PARENT SESSION
Oral Session #42: Carbon and Nitrogen Cycling. Presiding: S. Hobbie.
Wednesday, August 8, 2001. 8:00 AM to 11:30 AM. Hall of Ideas E.
The relative role of water and nitrogen in regulating productivity in tallgrass prairie.
Johnson, Loretta1, Behrens, Sarah1, Koelliker, James1, Knapp, Alan1, 1
ABSTRACT- Water limitation is a defining characteristic of prairie. However, the degree to which nitrogen co-limits productivity is not clear. Irrigation to eliminate water limitation, combined with N additions, allow us to assess the relative role of water and N in controlling productivity and to test for water-N interactions. The experiment is comprised of 3 levels of water (250, 150 mm in excess of natural precipitation, no water added) combined factorially with 4 levels of N (0, 2.5, 5, 10 g/m2/season). Objectives were to determine the relative importance of water and N in limiting productivity, test for water-N interactions, and identify functional group response. We hypothesized a synergistic water-N interaction, uplands will show a greater response than lowlands to water than N, and forbs will respond more strongly than grasses to water and N. First year results indicated that only grasses responded to water and N. Grasses in uplands responded to water and in lowlands to N but only to the highest levels of water or N. Contrary to hypotheses, we did not detect any water-N interaction. Year 2 results were similar. However, in year 2, we found a water-N interaction, but only in uplands. Forb biomass declined with added N in lowlands. We conclude that uplands experience water-N co-limitation, while N limits lowland productivity. Furthermore, naturally low N availability allows grass and forb coexistence in lowlands.
KEY WORDS: prairie, productivity, water, nitrogen