Rhizosphere priming can promote mobilisation of N-rich compounds from soil organic matter

• The magnitude of C and N basal mineralisation varied between soils.
• Both soils supported real positive priming.
• Results suggest that priming is an N-mining response of the microbial community.
• Basal and primed mineralisation were distinct processes.
• Gross SOM C:N ratio is a poor predictor of N-flux associated with C mineralisation.

Soil organic matter (SOM) is the dominant store of nutrients required for plant growth, but the availability of these nutrients is dependent on transformations mediated by the microbial biomass. The addition of labile C to soil is known to alter SOM turnover (priming effect, PE), but understanding of this is limited, particularly with respect to impact on gross nitrogen (N) fluxes. Here we examined relationships between C and N fluxes from SOM under primed and non-primed conditions in two soils. Stable isotopes (13C and 15N) were used to measure gross C and N fluxes from SOM and to differentiate between SOM mineralised due to priming and that from basal mineralisation. 13C-glucose was added daily to simulate the effect of addition of labile C on SOM-C and –N mineralisation within the rhizosphere. Addition of glucose increased both gross N and C mineralisation from SOM. However, the C-to-N ratio of the mineralised flux from ‘primed’ SOM was 5:1, whereas the C-to-N ratio of the basal mineralised flux was 20:1 indicating that priming acted on specific organic matter pools. This result is consistent with the concept that priming is a distinct N-mining response of the microbial biomass, as opposed to an acceleration of the basal flux. Our data suggest that C and N fluxes are not directly linked through their gross stoichiometry in SOM. This is due to the heterogeneity and overall passiveness of OM relative to the dynamic nature of mineralisation fluxes and source pools, and in primed systems the mineralisation of N-rich compounds.