¹³C-discrimination during microbial respiration of added C₃-, C₄- and ¹³C-labelled sugars to a C₃-forest soil

We tested whether ¹³C-discrimination during microbial respiration, or during CO₂ sampling in the field, can explain changes observed in the δ¹³C of emitted CO₂ that follow the addition of C₄-sucrose, as a microbial substrate, to the soil of a C₃-ecosystem. We approached this problem by adding C₃-glucose (δ¹³C=-23.4‰), C₄-sucrose (-10.8‰) or ¹³C-labelled glucose (103.7‰) to the intact mor layer, the upper organic soil (-26.5‰, bulk soil organic matter), of a boreal Pinus sylvestris L. forest. If ¹³C-discrimination is significant, it should generate illusory differences in the calculated contributions from the added C and endogenous C₃-C to total soil respiration, when C₄-sucrose or ¹³C-labelled glucose is added. Further, if discrimination occurs, we should also be able to detect a shift in the δ¹³C of respired CO₂ after the addition of C₃-glucose. The addition of the three sugar solutions gave similar increases in soil respiration (up to a doubling 1 h after the additions), while the addition of water gave no increase in respiration. There was no change in δ¹³C of the emitted CO₂ after additions of H₂O or C₃-glucose. In contrast, the addition of C₄-sucrose and ¹³C-labelled glucose gave δ¹³C values of evolved CO₂ that were 4.5‰ and 30.3‰ higher than the pre-sugar values, respectively. The calculated respiration rates of the added carbon sources, C₄-C or ¹³C-labelled C, were very similar. Also, we found very similar sugar-induced increases in respiration of endogenous C₃-C in the plots supplied with C₄-sucrose and ¹³C-labelled glucose, accounting for about 50% of the total increase in respiration 1 h after addition. Our results confirm that any microbial ¹³C-discrimination during respiration is minor.