Radiocarbon and the Transit Time of Carbon in Terrestrial Ecosystems

Purpose of Review The transit time of carbon quantifies the time that it takes carbon atoms to travel through an ecosystem, from fixation of atmospheric CO 2 via photosynthesis until loss of carbon, mostly via respiration by plants and microorganisms, but also by methane emissions, and by leaching as dissolved inorganic or organic matter. Transit times are relevant to predict the future behavior of carbon sinks and how they would respond to changes in the environment. As most carbon is lost to the atmosphere as CO 2 , measurements of radiocarbon in respired carbon dioxide can be used to approximate the mean transit time of carbon in ecosystems. We review here an increasing number of studies that use radiocarbon to obtain the age of respired carbon from ecosystems, and their use as a constraint for carbon cycle models. Recent Findings Measurements of radiocarbon from vegetation pools and soils indicate a mixture of ages of carbon in ecosystem respiration. For example, respiration to support metabolism and growth in trees comes from very recently fixed substrates in leaves, but organs like stems and roots can mix recent substrates with older storage reserves. From detrital necromass and soils, the age of respired carbon integrates large variations in the age of carbon in substrates available for decomposition, ranging from less than one year (leaf litter in tropical systems) to a few hundred years for slowly decaying wood, or carbon stabilized by mineral sorption. Integrated at the soil or ecosystem level, the age of respired carbon is generally a highly ‘right’ skewed distribution, with most CO 2 released through faster cycling processes (plant respiration and rapid decomposition), but with a small component derived from very slow processes (slower decomposition). Thus, the mean age of respired CO 2 can be one to two decades old, while the median age can be much younger. Linking measured radiocarbon to modeled processes requires translation of either measured values to transit times or prediction of radiocarbon by models. Summary Evidence from radiocarbon observations and models shows that the distribution of transit times of carbon in terrestrial ecosystems is key to understand what fraction of fixed carbon will be respired quickly, what fraction can be stored for decadal to century timescales, and what fraction can accumulate over longer timescales and influence long-term carbon storage.