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

Abstract

[1] 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 (d 13 C R ) in order to better understand the processes controlling ecosystem isotope discrimination. In this paper we synthesize emergent patterns in d 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 d 13 C R . These include the use of regression calculations that account for error in the x variable, and constraining estimates of d 13 C R to nighttime periods. We then recalculate d 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 À19.0 to À32.6%. Mean C 3 ecosystem discrimination was 18.3%. Precipitation was a major driver of both temporal and spatial variability of d 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 d 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 d 13 C R as a useful tool for integrating environmental effects on dynamic canopy and ecosystem processes.

Mixing ratio and carbon isotopic composition investigation of atmospheric CO 2 in Beijing, China

“…The δ 13 C S differences in different cities are related to local energy usage patterns and plant photosynthetic pathways. In general, the stable carbon isotope composition of ecosystem respiration within C 3 ecosystems ranged from − 19.0‰ to − 32.6‰ with an average value of − 26.2 ± 0.2‰ (Pataki et al, 2003b), and a mean value of − 12‰ has been observed in C 4 ecosystems (Fassbinder et al, 2012). The δ 13 C values of coal, gasoline and natural gas combustion ranged between − 27 to − 25‰, − 32 to − 28‰ and − 43.8 to − 37‰, respectively (Wada et al, 2011;Bush et al, 2007;Clark-Thorne and Yapp, 2003;Takahashi et al, 2002).…”

“…The basic assumption of the Keeling plot method is that the isotopic compositions of the background and source CO 2 do not change over the observation period (Pataki et al, 2003b). where δ 13 C is the isotopic composition of CO 2 ; [CO 2 ] represents the CO 2 mixing ratio; and the subscripts t, b and s indicate the total CO 2 , background CO 2 and additional CO 2 produced by the source, respectively.…”

“…δ 13 C s is the y-intercept when 1/[CO 2 ] t is zero. Geometric mean regression was used for the Keeling plot intercept estimation (Pataki et al, 2003b;Bowling et al, 2002). The intercept values were removed based on the following quality criteria: (1) the range of CO 2 mixing during the measured period was less than 50 ppm and (2) the standard error of the intercept was greater than 1‰ (Schaeffer et al, 2008).…”

A non‐native invasive grass increases soil carbon flux in a Hawaiian tropical dry forest

“…The 2% enrichment assumption for mineral soil end member designation is based on: (i) a nearby (o5 km), uninvaded dry forest with similar vegetation (see Litton et al, 2006), where the isotopic signature of bulk mineral soil is 2.04% enriched compared with in situ tree foliage (C. M. Litton, unpublished data); and (ii) the commonly observed phenomenon in forest ecosystems that mineral soil is enriched 1-3% compared with foliage (Ehleringer et al, 2000). We also quantified the grass (C 4 ) contribution to R soil using a Keeling plot approach (Pataki et al, 2003) on two locations in one plot of each treatment in July of 2004. A custom soil chamber (0.02 m 3 ) was placed over the soil surface and sealed with an O-ring made from nylon hose filled with fine sand.…”

Flow rate and source reservoir identification from airborne chemical sampling of the uncontrolled Elgin platform gas release

“…This technique needs rapid sampling during the brief fly-through. Figure 8 shows  13 C CH4 5 versus 1/CH 4 in air samples from the first two flights, following the Keeling plot methodology of (Pataki et al, 2003). The source gas has  13 C CH4 of -42.3±0.7 ‰ (±2 using geometric mean regression and a Monte Carlo style simulation to determine the propagation of errors into the fitting process where a geometric mean regression defines a line whose intercept on the  13 C CH4 axis gives the end-member source value.…”