PS 4-45 - Is the soil organic matter mineralisation response to root exudation controlled by microbial stoichiometric demand in subarctic soils?

Monday, August 7, 2017
Exhibit Hall, Oregon Convention Center
Lettice C. Hicks1, Ainara Leizeaga1, Kathrin Rousk2, Anders Michelsen2 and Johannes Rousk1, (1)Microbial Ecology, Lund University, Lund, Sweden, (2)Department of Biology, University of Copenhagen, Denmark

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.