Abstract
Herbivores impact nutrient availability and cycling, and the net effect of herbivory on soil nutrients is generally assumed to be positive in nutrient-rich environments and negative in nutrient-poor ones. This is, however, far from a uniform pattern, and there is a recognized need to investigate any interactive effects of herbivory and habitat fertility (i.e., plant C/N ratios) on soil nutrient availabilities. We determined long-term effects of reindeer on soil extractable nitrogen (N) and phosphorus (P) and their net mineralization rates along a fertility gradient of plant carbon (C) to N and P ratios in arctic tundra. Our results showed that reindeer had a positive effect on soil N in the more nutrient-poor sites and a negative effect on soil P in the more nutrient-rich sites, which contrasts from the general consensus. The increase in N availability was linked to a decrease in plant and litter C/N ratios, suggesting that a shift in vegetation composition toward more graminoids favors higher N cycling. Soil P availability was not as closely linked to the vegetation and is likely regulated more by herbivore-induced changes in soil physical and chemical properties. The changes in soil extractable N and P resulted in higher soil N/P ratios, suggesting that reindeer could drive the vegetation toward P-limitation. This research highlights the importance of including both the elements N and P and conducting studies along environmental gradients in order to better understand the interactive effects of herbivory and habitat fertility on nutrient cycling and primary production.
Highlights
Large mammalian herbivores influence nutrient cycling in ecosystems throughout the world (Milchunas and Lauenroth 1993; Hobbs 1996; Bardgett and Wardle 2003)
It has been assumed that herbivores have mainly positive effects on N availability and primary production in nutrient-rich environments dominated by plants of high quality (i.e., low carbon (C) to N ratios), and mainly negative ones in nutrient-poor environments dominated by plants of low quality (Hobbs 1996; Bardgett and Wardle 2003; Pastor and others 2006)
We analyzed the effects of grazing regime, topographic position and their interaction on plant, litter and soil elemental ratios, soil extractable N and P, net N and P mineralization rates, microbial N and P with linear-mixed effects models, whereby we used the paired plot number as random factor to account for the spatial autocorrelation of the paired study design
Summary
Large mammalian herbivores influence nutrient cycling in ecosystems throughout the world (Milchunas and Lauenroth 1993; Hobbs 1996; Bardgett and Wardle 2003). Herbivores influence the rate and form of nutrients that are returned to the soil directly by returning nutrients consumed as plant material in the form of dung and urine or indirectly by altering plant litter quantity and its nutrient content and degradability (McNaughton and others 1997; Ritchie and others 1998; Bardgett and Wardle 2003; Bakker and others 2009) These mechanisms occur simultaneously and can have contrasting effects on soil nutrient availability and primary production. These studies emphasize the need to investigate the effects of herbivores across environmental gradients
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