Response of macrophyte litter decomposition in global blue carbon ecosystems to climate change

Blue carbon ecosystems (BCEs) are important nature-based solutions for climate change--mitigation. However, current debates question the reliability and contribution of BCEs under future climatic-scenarios. The answer to this question depends on ecosystem processes driving carbon-sequestration and -storage, such as primary production and decomposition, and their future rates. We performed a global meta-analysis on litter decomposition rate constants (k) in BCEs and predicted changes in carbon release from 309 studies. The relationships between k and climatic factors were examined by extracting remote-sensing data on air temperature, sea-surface temperature, and precipitation aligning to the decomposition time of each experiment. We constructed global numerical models of litter decomposition to forecast k and carbon release under different scenarios. The current k averages at 27 +/- 3 x 10(-2) day(-1) for macroalgae were higher than for seagrasses (1.7 +/- 0.2 x 10(-2) day-1), mangroves (1.6 +/- 0.1 x 10(-2) day(-1)) and tidal marshes (5.9 +/- 0.5 x 10(-3) day(-1)). Macrophyte k increased with both air temperature and precipitation in intertidal BCEs and with sea surface temperature for subtidal seagrasses. Above a temperature threshold for vascular plant litter at similar to 25 degrees C and similar to 20 degrees C for macroalgae, k drastically increased with increasing temperature. However, the direct effect of high temperatures on k are obscured by other factors in field experiments compared with laboratory experiments. We defined fundamental and realized temperature response to explain this effect. Based on relationships for realized temperature response, we predict that proportions of decomposed litter will increase by 0.9%-5% and 4.7%-28.8% by 2100 under low-( 2 degrees C) and high-warming conditions (4 degrees C) compared to 2020, respectively. Net litter carbon sinks in BCEs will increase due to higher increase in litter C production than in decomposition by 2100 compared to 2020 under RCP 8.5. We highlight that BCEs will play an increasingly important role in future climate change-mitigation. Our findings can be leveraged for blue carbon accounting under future climate change scenarios.

Automated summary

Blue carbon ecosystems (BCEs) serve as crucial nature-based solutions for mitigating climate change, but their reliability and contribution under future climatic scenarios have been questioned. This study conducted a global meta-analysis to investigate litter decomposition rate constants (k) in BCEs and predict changes in carbon release. The results showed that k varied among different types of BCEs and increased with temperature and precipitation. By considering temperature thresholds, the study predicted an increase in decomposed litter and net litter carbon sinks by 2100 compared to 2020. These findings emphasize the growing importance of BCEs in climate change mitigation and provide insights for blue carbon accounting in future scenarios.