Partitioning net ecosystem carbon exchange into net assimilation and respiration using ¹³CO₂ measurements: A cost‐effective sampling strategy

Abstract

The current emphasis on global climate studies has led the scientific community to set up a number of sites for measuring the long‐term biosphere‐atmosphere net CO2 exchange (net ecosystem exchange, NEE). Partitioning this flux into its elementary components, net assimilation (FA), and respiration (FR), remains necessary in order to get a better understanding of biosphere functioning and design better surface exchange models. Noting that FR and FA have different isotopic signatures, we evaluate the potential of isotopic 13CO2 measurements in the air (combined with CO2 flux and concentration measurements) to partition NEE into FR and FA on a routine basis. The study is conducted at a temperate coniferous forest where intensive isotopic measurements in air, soil, and biomass were performed in summer 1997. The multilayer soil‐vegetation‐atmosphere transfer model MuSICA is adapted to compute 13CO2 flux and concentration profiles. Using MuSICA as a “perfect” simulator and taking advantage of the very dense spatiotemporal resolution of the isotopic data set (341 flasks over a 24‐hour period) enable us to test each hypothesis and estimate the performance of the method. The partitioning works better in midafternoon when isotopic disequilibrium is strong. With only 15 flasks, i.e., two 13CO2 nighttime profiles (to estimate the isotopic signature of FR) and five daytime measurements (to perform the partitioning) we get mean daily estimates of FR and FA that agree with the model within 15–20%. However, knowledge of the mesophyll conductance seems crucial and may be a limitation to the method.

Diel variations in the carbon isotope composition of respired CO<sub>2</sub> and associated carbon sources: a review of dynamics and mechanisms

“…A highly variable range in soil δ 13 C R of 0.3-12.5 ‰ occurred in an experimental garden with deciduous trees (Moyes et al, 2010). The information on dynamics of ecosystem respiration (δ 13 C R assessed by Keeling-plot approaches) presents again a very heterogeneous picture: while Ogée et al (2003) and Schnyder et al (2004) found only minor nocturnal variation of δ 13 C R (<3 ‰), others report that nocturnal ecosystem δ 13 C R presented the largest variation among different respiratory components (Kodama et al, 2008;Unger et al, 2010a). Nocturnal variations in δ 13 C R were 6.4 ‰ in a grassland (Bowling et al, 2003), 4.2-8.1 ‰ in a Mediterranean woodland (Werner et al, 2006;Unger et al, 2010a), 6.1 ‰ in a Pinus sylvestris stand (Kodama et al, 2008), 2.6-3.6 ‰ in a subalpine forest (Bowling et al, 2005;Riveros-Iregui et al, 2011), and 3.8 ‰ a beech-dominated deciduous forest (24 h-cycle, Knohl et al, 2005).…”

A multi-level canopy radiative transfer scheme for ORCHIDEE (SVN r2566), based on a domain-averaged structure factor

“…In one study in particular, ignoring such gradients resulted in sensible heat 30 fluxes that were 61% of reference measured values (Baldocchi and Wilson, 2001). Other studies (Ogée et al, 2003;Carrer et al, 2013) also found that simulations of the energy budget were improved with the simulation of vertical profiles.…”

Long-term field performance of a tunable diode laser absorption spectrometer for analysis of carbon isotopes of CO&lt;sub&gt;2&lt;/sub&gt; in forest air

“…The relatively short measurement time and field-capable operation of TDLAS instruments provides greater temporal and spatial resolution of [CO 2 ] and δ 13 C of CO 2 than flask-based techniques (Ogee et al, 2003;Mortazavi et al, 2005Mortazavi et al, , 2006. Observation of seasonal variation in the mole fraction and isotopic composition of atmospheric CO 2 is an obvious application that would benefit from increased temporal resolution (Lai et al, 2005(Lai et al, , 2006 cycles for [CO 2 ] and δ 13 C that are similar to those collected from the alpine tundra site (NWR) (smoothed line in Fig.…”

“…Conversely, at leaf to ecosystem scales the δ 13 C of CO 2 is mostly affected by photosynthesis (Farquhar et al, 1989) and by respiration from all autotrophic and heterotrophic organisms . Studies at these scales using flask-based sampling approaches may have temporal resolution on the order of hours (Knohl, 2003;Ogee et al, 2003), but lack seasonal or inter-annual resolution, and those that do address these timescales lack diurnal resolution (Lai et al, 2005;Alstad et al, 2007). TDLAS has the potential to greatly increase the number of measurements made at a single location and bridge the gap between diurnal and inter-annual timescales.…”