Effects of the clear-cutting of a Douglas-fir plantation (Pseudotsuga menziesii F.) on the chemical composition of soil solutions and on the leaching of DOC and ions in drainage waters

The effects of the clear-cutting of a 70-year-old Douglas-fir plantation on the chemical composition of soil solutions and on leaching of nutrients in drainage waters were observed by a continuous monitoring, six years before and three years after the cutting. Forest harvesting was made with very limited soil disturbances. Results showed that the concentration of weakly fixed solutions did not change but that the concentration of gravitational solutions of the upper soil layers drastically fell down after the cutting. The limited increase in nutrients leached with drainage waters was only due to the increase in the water flux, which is difficult to quantify because of the presence of ground vegetation. The monitoring of numerous fluxes before and after the clear-cutting could explain the specific behaviour of the soil solutions. The limited losses of nutrients the after clear-cutting in a potentially responsive ecosystem were unexpected. The initial hypothesis was that the decrease in the mineralization and nitrification rates observed after the cutting was related to a stimulating effect of Douglas-fir on the activity of soil nitrifyers.