Short‐Term Competition for Ammonium and Nitrate in Tallgrass Prairie

The availability of N limits productivity in tallgrass prairie. Spring burning is common because it results in greater plant productivity despite reducing net N mineralization. To better explain how burning affects inorganic N availability in tallgrass prairie, the partitioning of 15 N among plant and soil pools was measured in June and August 1994. Approximately 2.5 μg N g −1 soil was injected as either NH 4 or NO 3 to a depth of 15 cm within cores in burned and unburned prairie. Cores were removed from the field 6 d after injection, and 15 N recovery in plant and soil N pools was determined. No more than 14% of the applied 15 N remained in inorganic form 6 d after application. The largest portion of the applied 15 N (35–80%) was recovered in the soil organic nitrogen pool (N o ). Burning significantly increased the immobilization of both NH 4 and NO 3 within N o , and microbial biomass accounted for ≥50% of the 15 N recovered in N o Accumulation of 15 N in plants accounted for ≤35% of the applied 15 N with a majority recovered from roots. Burning had little effect on 15 N recovery in plants; however, 15 N accumulations in roots were significantly greater when NO 3 was used. Results indicate that immobilization within soil organic matter (SOM) controls the availability of both NH 4 and NO 3 to plants. Increased immobilization in soils with burning probably results largely from increased microbial N demand resulting from greater litter inputs with wider C to N ratios. Further research is needed to determine if abiotic mechanisms for N immobilization also significantly influence N availability in prairie soils.