Substrate-induced respiration measured in situ in a C3-plant ecosystem using additions of C4-sucrose

We added sucrose derived from sugar cane, a tropical C4-plant, to the soil of a temperate C3-forest plant system. The combined measurement of CO2 respiration rate and 13C natural abundance of CO2 enabled a distinction to be made between C3- and C4-respiration, which offered new possibilities to analyze basal respiration and substrate-induced respiration (SIR) in the field. In tests in the laboratory, through-flow systems were used, while in the field the stationary gas phase under soil covers was sampled. Results from the laboratory and in the field were similar with an average SIR response of 2.2 (range 1.7–2.7) times the basal respiration. The change in δ13C after addition of C4-surcrose was less than expected from the increase in respiration rate. Calculations showed that there was an increased efflux of C3-carbon after the C4-sucrose addition. We describe mathematical models, by which we calculated the various source effects contributing to the measured response. The method has numerous advantages, e.g. it uses naturally labelled inexpensive non-hazardous compounds and measurements are non-destructive to the studied system.