Soil Microbial Responses to 28\xa0Years of Nutrient Fertilization in a Subarctic Heath

Lettice C Hicks,Kathrin Rousk,Riikka Rinnan,Johannes Rousk

doi:10.1007/s10021-019-00458-7

Lettice C Hicks, Kathrin Rousk + Show 2 more

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https://doi.org/10.1007/s10021-019-00458-7

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Journal: Ecosystems	Publication Date: Nov 14, 2019
Citations: 15	License type: open-access

Affiliation: Lund University, University of Copenhagen

Abstract

Arctic and subarctic soils are typically characterized by low nitrogen (N) availability, suggesting N-limitation of plants and soil microorganisms. Climate warming will stimulate the decomposition of organic matter, resulting in an increase in soil nutrient availability. However, it remains unclear how soil microorganisms in N-limited soils will respond, as the direct effect of inorganic N addition is often shown to inhibit microbial activity, while elevated N availability may have a positive effect on microorganisms indirectly, due to a stimulation of plant productivity. Here we used soils from a long-term fertilization experiment in the Subarctic (28 years at the time of sampling) to investigate the net effects of chronic N-fertilization (100 kg N ha−1 y−1, added together with 26 kg P and 90 kg K ha−1 y−1, as expected secondary limiting nutrients for plants) on microbial growth, soil C and N mineralization, microbial biomass, and community structure. Despite high levels of long-term fertilization, which significantly increased primary production, we observed relatively minor effects on soil microbial activity. Bacterial growth exhibited the most pronounced response to long-term fertilization, with higher rates of growth in fertilized soils, whereas fungal growth remained unaffected. Rates of basal soil C and N mineralization were only marginally higher in fertilized soils, whereas fertilization had no significant effect on microbial biomass or microbial community structure. Overall, these findings suggest that microbial responses to long-term fertilization in these subarctic tundra soils were driven by an increased flow of labile plant-derived C due to stimulated plant productivity, rather than by direct fertilization effects on the microbial community or changes in soil physiochemistry.

Highlights

Arctic and subarctic ecosystems are often characterized by low nutrient availability, due to low temperatures, which slow rates of organic matter decomposition (Hobbie 1996)
At the studied subarctic tundra site, soils have been fertilized with inorganic NPK for 28 years, with N addition equivalent to 100 kg N ha-1 y-1
We found evidence for higher plant productivity with fertilization, as rates of gross ecosystem productivity measured after 26 years of fertilization were about four times higher in fertilized plots than controls (Figure 1A)

Summary

Introduction

Arctic and subarctic ecosystems are often characterized by low nutrient availability, due to low temperatures, which slow rates of organic matter decomposition (Hobbie 1996). Rather than relieving nutrient limitation and increasing microbial activity, the addition of inorganic N is often found to reduce microbial growth and respiration (Ramirez and others 2010; Rousk and others 2011; Kristensen and others 2018). This suggests that increased inorganic N availability can inhibit soil microbial activities (see reviews by Fog 1988; Treseder 2008; Pregitzer and others 2008; Whalen and others 2018). Increased N availability might have a positive effect on soil microorganisms indirectly, via stimulation of plant productivity which in turn provides organic matter to fuel microbial activity (Wardle 2002; Mack and others 2004)

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