Hydrogeomorphic response of charcoals during river transits and its impact on the carbon cycle

Abstract

Natural or anthropogenic-induced biomass burning produces large amounts of charcoals, which enter riverine or lacustrine systems mostly via surface runoff processes. Charcoal storage and cycling within large riverine systems can play a crucial role in mediating long-term carbon sink across transient reservoirs, thereby influencing the global carbon budget. However, natural processes governing the transport and storage of charcoal particles in large terrestrial reservoirs such as the Indo-Gangetic region still remain unknown. To understand charcoal movement and accumulation across upland and lowland transient reservoirs, we characterized spatial distribution and morphology of different charcoal forms (>125Mu m and <125Mu m) from bedload and floodplain sediments of the Yamuna sub-basin (YSB), India. Both >125?m and <125Mu m charcoal forms in bedload and floodplain sediments did not exhibit similar spatial distribution patterns, indicating segregation of charcoal particles influenced by variable flow regimes. Attrition with sediments breaks down fragile charcoals (leaves) quickly compared to the woody forms, resulting in dominant woody microforms in transient deposits. Higher stream power and limited stable bedform development in upland mountainous regions restrict charcoal storage. During lowland riverine transits, reduced stream power conditions allow increased floodwater inundation and finer clay substrate availability, facilitating an exponential increase in charcoal storage. However, increased discharge from peninsular rivers into the YSB lead to erosion and redistribution of sediment, including charcoal particles, and reduced charcoal storage in lowland transient areas. Such diverse dispersal pathways and fate of charcoal particles across riverine transits highlight the influence of regional hydrogeomorphic processes on the overall carbon cycle within transient reservoirs.