LONG-TERM AND SHORT-TERM EFFECTS OF FIRE ON NITROGEN CYCLING IN TALLGRASS PRAIRIE

Fires in the tallgrass prairie are frequent and significantly alter nu trient cycling processes. We evaluated the short-term changes in plant production and microbial activity due to fire and the long-term conse quences of annual burning on soil organic matter (SOM), plant producti on, and nutrient cycling using a combination of field, laboratory, and modeling studies. In the short-term, fire in the tallgrass prairie en hances microbial activity, increases both above- and belowground plant production, and increases nitrogen use efficiency (NUE). However, rep eated annual burning results in greater inputs of lower quality plant residues causing a significant reduction in soil organic N, lower micr obial biomass, lower N availability, and higher C:N ratios in SOM. Cha nges in amount and quality of belowground inputs increased N immobiliz ation and resulted in no net increases in N availability with burning. This response occurred rapidly (e.g., within two years) and persisted during 50 years of annual burning. Plant production at a long-term bu rned site was not adversely affected due to shifts in plant NUE and ca rbon allocation. Modeling results indicate that the tallgrass ecosyste m responds to the combined changes in plant resource allocation and NU E. No single factor dominates the impact of fire on tallgrass plant pr oduction.