Table 2. Potential causes of priming effects, resulting from the addition of C and N inputs.

Target	Priming effect	Potential causes
Carbon	Positive real priming effect	Acceleration of SOM mineralization as a substrate and energy source and N immobilization through an increase in microbial activity
	Negative real priming effect	Reduction in C:N ratio
		Microbial immobilization of C
		Microbial immobilization of N due to readily-available C-rich substrate in the soil
Nitrogen	Positive real priming effect	Increase in atmospheric N2-fixation
	Negative real priming effect	Decrease in atmospheric N2-fixation Microbial immobilization of N
	Positive apparent priming effect	Stimulation of N uptake by roots
	Negative apparent priming effect	NH4+ fixation by clay minerals
Carbon and nitrogen	Positive real priming effect	Increase mineralization of SOM through a lower C:N ratio
		Increase in microbial activity and facilitation of SOM mineralization by means of co-metabolism
	Negative real priming effect	Switching of microbial biomass from soil organic matter on the easily available C and N sources
		Preferred uptake of C-rich substrates by microorganisms
	Negative apparent priming effect	Incomplete decomposition of C and N sources
		Sorption of physico-chemical protection and immobilization of added substrates

Notes.

Source: Adapted from Kuzyakov, Friedel & Stahr (2000).