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

We tested whether C-13-discrimination during microbial respiration, or during CO2 sampling in the field, can explain changes observed in the delta(13)C of emitted CO2 that follow the addition of C-4-sucrose, as a microbial substrate, to the soil of a C-3-ecosystem. We approached this problem by adding C-3-glucose (delta(13)C=-23.4parts per thousand), C-4-sucrose (-10.8parts per thousand) or C-13-labelled glucose (103.7parts per thousand) to the intact mor layer, the upper organic soil (-26.5parts per thousand, bulk soil organic matter), of a boreal Pinus sylvestris L. forest. If C-13-discrimination is significant, it should generate illusory differences in the calculated contributions from the added C and endogenous C-3-C to total soil respiration, when C-4-sucrose or C-13-labelled glucose is added. Further, if discrimination occurs, we should also be able to detect a shift in the delta(13)C of respired CO2 after the addition of C-3-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 delta(13)C of the emitted CO2 after additions of H2O or C-3-glucose. In contrast, the addition of C-4-sucrose and C-13-labelled glucose gave delta(13)C values of evolved CO2 that were 4.5parts per thousand, and 30.3parts per thousand higher than the pre-sugar values, respectively. The calculated respiration rates of the added carbon sources, C-4-C or C-13-labelled C, were very similar. Also, we found very similar sugar-induced increases in respiration of endogenous C-3-C in the plots supplied with C-4-sucrose and C-13-labelled glucose, accounting for about 50% of the total increase in respiration 1 h after addition. Our results confirm that any microbial C-13-discrimination during respiration is minor.