Simulation of Carbon and Nitrogen Cycling in an Alpine Tundra

Abstract

Simulations of an alpine tundra ecosystem using the CENTURY ecosystem model were conducted to test model descriptions of carbon and nitrogen cycling and to explore the alpine ecosystem response to physical and chemical components of global change. The parameterization of the alpine tundra for CENTURY was updated to reflect current knowledge of the site, and sensitivity analyses were conducted. Verification of results from a 6-yr fertilization experiment in the alpine tested the predictive capabilities of the parameterization. Simulations with increased winter precipitation and with the climate predicted under doubled atmospheric carbon dioxide concentrations were then conducted.

Modifications to the parameterization necessary to describe carbon and nitrogen cycling included decreasing the C:N ratios of plant tissues, increasing the amount of nitrogen retranslocated at the end of the growing season, extending the length of the growing season, and lowering the rate of decomposition. The updated parameterization requires 30% greater than observed inputs of net primary productivity to simulate observed levels of total soil carbon suggesting that soil carbon sequestration is not well represented in the model. Carbon and nitrogen cycling showed greatest sensitivity to the length of the growing season and to the temperature regulation of decomposition. Simulation of the nitrogen fertilization experiment resulted in 11% greater productivity than observed empirically, a reasonable verification of the updated parameterization. The major impact from increasing winter precipitation was a 30% increase in the amount of nitrogen in stream flow. Simulation with the climate predicted for a doubling of current carbon dioxide levels reduced production 10% while total soil carbon remained constant. This response was largely controlled by reduced soil moisture during the growing season.