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

We tested whether C-discrimination during microbial respiration, or during CO sampling in the field, can explain changes observed in the δC of emitted CO that follow the addition of C-sucrose, as a microbial substrate, to the soil of a C-ecosystem. We approached this problem by adding C-glucose (δC=-23.4‰), C-sucrose (-10.8‰) or C-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 C-discrimination is significant, it should generate illusory differences in the calculated contributions from the added C and endogenous C-C to total soil respiration, when C-sucrose or C-labelled glucose is added. Further, if discrimination occurs, we should also be able to detect a shift in the δC of respired CO after the addition of C-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 δC of the emitted CO after additions of HO or C-glucose. In contrast, the addition of C-sucrose and C-labelled glucose gave δC values of evolved CO that were 4.5‰ and 30.3‰ higher than the pre-sugar values, respectively. The calculated respiration rates of the added carbon sources, C-C or C-labelled C, were very similar. Also, we found very similar sugar-induced increases in respiration of endogenous C-C in the plots supplied with C-sucrose and C-labelled glucose, accounting for about 50% of the total increase in respiration 1 h after addition. Our results confirm that any microbial C-discrimination during respiration is minor.