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 (δ13CSR) 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 δ13CSR fluxes from the different treatments were then compared with respect to label re-appearance, first peak time and magnitude. By determining the δ13CSR decay of physical 13CO2 back-diffusion from bare soils (contamination), it was possible to separate biological and physical components in δ13CSR 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 δ13CSR dynamics.