Simulation of the long-term carbon and nitrogen dynamics in Dutch forest soils under Scots pine

Dynamics of C and N in forest soils in the Nutrient Cycling and Soil Acidif ication Model (NUCSAM) are described by the transformation and decompositio n of three organic matter compartments, litter, fermented material and humi c material. These three compartments are allocated to the morphological dis tinguishable L, F and H horizons of the organic layer. Changes in the pools of these organic compartments are described with first order equations for decomposition and transformation. Rate constants for decomposition and tra nsformation were derived by calibrating the model to measured organic matte r pools in organic layers of a chronosequence of five first succession Scot s pine stands between 15 and 120 years old. Simulated pools of organic matter in the organic layers were in agreement w ith measured pools in the five pine stands, except for the first thirty yea rs of the H-horizon. During this period, an increase in organic matter in t he H horizon was simulated while no H horizons were observed in the field. The simulated total pool of organic matter in the organic layer agreed well with values from a field inventory in 20 other Scots pine stands, but the simulated distribution over the three horizons differed from the field meas urements which varied among sites. For the Scots pine stands the model was able to simulate the organic matter accumulation in the top 40-cm of the mi neral soil; derived almost completely from fine root turnover. The accumula ted pool of nitrogen in the organic layer was in agreement with measured po ols for the oldest Scots pine stand but was too high for the younger stands . Especially, the accumulation of N in the F-horizon was too fast, presumab ly due to an overestimated retention of nitrogen.