Partitioning respiration of C3-C4 mixed communities using the natural abundance 13C approach--testing assumptions in a controlled environment.

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 (deltaR), and of its members (delta3 and delta4). Unfortunately, experimental determination of representative delta3 and delta4 values is virtually impossible in nature, generating a need for proxies (surrogates) of delta3 and delta4 values (e.g., the delta of leaf biomass). However, recent evidence indicates that there may be systematic differences among the delta of respiratory and biomass components. Thus, partitioning may be biased depending on the proxy. We tested a wide range of biomass- and respiration-based delta 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 delta of whole plant respiratory CO2 (delta3 and delta4) 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 delta of shoot or root respiratory CO2 produced the worst bias, because shoot respiratory CO2 was enriched in 13C by several per thousand and root respiratory CO2 was depleted by several per thousand relative to whole plant respiratory CO2. Use of whole plant or whole shoot biomass delta 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.