EFFECTS OF IRRIGATION AND BALANCED FERTILIZATION ON NUTRIENT CYCLING IN A DOUGLAS-FIR STAND

The effects on growth and nutrient status of an increased availability of all major nutrients was studied in a 40 year old Douglas fir stand . The nutrient amounts were applied by daily sprinkling and were in fi xed, optimal proportions to the estimated annual uptake of N. Irrigati on was included to avoid drought stress. The nutrient applications wer e also done in combination with a lower NH4 load, realized under a roo f that was placed above the ground. According to nutritional standards nitrogen supply was optimal and was related to the moderately high at mospheric N inputs. Nutrition of P, K acid in some years Ca seemed cri tical. In all four studied growing seasons water additions resulted in a growth increase, of 30 to 40%. Irrigation decreased needle fall. Nu trient applications increased the needle concentrations of P and K con siderably, but this did not result in a growth enhancement. Fertilizat ion also increased K and P in shedded needles. Only minor leaching los ses were calculated. The recovery in trees of fertilizer K and P was m aximally 15%. Input-output budgets suggested that P was strongly retai ned in the soil. The decrease in NH4 load had slightly decreased N ava ilability and soil acidification, and seemed to enhance tree growth. T he use of tree growth as a parameter to evaluate the effects of an imp roved nutritional balance is discussed.