**2.6 Calculation of priming effects**

The addition of exogenous test substrate (oil hydrocarbons) to soil was accompanied by the change in soil microbiota activity: the rate of CO2 production initially increased as a result of substrate and probably SOM mineralization and then, on depletion of the substrate, gradually decreased. The amount of CO2 evolved was divided by means of mass isotope balance into two fractions: from the substrates (oil hydrocarbons) and from SOM mineralization. Thus, the difference between CO2 evolved from SOM mineralization in oil hydrocarbons amended soil (C\*SOM) and in the control soil (CSOM) relative to the control (in percentage) was used to estimate the magnitude of the priming effect (PE) induced by oil hydrocarbons (denoted as SUB). The PE value was determined in two notations as *kinetic* PE(Δti ) calculated as a value for Δti–time intervals using equation [11] and the PE(*total*) calculated as a weighted average value for the whole period of observation using equation [12].

$$\text{PE(\Delta\text{tj})} \left[ \text{\%} \right] = 100 \times \left( \text{C}\_{\text{-SCM(i)}} - \text{C}\_{\text{-SCM(i)}} \right) / \text{C}\_{\text{-SCM(i)}} \tag{11}$$

where C\*SOM(i) = Fi×C(SUB+SOM)I; C(SUB+SOM)i is the total C evolved as CO2 in the amended soil during Δti-time; and Fi is the share of CO2-C resulting from the SOM in crude oil amended soil in Δti-time, which was calculated by equation [8].

$$\text{PE(total)} \left[ \% \right] = \Sigma(\text{PE(Att)} \cdot \Delta t \text{t}) / \Sigma(\Delta t \text{t}) \tag{12}$$

where PE(Δti) was calculated according to Eq. [11].
