Carbon isotopes in terrestrial ecosystem pools and CO 2 fluxes

Summary Stable carbon isotopes are used extensively to examine physiological, ecological, and biogeochemical processes related to ecosystem, regional, and global carbon cycles and provide information at a variety of temporal and spatial scales. Much is known about the processes that regulate the carbon isotopic composition (δ 13 C) of leaf, plant, and ecosystem carbon pools and of photosynthetic and respiratory carbon dioxide (CO 2 ) fluxes. In this review, systematic patterns and mechanisms underlying variation in δ 13 C of plant and ecosystem carbon pools and fluxes are described. We examine the hypothesis that the δ 13 C of leaf biomass can be used as a reference point for other carbon pools and fluxes, which differ from the leaf in δ 13 C in a systematic fashion. Plant organs are typically enriched in 13 C relative to leaves, and most ecosystem pools and respiratory fluxes are enriched relative to sun leaves of dominant plants, with the notable exception of root respiration. Analysis of the chemical and isotopic composition of leaves and leaf respiration suggests that growth respiration has the potential to contribute substantially to the observed offset between the δ 13 C values of ecosystem respiration and the bulk leaf. We discuss the implications of systematic variations in δ 13 C of ecosystem pools and CO 2 fluxes for studies of carbon cycling within ecosystems, as well as for studies that use the δ 13 C of atmospheric CO 2 to diagnose changes in the terrestrial biosphere over annual to millennial time scales. Contents Summary 1 I. Introduction 2 II. Isotopic composition of plant metabolites and autotrophic respiration 3 III. Isotopic composition of plant and ecosystem carbon pools 8 IV. Isotopic composition of assimilation and respiration fluxes 9 V. Ecosystem to global scale issues – isotopic composition of gross primary production, net primary production, and net ecosystem production 11 VI. Conclusions and suggestions for further research 12 Acknowledgements 13 References 13