Climate change will expose high-latitude systems to warming and a shift towards plant communities with more labile carbon (C) input. Labile C can increase the mineralisation of native soil organic matter (SOM); a phenomenon termed ‘priming’. We investigated how warming (+1.1°C above ambient using open top chambers) and plant litter or organic nitrogen (N; fungal sporocarps) addition (90 g m-2 y-1) in the Subarctic influenced the susceptibility of SOM mineralisation to priming, and its microbial underpinnings. Labile C was added either in the form of glucose (labile C only) or alanine (labile C + N).
We hypothesized that glucose addition would induce greater mineralization of N than C sourced from SOM (“N mining”); a response unrelated to microbial growth responses. Moreover, we expected that the N mining effect would be more pronounced in climate change simulation treatments of higher C/N (plant litter) than treatments with lower C/N (fungal sporocarps and warming), with the control treatments intermediate. We also hypothesized that addition of alanine (labile C + N) would not result in selective N mining, but instead coupled responses of C and N mineralisation sourced from SOM; a response that would coincide with stimulated microbial growth responses.
Labile C appeared to inhibit the mineralisation of C from SOM by up to 60 % within hours. By contrast, the mineralisation of N from SOM was stimulated by up to 300 %. These responses occurred rapidly and were unrelated to microbial successional dynamics, suggesting catabolic responses. Considered separately, labile-C inhibition of C mineralisation is compatible with previously reported findings termed ‘preferential substrate utilisation’ or ‘negative apparent priming’, while stimulated N mineralisation echoes reports of ‘real positive priming’ of SOM. However, C and N mineralisation responses derived from the same SOM source must be interpreted together, suggesting that microbial SOM-use decreased in magnitude and shifted to components richer in N. Selective mining for N increased in climate change treatments with greater fungal dominance.
Overall, these results highlight that only considering SOM in terms of C may be simplistic, and will not capture all changes in SOM decomposition. Labile C appeared to trigger catabolic responses of the resident microbial community that shifted SOM mineralisation to N-rich components; an effect that increased with greater fungal dominance. The predicted shrub expansion in the Subarctic could therefore result in altered microbial use of SOM, with selective mining for N-rich components but reduced total SOM-use.