Vegetation marginally offsets the CO2 emissions from dry inland waters

Exposed sediments from dry inland waters are an important component of the global carbon cycle, and their extent is increasing worldwide due to climate change and intensified human water use. These dry sediments often become colonized by terrestrial vegetation, which can counterbalance the mineralization of exposed organic matter through photosynthesis, thereby reducing overall CO₂ emissions. However, current CO₂ flux estimates from dry sediments are largely derived from bare sediments, meaning that the potential role of vegetation has been overlooked. To assess the role of vegetation in modulating CO₂ fluxes from dry sediments, we conducted a global study across 164 dry inland waterbodies (including lakes, ponds, reservoirs, streams, and wetlands) spanning a wide range of climatic regions from arid to polar. At each site, we measured CO₂ fluxes from vegetated and bare dry sediments using a standardized chamber-based method under two conditions: dark (capturing respiration only) and light (capturing both respiration and photosynthesis). On average, within vegetated zones, vegetation occupied 47 ± 35% in measured biomass quadrants.

Our results showed that under light conditions, instantaneous CO₂ fluxes were lower in vegetated sediments (mean ± SD = – 3.7 ± 12.9 mmol CO₂ m⁻² h⁻¹) compared to bare sediments (5.4 ± 12.7 mmol CO₂ m⁻² h⁻¹), suggesting that photosynthesis contributed to decrease CO₂ emissions to the atmosphere. In contrast, under dark conditions, vegetated sediments exhibited larger positive CO₂ fluxes (14.7 ± 20.1 mmol CO₂ m^-2 h^-1) than bare sediments (5.4 ± 8.2 mmol CO₂ m^-2 h^-1), likely due to plant respiration. Across ecosystem types and climatic zones, average net CO₂ emissions over a full diel cycle were 25% (± 358) lower from vegetated than from bare sediments, indicating that vegetation can partially offset sediment respiration.

Upscaling these fluxes to the ecosystem level considering vegetation cover, revealed that all waterbody types still function as net carbon sources. When exploring the potential effect of vegetation on previously published estimates only based on bare sediments, we found that global CO₂ fluxes from dry sediments could be suppressed by 10% (± 111%) due to the effect of vegetation.