Semi-continuous C supply reveals that priming due to N-mining is driven by microbial growth demands in temperate forest plantations

Nitrogen (N) availability is a powerful controller of soil carbon (C) cycling in temperate forests, affecting plant productivity and microbial activities. Enhanced rhizosphere input from increased plant productivity can stimulate the decomposition of native soil organic matter (SOM), termed the “priming effect”, with an effect size that is affected by N availability. Using soils from N-fertilization field-experiments conducted in larch and mixed forest plantations, we investigated how N availability influenced the priming of soil organic C (SOC) mineralization and soil organic N (SON) mineralization by adding 13C-labelled glucose semi-continuously (every third day) to simulate the semi-continuous delivery of rhizosphere inputs. We found that semi-continuous additions of glucose induced a repeating pattern for the priming of SOC mineralization with an initial decrease followed by an increase. This repeating pattern of SOC mineralization reversely coincided with repeating dynamics in the level of bacterial growth which first increased followed by a decrease. The labile C additions induced a gradually increasing priming of SON mineralization, which was greater than the priming of SOC mineralization (i.e. “selective microbial N-mining”). The priming of SOC mineralization and SON mineralization were both reduced by N fertilization. Increased priming of SON mineralization was related to stimulated bacterial growth and changes in the microbial C use efficiency. These responses suggested that increased microbial demand for N drove the observed N-mining responses. This microbial N-mining could not be explained by increased oxidative enzyme activities, and was instead linked to microbial growth. In the larch forest with high C-quality, the high priming of SOM mineralization were linked to bacterial growth, while in the mixed forest with low C-quality, the high levels of priming of SOM mineralization was linked to fungal growth. We also observed that labile C input initially decoupled the C and N mineralization from SOM. Within 16 days, however, a new equilibrium developed, where both C and N mineralization from SOM were similarly enhanced by labile C input. Overall, our results suggest that semi-continuous rhizosphere inputs can induce a sustained priming of SOM mineralization driven by the microbial demand for N – increasing the release of CO2 – but that N fertilization could reduce the soil C loss, contributing to enhanced soil C sequestration.