bedrock may also contribute to the ¹⁴C-depleted OM in sediments, accounting for 6% ± 6%. Sediments in most rivers are featured by low OC loadings (OC/SA ratio < 0.4 mg C m⁻²), owing either to a less OM input or intensive OM degradation. The estimated yields of particulate OC from Luzon vary between 3.2 and 3.7 t km⁻² yr⁻¹, which is higher than most tropical large rivers.

中文翻译：

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菲律宾潮湿热带火山吕宋岛河流沉积物中的有机物组成和负荷

热带河流向世界海洋输送了约 60% 的颗粒有机碳。然而，热带小型山区河流的沉积有机质 (OM) 的组成和负荷在很大程度上是未知的。在这里，我们通过测量总有机碳 (TOC)、总氮 (TN)、稳定碳同位素 (δ ¹³ C) 和 TOC 的放射性碳活动，初步限制了菲律宾吕宋岛 28 个河流系统中沉积 OM 的来源。表示为现代分数——F _m )，以及沉积物的粒度和矿物表面积 (SA)。结果表明，吕宋河沉积物同时含有当代和¹⁴ C 耗尽的 OM ( F _m : 0.71–1.06, 平均值 0.97 ± 0.07)，δ 范围很广¹³ C值（-28.3‰至-17.7‰，-24.9±2.2‰）。这归因于来自现代表层土壤和¹⁴ C 耗尽的底土和 C3 和 C4 植物植被的深层土壤中的 OM 来源，其中 C3 植物的平均分数为 80% ± 11%。来自基岩的少量输入也可能导致沉积物中¹⁴ C 耗尽的 OM，占 6% ± 6%。大多数河流中的沉积物的 OC 负载量较低（OC/SA 比率 < 0.4 mg C m ^-2），这是由于 OM 输入较少或 OM 降解严重。来自吕宋岛的颗粒有机碳的估计产量在 3.2 到 3.7 t km ^-2  yr ^-1之间变化，高于大多数热带大河。