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.

In situ net nitrogen mineralization response to regional and interannual variation in precipitation across the Central Great Plains of North America.

McCulley, Rebecca¹, Burke, Ingrid¹, Lauenroth, William¹, Knapp, Alan², ^{1 2}

ABSTRACT- Regional analyses and biogeochemical models predict that ecosystem nitrogen (N) pools and N cycling rates increase from the semi-arid shortgrass steppe to the sub-humid tallgrass prairie of the Central Great Plains, yet few field data exist to evaluate these predictions. We hypothesized that in situ net N mineralization would increase with increasing N pool sizes across this grassland community gradient and track interannual and regional differences in precipitation and temperature. We measured in situ net N mineralization monthly during two growing seasons at five sites across a precipitation gradient in the Great Plains region. Growing season precipitation varied more than two-fold across the gradient and by >25% at each site between years. Respectively, soil N pools and the N content of aboveground net primary production (ANPP-N) increased significantly from the shortgrass steppe (208 and 1.76 g N m ^-2) to the tallgrass prairie (409 and 3.27 g N m ^-2), and ANPP-N at the shortgrass steppe sites tracked the differences in interannual precipitation (0.88 g N m ^-2 in a dry year to 2.12 g N m ^-2 in a wet year). However, measured in situ net N mineralization rates did not increase across the community gradient or reflect regional and/or interannual differences in precipitation and temperature. These data suggest that current methods of measuring in situ net N mineralization may not be effective for soils with large immobilization potentials.

KEY WORDS: in situ net nitrogen mineralization, nitrogen pools, precipitation gradient, grasslands