The application and interpretation of Keeling plots in terrestrial carbon cycle research

Abstract

Photosynthesis and respiration impart distinct isotopic signatures to the atmosphere that are used to constrain global carbon source/sink estimates and partition ecosystem fluxes. Increasingly, the "Keeling plot" method is being used to determine the carbon isotope composition of ecosystem respiration (δ 13 C R ) in order to better understand the processes controlling ecosystem isotope discrimination. In this paper we synthesize emergent patterns in δ 13 C R by analyzing 146 Keeling plots constructed at 33 sites across North and South America. In order to interpret results from disparate studies, we discuss the assumptions underlying the Keeling plot method and recommend standardized methods for determining δ 13 C R . These include the use of regression calculations that account for error in the x variable, and constraining estimates of δ 13 C R to nighttime periods. We then recalculate δ 13 C R uniformly for all sites. We found a high degree of temporal and spatial variability in C 3 ecosystems, with individual observations ranging from - 19.0 to - 32.6‰. Mean C 3 ecosystem discrimination was 18.3‰. Precipitation was a major driver of both temporal and spatial variability of δ 13 C R , suggesting (1) a large influence of recently fixed carbon on ecosystem respiration and (2) a significant effect of previous climatic effects on δ 13 C R . These results illustrate the importance of water availability as a key control on atmospheric 13 CO 2 and highlight the potential of δ 13 C R as a useful tool for integrating environmental effects on dynamic canopy and ecosystem processes.