Hydrogeomorphic Response of Charcoals During River Transits and Its Impact on the Carbon Cycle.

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 (>125 μm and <125 μm) from bedload and floodplain sediments of the Yamuna sub‐basin (YSB), India. Both >125 μm and <125 μm charcoal forms in bedload and floodplain sediments did not exhibit similar spatial distribution patterns, indicating the 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 leads 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. Plain Language Summary: Rivers and wind carry a lot of charcoal, which comes from burned plants, to temporary storage places. Rivers play an essential role in charcoal distribution across mountain and lowland regions before it ends up to the ocean. Impact of river in charcoal movement is an important component of nature that helps in the recycling of carbon globally and this process still remains poorly understood. This study attempts to understand the natural response of charcoal particles across the Yamuna River (India) and its tributaries. It is observed that the river flow influences the spatial distribution of smaller charcoals (<125 μm) more than bigger (>125 μm) charcoal. Higher river flow provided less time to settle charcoals in the mountain areas, allowing limited charcoal accumulation. In lowland areas, charcoal tends to gather where there is more sand and dirt piling up. But when lots of water rushes in from rivers, it can wash away the charcoal, making it harder for charcoals to stay in one place. Such responses of charcoal particles to complex river actions, such as sediment types, river flow, and landscape, impact the storage of charcoal particles and can contribute to variation in the carbon recycling across Earth surface. Key Points: Regional geomorphic and hydrodynamic processes influence charcoal segregation in large river complexesSusceptibility of charcoals to different flow regimes impacts distribution of carbon fluxes in transient areasTransient lowlands favored more charcoal storage than upland areas due to higher availability of finer substrate in lowland floodplains [ABSTRACT FROM AUTHOR]