Understanding ¹⁴C dynamics: Local carbon cycle influences on isotopic disequilibrium and reservoir residence times

Radiocarbon (¹⁴C) is a powerful tracer of carbon cycling and source attribution, yet the influence of reservoir age and carbon turnover on local ¹⁴C dynamics remains poorly understood. To address this gap, we investigated ¹⁴C spatial variability across atmospheric, terrestrial, and aquatic reservoirs in two contrasting ecosystems: a coastal mangrove and an inland peri-urban forest, using samples analyzed by Accelerator Mass Spectrometry. Iso-fluxes of different reservoirs relative to the atmosphere were estimated and carbon turnover dynamics were derived using steady state modelling to evaluate reservoirs as sources or sinks of ¹⁴C. Results revealed clear carbon source partitioning: mangrove leaves exhibited enriched modern ¹⁴C signatures (Fm = 1.02) and positive isofluxes, demonstrating mangroves distinct role as hotspots of modern carbon fixation and active modulators of atmospheric Δ¹⁴CO₂, a novel outcome with implications for carbon budget modelling. In contrast, extremely negative Δ¹⁴C values and large isofluxes from coastal sediments and soils suggested significant export of pre-bomb or fossil carbon into biogeochemical contemporary cycles, highlighting land-ocean carbon coupling. Inland reservoirs displayed uniformly negative isofluxes, slower turnover, and aged carbon emissions. Comparative analysis indicated that stressed mangroves releases modern carbon, while inland ecosystems are dominated by aged carbon export, reflecting site-specific differences in vegetation, connectivity and anthropogenic influence. Turnover analysis showed clear differences in release rates constant (k) and residence times (τ), highlighting carbon cycle heterogeneity and the limits of steady-state models under transient conditions. Overall, findings demonstrate that ¹⁴C dynamics are highly reservoir and location-specific, with significant implications for local atmospheric ¹⁴C budgets.