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Insensitivity of Soil Microbial Activity to Temporal Variation in Soil N in Subarctic Tundra: Evidence from Responses to Large Migratory Grazers

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Abstract

Large migratory grazers commonly influence soil processes in tundra ecosystems. However, the extent to which grazing effects are limited to intensive grazing periods associated with migration has not previously been investigated. We analyzed seasonal patterns in soil nitrogen (N), microbial respiration and extracellular enzyme activities (EEAs) in a lightly grazed tundra and a heavily grazed tundra that has been subjected to intensive grazing during reindeer (Rangifer tarandus L.) migration for the past 50 years. We hypothesized that due to the fertilizing effect of the reindeer, microbial respiration and EEAs related to microbial C acquisition should be higher in heavily grazed areas compared to lightly grazed areas and that the effects of grazing should be strongest during reindeer migration. Reindeer migration caused a dramatic peak in soil N availability, but in contrast to our predictions, the effect of grazing was more or less constant over the growing season and the seasonal patterns of microbial activities and microbial N were strikingly uniform between the lightly and heavily grazed areas. Microbial respiration and the EEAs of β-glucosidase, acid-phosphatase, and leucine-aminopeptidase were higher, whereas that of N-acetylglucosamidase was lower in the heavily grazed area. Experimental fertilization had no effect on EEAs related to C acquisition at either level of grazing intensity. Our findings suggest that soil microbial activities were independent of grazing-induced temporal variation in soil N availability. Instead, the effect of grazing on soil microbial activities appeared to be mediated by substrate availability for soil microorganisms. Following a shift in the dominant vegetation in response to grazing from dwarf shrubs to graminoids, the effect of grazing on soil processes is no longer sensitive to temporal grazing patterns; rather, grazers exert a consistent positive effect on the soil microbial potential for soil C decomposition.

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Acknowledgments

We would like to thank Henni Ylänne for helping with soil sampling, Aarno Niva for helping with field manipulations, and Sirkka Aakkonen for help in the laboratory. We are also grateful to Dr. Minna Männistö for inspiring discussions about the results. This study was funded by the Academy of Finland (decision numbers 218121 and 130507 to S. Stark).

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  1. Arctic Centre, University of Lapland, P.O. Box 122, 96301, Rovaniemi, Finland

    Sari Stark & Maria Väisänen

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  1. Sari Stark
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  2. Maria Väisänen
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Correspondence to Sari Stark.

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Author contributions

SS and MV designed the experiment; MV and SS performed field manipulations and sample collection; SS and MV conducted soil analyses; SS conducted statistical tests and wrote the paper with a contribution from MV.

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Stark, S., Väisänen, M. Insensitivity of Soil Microbial Activity to Temporal Variation in Soil N in Subarctic Tundra: Evidence from Responses to Large Migratory Grazers. Ecosystems 17, 906–917 (2014). https://doi.org/10.1007/s10021-014-9768-2

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