Production and root uptake of mineral nitrogen in a chronosequence of Douglas-fir ( Pseudotsuga menziesii) in the Beaujolais Mounts

Nitrogen cycle dynamics were studied in a chronosequence of Douglas-fir ( Pseudotsuga menziesii) plantations, aged 20, 40 and 60 years, respectively. Mineralizing capacity was high in the three stands. After mineralization, nitrogen was mainly nitrified. Mineralization and nitrification were independent of stand age. Root uptake of nitrogen was high but lower than mineralization. Consequently, losses by leaching were high, especially in the younger stand. Nitrate leaching was accompanied by cation leaching, resulting from soil acidification. The destabilization of organic matter following the introduction of a non-native species may have been reinforced by former land use. Soils were in the process of losing nutrients, especially in the young stand. As nutrient losses decrease with increasing stand age, forest rotations of over 60 years are recommended, as fertility budgets are closer to equilibrium.