Ungulate and topographic control of nitrogen: phosphorus stoichiometry in a temperate grassland; soils, plants and mineralization rates

Although the link between the nitrogen (N): phosphorus (P) stoichiometry of biota and availability has received considerable attention in aquatic systems, there has been relatively little effort to compare the elemental composition of biota and supply in terrestrial habitats. In this study, I explored the effects of a prominent topo-edaphic gradient, from dry hilltop to wet slope-base, and native ungulates on N and P of soils, plants, and rates of in situ net mineralization in grasslands of Yellowstone National Park. Nitrogen and P measurements were made May-September, 2000, in paired, grazed and 38-42 year fenced, ungrazed grassland at five topographically variable sites. Similar to findings from other grassland ecosystems, several site factors associated with organic activity, including soil moisture, C, and plant biomass, covaried with soil N concentration and/or net N mineralization. Soil P concentration and net P mineralization, however, were unrelated to those factors. Instead, net P mineralization was negatively related to soil pH, which is known to control the form of inorganic P and its availability, and soil P was uncorrelated with any soil or plant variable measured in the study. Because of being influenced by different soil properties, N and P net mineralization were unrelated among grasslands. Furthermore, supply and plant N:P ratios were uncorrelated in this grassland system. Based on critical N:P ratios reflecting nutritional limitation of plants, Yellowstone grassland vegetation ranged from being N limited to N-P co-limited. Grazers increased N-P co-limitation by enhancing plant N concentrations and the soil pH gradient across grassland sites regulated plant nutritional limitation by affecting plant-available P. These findings showed how ungulates and a landscape factor, i.e. soil pH, determined plant nutrient status among YNP grasslands differently by influencing plant N concentration versus plant P concentration, respectively.