Soil Microbial Responses to Herbivory in an Arctic Tundra Heath at Two Levels of Nutrient Availability

Abstract

We tested the hypothesis that herbivory accelerates nutrient cycling through nutrient-rich soils and decelerates nutrient cycling through nutrient-poor soils in a welldrained arctic tundra heath by measuring effects of herbivory on soil and plant properties in control areas and areas treated with NPK fertilizer. The impact of herbivores was studied with two types of exclosures, one excluding reindeer and the other excluding both rodents and reindeer. We predicted that herbivores would enhance soil microbial processes in the fertilized areas, but retard them in the unfertilized ones.Microbial respiration and microbial C were significantly lower in grazed areas than in herbivore exclosures in both unfertilized and fertilized treatments, indicating that herbivores limit the C available for the soil microbes. Microbial N was significantly increased in the exclosures in the fertilized treatment, but there were no effects in the unfertilized one. This reflects both an increase in resources by fertilization and the effect of herbivores on microbial N acquisition: there was surplus N in the fertilized areas to be immobilized by microbial biomass only when access by herbivores was prevented. Thus, mammalian grazers affect the resource coupling between the plant and microbial trophic levels. Fertilization did not affect the soil inorganic, organic, or microbial N contents, or litter decomposition rates, but it significantly increased soil and microbial P. Microbial C was significantly decreased by fertilization, contradicting some earlier fertilization studies in the arctic.One reason for deceleration of soil nutrient cycling in our study area may be that herbivory by both reindeer and rodents occurs mainly outside the growing season; hence, mammalian waste products do not contribute to soil nutrient availability at the times of highest nutrient demands by plants. In addition, grazing during reindeer migrations is likely to cause net resource output from the system.

Interactive Effects Between Reindeer and Habitat Fertility Drive Soil Nutrient Availabilities in Arctic Tundra

“…These mechanisms occur simultaneously and can have contrasting effects on soil nutrient availability and hence primary production. For instance, empirical studies have found positive (Hobbs and others 1991;Frank and Evans 1997;McNaughton and others 1997), negative (Pastor and others 1993;Ritchie and others 1998;Fornara and Du Toit 2008) or mixed effects (Olofsson and others 2001;Olofsson and Oksanen 2002;Stark and Grellmann 2002;Stark and others 2003) of herbivores on nitrogen (N) cycling and primary production.…”

“…For two reasons this is a suitable system to test the effects of herbivory on nutrient availability across a productivity gradient: (1) plant community structure and stoichiometry is known to vary dramatically along topographic gradients at relatively small spatial scales (Bjö rk and others 2007; Chu and Grogan 2010; Sundqvist and others 2011a), and (2) reindeer have been shown to have strong but varying effects on nutrient availability (Olofsson and others 2001;Stark and Grellmann 2002;Stark and others 2002;Olofsson and others 2004;Van der Wal and Brooker 2004;Stark and others 2010). So far, studies on the effects of reindeer on nutrient availability have focused primarily on N, likely because it is considered to be the main limiting nutrient in tundra ecosystems (Shaver and Chapin 1980;Tamm 1991;Aerts and Chapin 2000).…”

Moth herbivory enhances resource turnover in subarctic mountain birch forests?

“…In arctic communities, intensive mammalian herbivory on aboveground plant parts has been reported to result in either acceleration (Stark and Grellmann 2002) or deceleration (Olofsson and Oksanen 2002) of nutrient cycling rates. Whether herbivory accelerates or decelerates nutrient cycling may be crucial to the recovery of ecosystems after herbivore infestation.…”

Effects of reindeer density on vascular plant diversity on North Scandinavian mountains

“…Studies on the impact of reindeer on Fennoscandian arctic-alpine nature have improved our knowledge on the impact of reindeer on the coverage of lichens (Johansen et al, 1995;Ihse & Allard, 1995;Johansen & Karlsen, 1996;Allard et al, 1998), on nutrient mineralization rate and primary production (Stark et al, 2000;Olofsson et al, 2001;Stark & Grellmann, 2002), on ground temperatures (Olofsson et al, 2003) and on abundances of ground-dwelling (Suominen et al, 1999a;Suominen et al, 1999b) and galling (Olofsson & Strengbom, 2000;Herder et al, 2004) invertebrates. Vegetation studies have corroborated the predictions of IDH in the context of lichen rich heathlands, which become overwhelmingly dominated by Cladina stellaris in the absence of reindeer, whereas only crustose lichens survive on the most grazed and trampled sites along reindeer fences.…”