Christina Biasi

Rationale Land‐use changes, e.g., forestry drainage, modify the characteristics of peatland soil and affect the peatland carbon (C) balance. Peat soil nutrient status, related mainly to the original peatland type, also has an impact on the C balance after drainage, as observed earlier at the ecosystem scale for two forestry‐drained sites in Southern Finland. Here the aim was to compare the soil CO 2 fluxes from the two sites, nutrient‐poor and nutirent‐rich forestry‐drained peatlands, and study the effect of plant photosynthates on the decomposition of peat C. Therefore, the respiration rates and priming effect (PE) of peat soils with variable nutrient status were examined in the laboratory. Methods Half of the samples were labelled with 13 C‐glucose to study the effect of fresh C addition on the soil decomposition. The 13 CO 2 ‐samples were analysed with isotope ratio mass spectrometry. A two‐pool mixing model was applied to separate the soil‐ and sugar‐derived respirations and to determine the PE. Results The nutrient‐rich peat soil respired generally more than the nutrient‐poor peat. A negative PE was observed in both peat soils, suggesting that the addition of fresh C did not increase the soil decomposition, but on the contrary decreased it. The negative PE was significantly more pronounced in nutrient‐poor peat soil than in the nutrient‐rich peat treatments, suggesting that the higher nutrient availability suppresses the negative PE. Conclusions These results imply that microbes prefer utilizing fresh C instead of old C in the short term and that the peat decomposition is suppressed in the presence of fresh C inputs from vegetation at forestry‐drained peatlands. These effects are even stronger in peat soils with less nutrients available. Ecosystem scale and soil process models could be improved with the help of these results.