Addition of easily available organic substances (e.g. glucose) to soil can result in a change in mineralization of native soil organic carbon (SOC), termed as priming effect (PE). In C and N dynamics models and C studies, PE are not considered, even though very low amounts of added substances can cause considerable PE. The effects of the amounts of the added substances inducing PE are not investigated well. Also little is known about the origin of the additionally mineralised carbon (C), although it is proposed that this C is released from not stabilized SOC pools. So, the aims of our study were to investigate the effect of strongly increasing glucose amounts on 1) its mineralization and 2) the induced PE on native SOC. Additionally, application of ¹⁴C labelled substrate (glucose) to a soil with C₃/C₄ vegetation changes before 10 years and so using ¹³C natural abundance should allow to differentiate between PE on old SOC and new C incorporated into SOC during the last 10 years.

Samples of the Ah of a loamy soil 1) from a field cultivated with Miscanthus x giganteus (a perennial C₄ plant) for the last 10 years and 2) from a neighboured plot covered with C₃ grassland without vegetation changes were used for the incubation experiment (T= 18 °C, 60% WHC, 22 days). Increasing glucose amounts (0.25, 2.5, 25, and 250 µg glucose-C g^-1 soil) were added to the soil as aqueous solution. Each replicate received ¹⁴C labelled glucose with an activity of 243 Bq g^-1 soil and nutrient solution. Evolved CO₂ was trapped in 1 M NaOH solution, where 1) the amount of total CO₂ was measured by titration, 2) the amount of glucose derived CO₂ was measured as ¹⁴C activity by scintillation counting and 3) the d¹³C values were measured as SrCO₃ on a mass spectrometer. So, combining ¹⁴C and d¹³C measurements allowed partitioning of evolved CO₂ into three sources: 1) glucose-derived CO₂, 2) C₃-derived CO₂ from old SOC and 3) C₄-derived CO₂ from new, plant-derived and less stabilized C. Amount and ¹⁴C activity of microbial biomass were determined 5 times during the incubation.

With increasing glucose amounts, the proportion of glucose mineralised during the 22 days increased significantly from 29% of added ¹⁴C after addition of 0.25 µg glucose-C g^-1 soil to 44% after addition of 250 µg glucose-C g^-1 soil. The addition of glucose induced an extra mineralization of SOC from 87 µg C g^-1 soil (0.25 µg glucose-C g^-1 soil) up to 174 µg C g^-1 (250 µg glucose-C g^-1 soil) during the 22 days. So, the addition of increasing glucose amounts induced higher PE. They increased from 21% of control after addition of the smallest amount of glucose to 43% after addition of the highest amount of glucose.

Before glucose addition, 68% of evolved CO₂ was derived from the new C, which was sequestered in the soil during the last 10 years. After the glucose addition, less new C was evolved in treatments with glucose addition (21-28% of total evolved C) than in the control treatment (40% of total evolved C). So, the addition of higher glucose amounts caused greater decreases in mineralization of new C accompanied by greater amounts of additionally mineralised old C. So, we conclude, that during priming effect induced by addition of easily available C source, the additionally mineralised C originated from old, better stabilized SOC.

Combining different methods using stable and radioactive C isotopes is a useful tool for studying the sources of extra mineralised C after addition of easily available substances to soil. This is important for implying PE in C models and investigations of C turnover in soil.

Keywords: SOC, priming effects, glucose, 14C, natural 13C abundance.