ANAEROBICALLY MINERALIZABLE SOIL-N AS A PREDICTOR OF GROWTH-RESPONSE TO FERTILIZATION IN LODGEPOLE PINE

The relationship between the amount of mineralizable N in mineral soil and the basal area response of trees to N fertilization was examined in 24 stands of lodgepole pine (Pinus contorta var. latifolia Engelm.) in interior British Columbia. Plots received a single application of either NH4NO3 or urea between 1980 and 1983. Increment cores taken from trees on fertilized and control plots 4 years after fertilization were used to estimate basal area response of fertilized trees relative to controls. Amounts of N mineralized during a 1-week anaerobic incubation of mineral soil from control plots at each site were used as an estimate of N availability in each stand. The relationship between N availability and growth response of trees was weak, except that stands with mineralizable soil N greater than 40 ppm were not responsive to fertilization. Measurement of mineralizable soil N might be useful prior to screening trials to eliminate stands with very high levels of available N.