Quality and potential biodegradability of soil organic matter preserved in permafrost of Siberian tussock tundra

Abstract

Soil organic matter (SOM) quality and biodegradability the permafrost underlying Siberian wet tussock tundra (Kolyma river basin, northeast Siberia) were analyzed and compared to the characteristics of the contemporary active layer. For this purpose, three permafrost affected soil cores (down to 3 m depth) and seven active layer soil cores (down to 0.3 m depth) were sampled. The samples were divided into particular layers, which were analyzed separately. SOM stability was assessed using a simple chemical fractionation (sequential extraction by cold and hot water, and hot acid). SOM biodegradability and soil mineralization potentials were tested in short-term laboratory incubations. The active layer contained 24 kg C m −2 and 70 kg C m −2 was preserved in 3 m of permafrost. The chemical quality and biodegradability of permafrost SOM were very similar to that of the active layer mineral horizon, and independent from depth. The only exceptions were (1) higher solubility of permafrost SOM in water, indicating its higher mobility and potential leakage after permafrost thawing and (2) higher nutrient (N, P) concentrations available to a dense permafrost microbial community, which could support decomposition of more complex substrates under suitable temperature conditions after thawing. The mineralization potential of the upper 1 m deep permafrost, which could melt by 2100 according to permafrost degradation models, was 6.7 g C m −2 d −1 (optimum conditions of 20 °C, field water capacity), which is comparable to that of the contemporary active layer of 0.5 m depth (7.5 g C m −2 d −1). Under field conditions, SOM mineralization rate would reasonably be significantly lower due to prevailing anoxia (high water table) and diffusion constraints in the deep and flooded soil profile. We conclude from our results that the permafrost (1) cryopreserves a high SOM amount, which is distributed to considerable depths, being of similar chemical quality and biodegradability to that of the active layer mineral horizon SOM, and (2) contains a dense living microbial community, which is able to decompose the present SOM rapidly without any obvious chemical limitation under suitable conditions.