SHORT-TERM COMPETION FOR AMMONIUM AND NITRATE IN TALLGRASS PRAIRIE

Authors: Dell, Curtis; RICE, CHARLES - KANSAS STATE UNIV.

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/23/2004
Publication Date: 4/4/2005
Citation: Dell, C.J., Rice, C. 2005. Short-term competion for ammonium and nitrate in tallgrass prairie. Soil Science Society of America Journal. 69:371-377.

Interpretive Summary: The availability of inorganic nitrogen (NH4+ and NO3-) is a primary factor limiting both plant productivity and soil microbial activity in tallgrass prairie, therefore competition for that limited resource is likely to influence productivity. Annual spring burning increases plant productivity but appears to result in greater N demand. Tracer experiments with 15NH4+ and 15NO3- were conducted to follow the partitioning of available N and to determine if the competition changed with burning. Additionally, the experiment was conducted in June and August to determine if the competition was affected by plant growth stage. Cores were placed into annually burned and unburned prairie and injected with 15N solutions. Cores were removed from the field six days after injection, and the 15N content of plant and soil N pools was determined. In both months, the majority 15N applied as either 15NH4+ or 15NO3- was recovered from the soil microbes regardless of burning. Burning increased the microbial uptake of NO3-, further limiting the N availability to plants. The results show that soil microbes control the flow of N in the tallgrass prairie. Given the very limited supply of inorganic N available to the plants, productivity in tallgrass prairie soils is highly dependent on N conservation mechanisms within the plant.

Technical Abstract: The tallgrass prairie is a N-limited ecosystem, therefore competition between plants and soil microorganisms is likely. Annual spring burning increases plant productivity but appears to result in greater N demand. To determine partitioning of N between plants and microorganisms, 15N was applied as either KNO3 or (NH4)2SO4 in June and August, 1994. The tracer was injected to a depth of 15 cm within 25-cm-diameter cores, which had been inserted into burned and unburned prairie. Cores were removed from the field six days after injection, and the 15N content was determined for the foliage, rhizomes, roots, and the soil inorganic, organic, and microbial biomass N pools. Distribution of recovered N was similar in June and August. No more than 14% of the 15N remained in inorganic form, with the least remaining in the burned treatments. Regardless of burning or N source, the largest portion of the tracer was recovered in the soil organic fraction. The amount of 15N recovered in the soil organic fraction was significantly lower in the unburned/NO3- treatment. Most of the 15N recovered in plants was found in the roots, with the greatest accumulations when N was applied as NO3-. Burning did not significantly affect the quantities of 15N recovered in the plant components. Results indicate microbes dominate the supply of available N in the short term. Greater microbial uptake of NO3- with burning showed that increased microbial N demand further limits the availability of N to plants. Sustained increases in plant productivity with burning, despite greater microbial N immobilization, must be linked to higher N-use-efficiency related to internal cycling.