Simulation of soil chemistry and nutrient availability in a forested ecosystem of southern Quebec — I. Reconstruction of the timeseries files of nutrient cycling using the Makedep model

The Makedep model is used to reconstruct the time-series input files of deposition, forest growth, uptake and litterfall needed to run the dynamic biogeochemical model SAFE. The amounts and timing of N made available for tree growth is what determines most of the model output. In this paper, the sensitivity of Makedep to various amounts and timing of N availability, including increased supplies of N prior to air pollution assuming increased biological N2 fixation, was assessed in a small hardwood stand of southern Quebec. The scenarios of N cycling were validated for burned and unburned conditions based on the simulated tree growth and N leaching rates as well as the model’s ability to recreate the trends in atmospheric deposition of major ions in northeastern North America. The N leaching rates were calculated as the residual component of all simulated processes acting as sources and sinks of N. The N availability scenarios that subtracted/added 5 mmol m 2 yr 1 of NH4 in the unburned and burned conditions, respectively, recreated N leaching rates and tree biomass with the most precision. Modifying the amounts of N made available prior to air pollution was also necessary for the trees to grow to reasonable values before 1900. Historical trends of deposition were linked to the changes in the continental emissions in northeastern North America. Observational data for 30 years at the Hubbard Brook Experimental Forest, New Hampshire, showed convergent trends for most ions with the deposition trends simulated with Makedep.