Soil matrix tracer contamination and canopy recycling did not impair ¹³CO₂ plant–soil pulse labelling experiments

Abstract

When conducting ¹³CO₂ plant–soil pulse labelling experiments, tracer material might cause unwanted side effects which potentially affect δ¹³C measurements of soil respiration (δ¹³C_SR) and the subsequent data interpretation. First, when the soil matrix is not isolated from the atmosphere, contamination of the soil matrix with tracer material occurs leading to a physical back-diffusion from soil pores. Second, when using canopy chambers continuously, ¹³CO₂ is permanently re-introduced into the atmosphere due to leaf respiration which then aids re-assimilation of tracer material by the canopy. Accordingly, two climate chamber experiments on European beech saplings (Fagus sylvatica L.) were conducted to evaluate the influence of soil matrix ¹³CO₂ contamination and canopy recycling on soil ¹³CO₂ efflux during ¹³CO₂ plant–soil pulse labelling experiments. For this purpose, a combined soil/canopy chamber system was developed which separates soil and canopy compartments in order to (a) prevent diffusion of ¹³C tracer into the soil chamber during a ¹³CO₂ canopy pulse labelling and (b) study stable isotope processes in soil and canopy individually and independently. In combination with laser spectrometry measuring CO₂ isotopologue mixing ratios at a rate of 1 Hz, we were able to measure δ¹³C in canopy and soil at very high temporal resolution. For the soil matrix contamination experiment, ¹³CO₂ was applied to bare soil, canopy only or, simultaneously, to soil and canopy of the beech trees. The obtained δ¹³C_SR fluxes from the different treatments were then compared with respect to label re-appearance, first peak time and magnitude. By determining the δ¹³C_SR decay of physical ¹³CO₂ back-diffusion from bare soils (contamination), it was possible to separate biological and physical components in δ¹³C_SR of a combined flux of both. A second pulse labelling experiment, with chambers permanently enclosing the canopy, revealed that ¹³CO₂ recycling at canopy level had no effect on δ¹³C_SR dynamics.

Keywords:

• biogeochemistry
• ¹³CO₂ pulse labelling
• plant–soil interactions
• carbon dioxide
• carbon-13
• laser spectroscopy
• isotope ecology
• stable isotope tracer techniques
• isotope measurements
• methods and equipment

Acknowledgements

The authors thank Peter Plüss and Thomas Baur for their excellent technical support and the Grassland Group at ETH Zürich for hosting this project. This study was financed by a Marie Curie Excellence Grant granted by the European Commission to A.K. (Project No.: MEXT-CT-2006-042268).