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Doctoral thesis2018Open access

Microbial life in boreal soils : on the availability and fate of carbon substrates for microbial activity in boreal soils

Segura, Javier

Abstract

The large pool of carbon (C) stored as soil organic matter (SOM) in soils of high-latitude ecosystems contains more organic C than all global vegetation and the atmosphere combined. Global climate change is expected to have especially pronounced effects in these ecosystems, and even small changes in the accumulation and decomposition of their soil C pool driven by heterotrophic microbial activity could profoundly affect atmospheric CO₂ levels and thus the global climate. Because such changes could trigger drastic shifts in the delicate balance of CO₂ between the biosphere and atmosphere, a better understanding of the key regulators of C cycling is urgently needed. Using advanced molecular and biochemical techniques, I investigated the availability of C substrates and their utilization by microorganisms under controlled but ecologically relevant conditions in soils representative of the boreal landscape. This molecular characterization of SOM revealed that tree species significantly influence SOM genesis by changing its rate of accumulation and organo-chemical composition. More importantly, a strong connection between SOM decomposition and microbial decomposers was observed and shown to be governed by the organo-chemical composition of the SOM. The structural arrangement of cellulose, and particularly its degree of crystallinity, emerged as a key factor determining rates of cellulose hydrolysis and subsequent C decomposition in boreal forest soils. This work provides some of the first empirical evidence that soil microbial communities in frozen boreal forest soils can hydrolyze cellulose and use the released substrate for both catabolic and anabolic metabolism. These findings, together with results from peat soil experiments, show that both persistent microbial degradation of C (biopolymers and monomers) and the synthesis of new microbial biomass during winter are widespread features in soils of the boreal landscape. More importantly, the results indicated that small differences in winter soil temperatures can have very large implications for the winter C fluxes of boreal soils. Over the studied temperature range, C substrates were readily utilized and microbial activity was never totally impeded. However, thermodynamic constraints caused strong reductions in metabolic rates at sub-zero temperatures. The rates of these processes at low temperatures are low but their importance should not be neglected given the spatial scale over which they can occur and the prolonged winters these ecosystems experience. Taken as a whole, this thesis provides a valuable contribution to our understanding of microbial C cycling in one of the world's major soil C pools.

Keywords

boreal forest, soil organic matter, organo-chemical composition, frozen soils, CO2, biopolymers, decomposition, hydrolysis, PLFA, 13C-NMR, Py-GC–MS

Published in

Acta Universitatis Agriculturae Sueciae
2018, number: 2018:32ISBN: 978-91-7760-204-0, eISBN: 978-91-7760-205-7
Publisher: Department of Forest Ecology and Management, Swedish University of Agricultural Sciences

    UKÄ Subject classification

    Forest Science
    Soil Science

    Permanent link to this page (URI)

    https://res.slu.se/id/publ/104190