Fire and Grazing in the Tallgrass Prairie: Contingent Effects on Nitrogen Budgets

Abstract

Fire and grazing occur together in many of the world's grasslands, but their effects on nutrient cycling have usually been studied as if they acted separately. We hypothesized that grazing by large herbivores results in conservation of nitrogen that would otherwise be lost from burned grasslands. We tested this hypothesis in a series of experiments on burned and unburned tallgrass prairie grazed by cattle. We manipulated grazing using exclosures and mowing. Combustion losses of N from ungrazed plots (1.8 g°m—2°yr—1) burned in the spring were double those from similarly burned, grazed plots (0.9 g°m—2°yr—1). These losses represented about half of the preburn, aboveground stocks of N. The magnitude of N loss was proportional to the standing crop biomass available for combustion. Fire temperatures and energy release were reduced by grazing. We used mowing to simulate locally heavy grazing in patches. In the absence of burning, mowing patches increased the likelihood that a patch would be regrazed and caused persistent reductions in the residual biomass remaining in a patch at the end of the growing season. Mowing did not influence patch utilization or residual biomass when pastures were burned. Thus, the effects of fire on grassland N budgets were modified by grazing, and the effects of grazing on the patch structure of grasslands were modified by fire. We conclude that accurately predicting volatile losses of nutrients from grassland ecosystems resulting from biomass burning may depend on understanding effects of grazing.

Global assessment of the non‐equilibrium concept in rangelands

“…Such variables include alterations to fire frequency (Thompson Hobbs et al 1991, Fuhlendorf and Smeins 1997, Harrison et al 2003, invasive plants (Huebner and Vankat 2003), or herbivores regimes (Tadey and Farji-Brener 2007), and the presence of snow (Begzsuren et al 2004) or frost (Buttolph and Coppock 2004). These variables should not be considered part of the traditional non-equilibrium concept, and therefore their effects need to be studied on a case-by-case basis.…”

Response of Nebraska Sand Hills Natural Vegetation to Drought, Fire, Grazing, and Plant Functional Type Shifts as Simulated by the Century Model

“…However, grassland fire events naturally occur approximately every 3 to 5 years on the High Plains (Parton et al, 1993). Fire can influence the nitrogen budgets in grassland ecosystems (Hobbs et al, 1991), and can alter nutrient cycling processes . To evaluate the impact of a change in summertime fire regime on biomass, a CENTURY simulation was run with a 'cold' fire every three years under the rescaled 1934 mild drought conditions.…”

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).…”