Unraveling the mechanistic interplay of carbon cycle and nitrogen transport within deep reservoirs: A perspective of δ13CDIC analysis

In the Lancang River Watershed (LRW), the establishment of substantial reservoirs has been implicated in the perturbation of biogeochemical carbon-nitrogen cycles, as well as their environmental repercussions, across both temporal and spatial dimensions. This work involved the procurement of water samples from various points within the Nuozhadu Reservoir (NZD). During the rainy season, pronounced alterations in stratification were observed: the surface mixed layer ranging from 0 to 5 m in depth; the thermocline layer extended from 5 to 40 m; and the profundal zone, spanning from 40 m to the riverbed, was designated as the stagnant temperature layer. The spatiotemporal heterogeneity of dissolved inorganic carbon (DIC) and its isotopic composition (δ¹³C_DIC) was pronounced, with δ¹³C_DIC values spanning from −9.04 ‰ to −3.00 ‰. Spatially, the concentrations of DIC exhibited an increment with increasing depth across various vertical profiles. This variation was attributed to the thermal stratification within reservoirs, which modulated the intensity of photosynthetic carbon fixation. The nitrogen content exhibited a parallel trend to DIC concentration, with lower values in the upper layers and elevated levels in the deeper strata. This work revealed that photosynthesis and carbonate sedimentation might be an important carbon sequestration process in the large reservoir. Future research endeavors should focus on elucidating the migratory and transformational dynamics of carbon and nitrogen elements, to enhance the management and environmental surveillance strategies for deep reservoirs.