Stream carbon fluxes are one of the major components in the global C cycle, yet the discrimination of the various sources of stream carbon remains to a large extent unclear and less is known about the biogeochemical transformations that accompany the transfer of C from soils to streams. Here, we used patterns in stream water and groundwater δ¹³C values in a small forested tropical headwater catchment to investigate the source and contribution from the soil carbon pools to stream organic and inorganic carbon behavior over seasonal scales. Stream organic carbon (DOC and POC) comes mainly from the upper rich soil organic carbon horizons and derived from total organic carbon (TOC) of biogenic source. The isotopic compositions δ¹³C_TOC, δ¹³C_DOC and δ¹³C_POC of these carbon species were very close (− 30‰ to − 26‰) and typical of the forested C3 vegetation. The relationship observed between DOC and log pCO₂ and δ¹³C_DIC indicated that besides the considerable CO₂ evasion that occurs as DIC is transported from soils to streams, there were also other processes affecting the stream DIC pool. In-stream mineralization of DOC and mixing of atmospheric carbon had a significant influence on the δ¹³C_DIC values. These processes which varied seasonally with hydrological changes represent the main control on DOC and DIC cycling in the wet tropical milieu. The rapid turnover of carbon on hillside soils, the transformation of TOC to DOC in wetland soils and further mineralization of stream DOC to DIC favor the evasion of C, making the zone a source of carbon to the atmosphere.

河流碳通量是全球碳循环的主要组成部分之一，但对不同来源的河流碳的区分在很大程度上仍不清楚，并且对伴随碳从土壤转移到河流的生物地球化学转化知之甚少。在这里，我们利用小型森林热带源头集水区溪流水和地下水 δ ¹³ C 值的模式来研究土壤碳库在季节性尺度上对溪流有机和无机碳行为的来源和贡献。溪流有机碳（DOC和POC）主要来自上层丰富的土壤有机碳层，来源于生物源的总有机碳（TOC）。这些碳物种的同位素组成 δ ¹³ C _TOC、δ ¹³ C _DOC和 δ ¹³ C _POC非常接近（− 30 ‰ 至 − 26 ‰），是森林 C3 植被的典型特征。观察到的 DOC 与 log pCO ₂和 δ ¹³ C _DIC之间的关系表明，除了 DIC 从土壤转移到溪流时发生大量 CO ₂逃逸之外，还有其他过程影响溪流 DIC 池。 DOC 的河内矿化和大气碳的混合对 δ ¹³ C _DIC值有显着影响。这些随水文变化而季节性变化的过程是潮湿热带环境中 DOC 和 DIC 循环的主要控制因素。山坡土壤中碳的快速周转、湿地土壤中 TOC 向 DOC 的转化以及溪流 DOC 向 DIC 的进一步矿化有利于 C 的逃逸，使该区域成为大气碳源。