Partitioning Respiration of C3-C4 Mixed Communities Using the Natural Abundance ¹³C Approach - Testing Assumptions in a Controlled Environment

Abstract

Contributions of C3 and C4 plants to respiration of C3-C4 ecosystems can be estimated on the basis of their contrasting ¹³C discrimination. But accurate partitioning requires accurate measurements of the isotope signature of whole system respiratory CO₂ (δ_R), and of its members (δ₃ and δ₄). Unfortunately, experimental determination of representative δ₃ and δ₄ values is virtually impossible in nature, generating a need for proxies (surrogates) of δ₃ and δ₄ values (e.g., the δ of leaf biomass). However, recent evidence indicates that there may be systematic differences among the δ of respiratory and biomass components. Thus, partitioning may be biased depending on the proxy. We tested a wide range of biomass- and respiration-based δ proxies for the partitioning of respiration of mixed Lolium perenne (C3) - Paspalum dilatatum (C4) stands growing at two temperatures inside large ¹³CO₂/¹²CO₂ gas exchange chambers. Proxy-based partitioning was compared with results of reference methods, including (i) the δ of whole plant respiratory CO₂ (δ₃ and δ₄) or (ii) respiration rate of intact C3 and C4 plants. Results of the reference methods agreed near perfectly. Conversely, some proxies yielded erroneous partitioning results. Partitioning based on either the δ of shoot or root respiratory CO₂ produced the worst bias, because shoot respiratory CO₂ was enriched in ¹³C by several ‰ and root respiratory CO₂ was depleted by several ‰ relative to whole plant respiratory CO₂. Use of whole plant or whole shoot biomass δ gave satisfactory partitioning results under the constant conditions of the experiments, but their use in natural settings is cautioned if environmental conditions are variable and the time scales of respiration partitioning differ strongly from the residence time of C in biomass. Other biomass-based proxies with faster turnover (e.g., leaf growth zones) may be more useful in changing conditions.

Key words

Respiration - ¹³C discrimination - C3 - C4 - flux partitioning - ¹³CO₂/¹²CO₂ exchange - ecosystem - Lolium perenne - Paspalum dilatatum.

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