Differences between priming and rhizosphere priming effects: Concepts and mechanisms

The addition of fresh carbon (C) substrates to the soil can result either in the acceleration or retardation of native soil organic matter (SOM) decomposition, referred to as the priming effect (PE). Related to PE is the rhizosphere priming effect (RPE), which involves the activity of plant roots. It remains unclear whether PE and RPE differ in their impacts on soil C dynamics and their response to soil nitrogen (N) availability. Here, we highlight drivers and mechanisms that distinguish RPE from PE and how they change with N addition. We develop different scenarios for PE and RPE separately and their responses to N addition. We describe temporal differences between PE and RPE, where microbes become more C limited with time in PE studies and more N limited in RPE studies. We then analysed published data to assess support for our scenarios. Without N addition PE and RPE were mostly positive and of similar magnitude, indicating that stoichiometric decomposition under microbial C limitation and N mining under microbial N limitation are major drivers of PE and RPE. Root effects on physical and chemical destabilization of native SOM and plant-microbe competition for N may also cause positive RPEs. With N addition, more than half of all observations showed reductions in PE (56%) and RPE (51%), possibly because of N-induced soil acidification. The RPEs and their responses to N could potentially last longer than for PE, due to ongoing root-induced destabilization of SOM and competition for N between plants and microbes thereby maintaining microbial N limitation and N mining. The distinctly different mechanisms and temporal dynamics between PE and RPE will have important implications for short- and long-term SOM dynamics and their responses to N addition. • Priming effect (PE) and rhizosphere priming effect (RPE) differ mechanistically. • Microbes become more C limited with time for PE but more N limited for RPE. • PE and RPE are mostly positive and of similar magnitude. • Stoichiometric decomposition and N mining are major drivers for PE and RPE. • N addition reduced PE in 56% and RPE in 51% of all observations.