Document: ANN-3-59-30

The impact of nitrogen availability on ecosystem carbon storage in deciduous forests of the Great Lakes region.

HARTLEY, A.E.* 1, D.W.KICKLIGHTER 2, J.M.MELILLO 2 and P.S.CURTIS 1

The Ohio State University, Columbus, OH 43210 USA 1
The Ecosystems Center, Woods Hole, MA 02543 USA 2

Abstract:
North American forests are predicted to be major sinks for atmospheric carbon (C). Understanding the dynamics of this sink is of central importance in studies of the global C cycle and for establishing compliance with the Kyoto protocols governing greenhouse gas emission. Climate warming is expected to accelerate nitrogen (N) mineralization, which could enhance C storage in temperate forests. This work has two components: On-going field studies in three northern temperate deciduous forests that are part of an international network of forest C exchange monitoring sites (the "AmeriFlux" network), and regional extrapolation and simulation modeling using the Terrestrial Ecosystem Model (TEM). During the first field season, we analyzed site-level differences in vegetation and soil characteristics. The three sites represent a gradient in climate, soil texture and N availability. Net N mineralization was lowest in Michigan (5.0 kg ha-1 growing season-1) and highest in Indiana (95.5 kg ha-1 growing season-1). Net primary production (NPP) ranged from 344 g C m-2 y-1 in Michigan to 862 g C m-2 y-1 in Indiana. In the model analysis, we simulated the impact of historical climate change (increases in temperature and precipitation from 1920 to 1992) and rising atmospheric CO2 concentrations (from 305 to 349 ppm) on C and N pools and fluxes in temperate deciduous forests. From 1920-1929 to 1983-1992, NPP increased at the site level by 9% in Indiana (to 794.6 g C m-2 y-1), by 22% in Ontario (to 344.3 g C m-2 y-1) and by 28% in Michigan (to 331.2 g C m-2 y-1). For the same time period, the model results indicate net N mineralization rates increased by 4% in Indiana (to 108.8 kg N ha-1 y-1), decreased 1% in Ontario (to 51.1 kg N ha-1 y-1) and decreased 8% in Michigan (to 43.1 kg N ha-1 y-1). We estimate that NPP for temperate deciduous forests in the Great Lakes region as a whole increased by 23% (to 187.1 Tg C y-1) from 1920-1929 to 1983-1992. Net ecosystem production increased by 84% (to 64.0 Tg C y-1), while net N mineralization rates remained unchanged. Long-term field measurements are planned to investigate the relationship between forest C storage and soil N cycling at AmeriFlux forest sites.

Keywords: nitrogen mineralization, carbon storage, temperate deciduous forests, ecosystem modeling

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