Carbon and Nutrient Limitation of Microbial Decomposition of Organic Matter in Permafrost Soils

Thursday, 18 December 2014: 9:00 AM
Andreas Richter1,2, Jiri Barta3, Petr Capek3, Norman Gentsch4, Georg Guggenberger4, Christina Kaiser5, Robert Mikutta4, Olga Shibistova4, Hana Santruckova3, Joerg Schnecker1,2 and Birgit Wild1,2, (1)University of Vienna, Vienna, Austria, (2)Austrian Polar Research Institute, Vienna, Austria, (3)University of South Bohemia, Ceske Budejovice, Czech Republic, (4)Leibniz Universit├Ąt Hannover, Hannover, Germany, (5)University of Vienna, Microbiology and Ecosystem Science, Vienna, Austria
Cryotubated horizons of permafrost soils contain as much as 410 Pg C, a significant proportion of which may be vulnerable to climate changes through permafrost thawing and subsequent decomposition by soil microorganisms. While numerous studies have addressed temperature and soil moisture controls, very little is known on energy and nutrient constraints of microbial decomposition in permafrost soils.

We investigated nutrient and carbon limitations of permafrost soils by analysing a wide range of soil samples for C:N stoichiometry and carbon isotope composition and incubating soil samples of different depths with carbon only or carbon and nitrogen amendments.

Utilising the carbon isotopic composition of soil organic matter (SOM) as a proxy for the stage of decomposition, we identified a breakpoint of about 1.5% C, below which decomposition was dominated by the recycling of microbial biomass, and above which decomposition of plant derived material was the main process. This breakpoint corresponds to a threshold element ration (TERC:N) between 15-20. Soils with a C:N ratio below the TERC:N (i.e., soil that can be considered as C or energy limited) make up for a significant proportion of the overall SOM stored in permafrost soils, even in the active layer.

To get more insight into the limitations of SOM decomposition, we incubated about 100 soil samples from 4 sites across Siberia with either cellulose or cellulose and protein for 180 days and calculated how the amendments affected the decomposition of native SOM. All incubated horizons, regardless of their depth or C:N ratio showed a significant increase in native SOM decomposition to a combined carbon and nitrogen amendment, that was higher than with C amendment alone. We conclude that nutrient and carbon limitations of soil microbial communities constitute important constraints on SOM decomposition and should be incorporated into models predicting future permafrost carbon storage.