Effects of repeated fertilization on nitrogen fluxes in a young lodgepole pine stand as measured by an in situ sequential coring technique

A young lodgepole pine stand has been fertilized four times since 1982 with four levels of nitrogen (N) fertilizer with or without other macro and micro nutrients ("PK"). The effect of these repeated fertilizer additions on N mineralization and uptake has been assessed using a sequential coring in situ technique. Both N and PK fertilization appear to have increased N mineralization and uptake (but not significant statistically), while N additions have significantly increased the inorganic N concentration in the soil. Because N mineralization has been increased, it appears as though repeated fertilization has resulted in a sustained increase in site quality as opposed to a short term increase in tree production. The increased uptake demand of the trees on the heavily fertilized plots can be partially met by increased mineralization; therefore, fertilizer application on these plots could theoretically be reduced without reducing the rate of uptake. Even though moisture appears to be limiting on the site, significant growth responses indicate that an improvement in site nutrient status has increased the degree to which growth potential for the site has been achieved. However, responses have probably been reduced by moisture limitations on the site. The sequential coring technique appears to offer several advantages over other field and lab techniques for assessing N fluxes. Because it results in minimal soil disturbance and is sensitive to environmental factors which affect N processes, it should be useful for testing models of N mineralization and evaluating more rapid indexes of N availability. Gross mineralizable N (incubated mineral N concentration) estimated by anaerobic incubation in the laboratory produced results consistent with those obtained in the field. Gross mineralizable N also correlated more closely with foliar N concentration and content (on a per needle basis) than in situ mineralization. However, net mineralizable N (gross minus initial N mineral concentration) did not produce results consistent with the sequential coring data from the field. Sequential coring also requires very intensive spatial and temporal sampling (at least following fertilization) which will restrict its use to research, and then only to studies which require accurate estimates of in situ N fluxes, as opposed to a simple index of N availability.Forestry, Faculty ofGraduat