Tuesday, 16 August 2016

Do shifts in life strategies explain microbial community responses to increasing nitrogen in tundra soil?

Published Date

May 2016, Vol.96:216–228, doi:10.1016/j.soilbio.2016.02.012

Title

Do shifts in life strategies explain microbial community responses to increasing nitrogen in tundra soil?

Author

Minna Männistö^a,^,

Lars Ganzert^a,b

Marja Tiirola^c

Max M. Häggblom^d

Sari Stark^e

aNatural Resources Institute Finland, P.O. Box 16, FI-96301 Rovaniemi, Finland
bDepartment of Experimental Limnology, Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB), Stechlin, Germany
cDepartment of Biological and Environmental Science, P.O. Box 35, FI-40014, University of Jyväskylä, Finland
dDepartment of Biochemistry and Microbiology, Rutgers University, NJ, USA
eArctic Centre, University of Lapland, P.O. Box 122, FI-96101 Rovaniemi, Finland

Received 27 October 2015. Revised 19 February 2016. Accepted 20 February 2016. Available online 3 March 2016.

Highlights

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Similar bacterial community structure in soils under different grazing intensities.
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N amendment decreased respiration in tundra soil.
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N amendment decreased biomass but increased rRNA copy numbers per unit DNA.
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Copiotrophic taxa were more abundant in N amended soils.

Abstract

Subarctic tundra soils store large quantities of the global organic carbon (C) pool as the decomposition of plant litter and soil organic matter is limited by low temperatures and limiting nutrients. Mechanisms that drive organic matter decomposition are still poorly understood due to our limited knowledge of microbial communities and their responses to changing conditions. In subarctic tundra large grazers, in particular reindeer, exert a strong effect on vegetation and nutrient availability causing drastic nutrient pulses in the soils located along the migratory routes. Here we studied the effect of increased nitrogen (N) availability on microbial community structure and activities by laboratory incubations of soil collected from two sites with contrasting grazing intensities. We hypothesized that heavily grazed soil experiencing nutrient pulses harbor more copiotrophic taxa that are able to respond positively to increases in available N leading to increased enzyme activities and respiration. Contrary to our hypothesis, there were only minor differences in the microbial community composition between the lightly and heavily grazed soils. N amendment shifted the bacterial community composition drastically, but the changes were similar at both grazing intensities. The relative abundance of diverse Actinobacteria and Rhodanobacter-affiliated Gammaproteobacteria increased in the N amended microcosms, while the abundance of Acidobacteria, Alphaproteobacteria, Deltaproteobacteria, Verrucomicrobia and Bacteroidetes decreased. Contrary to our hypotheses, increased N availability decreased respiration and microbial biomass at both grazing intensities, while increased N availability had little influence on the extracellular enzyme activities. We propose that similar to what has been reported in other systems, elevated N availability suppressed microbial respiration and biomass by favoring copiotrophic species with faster growth rates and with limited capabilities to decompose recalcitrant organic matter. Similar responses in soils from contrasting vegetation types, soil organic matter (SOM) quality and N availabilities in response to grazing intensity indicate that nutrient pulses may have a strong direct impact on the microbial communities. Responses detected using laboratory incubations are likely amplified in the field where the direct effect of increased N availability is combined with increase in labile C through changes in plant production and species composition.