Decreases in priming effect vary with labile carbon type under long-term nitrogen addition in a boreal forest

The priming effect induced by labile carbon inputs can influence soil organic carbon (SOC) decomposition, which can be regulated by nitrogen (N) availability. However, how long-term N addition affects the priming effect is still unclear. Here, we address this issue by combining results from an experiment and a data synthesis. For the experiment, soils were collected from a 12-year in-situ N addition in a boreal forest, and 13 C-labeled glucose, glycine or oxalic acid were added as a rate of 2.5% of SOC. Further, we conducted a global data synthesis with 188 observations from 50 studies from forests to compare priming in response to transient incubation addition versus continuous field N addition. The positive priming effect caused by glycine was highest among the three C types especially in the control treatment, i.e. , a significant interaction between N addition and C type. These variations could be attributed to differences in microbial substrate metabolic pathways. N addition significantly suppressed the priming effect, consistent with trends observed in the global data synthesis. However, the extent of N inhibition from field studies was marginally lower than that from incubation studies, suggesting that the N inhibition on priming may be to some extent overestimated. Overall, our results emphasize that the inhibition of long-term N addition on priming may depend on labile C type. More studies with soils from long-term N addition in the field are urgently needed for accurately assessing SOC decomposition via the priming effect in the context of chronic N deposition in boreal forests.