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Short-term carbon input increases microbial nitrogen demand, but not microbial nitrogen mining, in a set of boreal forest soils

Journal article
Authors Birgit Wild
S. Alaei
P. Bengtson
S. Bode
P. Boeckx
J. Schnecker
W. Mayerhofer
Tobias Rütting
Published in Biogeochemistry
Volume 136
Issue 3
Pages 261-278
ISSN 0168-2563
Publication year 2017
Published at Department of Earth Sciences
Pages 261-278
Language en
Links doi.org/10.1007/s10533-017-0391-0
https://gup.ub.gu.se/file/207266
Keywords Boreal forest, Priming, Microbial N mining, Organic N, Protein depolymerization, N mineralization, net primary productivity, organic-matter, root exudation, n, availability, elevated co2, nutrient availability, temperate forest, use, efficiency, decomposition, rhizosphere, Environmental Sciences & Ecology, Geology
Subject categories Geochemistry

Abstract

Rising carbon dioxide (CO2) concentrations and temperatures are expected to stimulate plant productivity and ecosystem C sequestration, but these effects require a concurrent increase in N availability for plants. Plants might indirectly promote N availability as they release organic C into the soil (e.g., by root exudation) that can increase microbial soil organic matter (SOM) decomposition ("priming effect"), and possibly the enzymatic breakdown of N-rich polymers, such as proteins, into bio-available units ("N mining"). We tested the adjustment of protein depolymerization to changing soil C and N availability in a laboratory experiment. We added easily available C or N sources to six boreal forest soils, and determined soil organic C mineralization, gross protein depolymerization and gross ammonification rates (using N-15 pool dilution assays), and potential extracellular enzyme activities after 1 week of incubation. Added C sources were C-13-labelled to distinguish substrate from soil derived C mineralization. Observed effects reflect short-term adaptations of non-symbiotic soil microorganisms to increased C or N availability. Although C input promoted microbial growth and N demand, we did not find indicators of increased N mobilization from SOM polymers, given that none of the soils showed a significant increase in protein depolymerization, and only one soil showed a significant increase in N-targeting enzymes. Instead, our findings suggest that microorganisms immobilized the already available N more efficiently, as indicated by decreased ammonification and inorganic N concentrations. Likewise, although N input stimulated ammonification, we found no significant effect on protein depolymerization. Although our findings do not rule out in general that higher plant-soil C allocation can promote microbial N mining, they suggest that such an effect can be counteracted, at least in the short term, by increased microbial N immobilization, further aggravating plant N limitation.

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