Quantifying ecosystem‐atmosphere carbon exchange with a 14C label

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Abstract
The role of terrestrial ecosystems as sources or sinks for carbon to the atmosphere and their contribution to inter‐annual variations in atmospheric CO2 remain hotly‐debated topics. Carbon enters terrestrial ecosystems through a single process, photosynthesis, but it is returned to the atmosphere by the combined metabolic activity of plants, animals, and microbes (Figure l). The largest uncertainties in our understanding of terrestrial carbon cycling are in these return processes, especially how CO2 losses from ecosystems are divided among respiration by living plants—termed autotrophic respiration—versus microbial and faunal decomposition of plant residues—termed heterotrophic respiration (Figure 1); and how seasonal and climatic factors that change plant physiological status and soil conditions influence that partitioning.