Direct and Indirect Effects of Herbivores on Nitrogen Dynamics: Voles in Riparian Areas

Abstract

Abstract. Herbivores can directly increase nitrogen mobility by increasing the quality of organic matter entering the decomposition cycle, but they also may decrease nitrogen mobility by decreasing the biomass of high-nitrogen species in the plant community. We assessed effects of voles (Microtus) on nitrogen dynamics using exclosures in two riparian meadows (Crystal Bench and Blacktail Deer Creek) in Yellowstone National Park (USA). At both sites, the quantity of plant litter was decreased by herbivory following a vole population peak in 1992. At Crystal Bench, removal of voles caused a decrease in the nitrogen concentration and an increase in the C:N ratio of plant litter over the four years of the study. The higher quality litter produced in the presence of voles at Crystal resulted in a larger pool of potentially mineralizable nitrogen in soil from control plots relative to soils from plots that had not been accessible to voles. At Crystal, vole removal did not cause a change in plant community composition. However, at Blacktail, after several years of vole exclusion, legumes became more common in exclosures than in control plots that were accessible to voles. Selective herbivory on high-nitrogen legumes kept the litter quality outside exclosures low, whereas higher legume biomass caused a decrease in C:N ratio of plant litter inside exclosures. The removal of voles at Blacktail caused a 15% increase in the fraction of the soil nitrogen that was rapidly mineralizable.Our results show that voles increased nitrogen mobility, especially during and after population peaks. However, that increase was offset by decreases in nitrogen mineralization over longer periods when voles caused a decrease in high-quality plant litter produced by preferred forage plants, especially legumes. Thus, both the mechanisms by which voles affected nitrogen dynamics and the net effects of voles varied over time and space. The balance of direct and indirect effects may provide a general mechanistic explanation of whether herbivores increase or decrease the rate of nitrogen cycling.

Short‐ and long‐term effects of changes in reindeer grazing pressure on tundra heath vegetation

“…This is not surprising as the effects of herbivory on forage quality and nutrient cycling (Pastor et al 1993;de Mazancourt et al 1998;Sirotnak & Huntly 2000) differ at different temporal scales. However, short-term grazing enhancement proved to be an effective indicator of long-term effects, although the same was not true of short-term grazing reduction.…”

The Stoichiometry of Nutrient Release by Terrestrial Herbivores and Its Ecosystem Consequences

“…Many studies on herbivore-driven nutrient recycling in terrestrial ecosystems have focused on N, both for invertebrates (e.g., Seastedt and Crossley, 1984;Lovett and Ruesink, 1995;Belovsky and Slade, 2000;Reynolds and Hunter, 2001;Hunter et al, 2003;Metcalfe et al, 2014) and vertebrates (e.g., Pastor et al, 1988Pastor et al, , 1993Pastor et al, , 2006McNaughton et al, 1988;Hobbs et al, 1991;Frank and McNaughton, 1993;Frank and Evans, 1997;McNaughton et al, 1997;Ritchie et al, 1998;Sirotnak and Huntly, 2000;Olofsson et al, 2001;Stark et al, 2003;Fornara and Du Toit, 2008). For invertebrate herbivores, the general view is that they speed up nutrient cycling in terrestrial systems by changing litter quantity and quality, modifying the nutrient content of throughfall, and releasing easily-available nutrients in frass and cadavers (Hunter, 2001).…”

“…All herbivores consume and digest autotroph biomass, and release nutrients, e.g., nitrogen (N) and phosphorus (P), in wastes through excretion (urine) or egestion (feces). Nutrient release by herbivores can strongly impact nutrient availability for autotrophs in terrestrial, marine, and freshwater ecosystems (Pastor et al, 1993;McNaughton et al, 1997;Covich et al, 1999;Sirotnak and Huntly, 2000;Hunter, 2001;Vanni, 2002;Bardgett and Wardle, 2003;McIntyre et al, 2007;Cech et al, 2008;Roman and McCarthy, 2010;Metcalfe et al, 2014;Turner, 2015;Doughty et al, 2016). The ratio of N to P released (i.e., waste N:P) may be crucial for mediating ecosystem impacts of herbivore-driven nutrient recycling (Sterner, 1990;Urabe et al, 1995;Elser and Urabe, 1999).…”

Burrowing herbivores alter soil carbon and nitrogen dynamics in a semi-arid ecosystem, Argentina

“…A potential mechanism accounting for these observations is that the increase in quality of litter produced by the shift in plant composition with grazing from perennial grasses to annual and perennial forbs compensated for the reduction in litter quantity in grazed zones (Sirotnak and Huntley, 2000;Villarreal et al, 2008;Vaieretti et al, 2013). Our field data also indicate that activities of vizcachas did not promote erosion of organic matter pools or result in soil compaction, potential mechanisms that affect C and N dynamics in other heavily grazed ecosystems (e.g., Schrama et al, 2013), as we observed no differences in soil organic matter content, bulk density or particle size in surface soil in zones with and without vizcacha activity.…”