Effects of Grazing and Fire Frequency on Floristic Quality and its Relationship to Indicators of Soil Quality in Tallgrass Prairie

Fire and grazing are widely used to manage grasslands for conservation purposes, but few studies have evaluated the effects of these drivers on the conservation value of plant communities measured by the floristic quality index (FQI). Further, the influence of fire and grazing on soil properties and functions are difficult for land managers and restoration practitioners to assess. The objectives of this study were to: (1) quantify the independent and interactive effects of grazing and fire frequency on floristic quality in native tallgrass prairie to provide potential benchmarks for community assessment, and (2) to explore whether floristic quality can serve as an indicator of soil structure and function for more holistic ecosystem assessments. A factorial combination of fire frequencies (1–2, 4, and 20 years return intervals) and grazing (by bison or ungrazed) treatments were sampled for plant species composition, and for several indicators of soil quality in lowland tallgrass prairie. Floristic quality, diversity, and richness were higher in grazed than ungrazed prairie over all fire frequencies (P < 0.05). Available inorganic N, microbial biomass N, total N, and soil bulk density were also higher in grazed prairie soil over all fire frequencies (P < 0.05). Microbial biomass C, total organic C, and total soil N were positively correlated with FQI (P < 0.05). This study shows that floristic quality and soil N pools are more strongly influenced by grazing than fire and that floristic quality can be an indicator of total soil C and N stocks in never cultivated lowland prairie.