Comparison of Fall and Spring Application of 15N-Labeled Urea to Douglas-Fir: I. Growth Response and Nitrogen Levels in Foliage and Soil

Movement, transformation, and recovery of fertilizer N by analysis of soil and foliage of seven- and nine-year-old Douglas-fir were followed over a two-year period at two relatively high quality sites in western Washington. Urea fertilizer labeled at approximately 8 atom percent ¹⁵N was applied at 224 kg N/ha at the end of November 1977 and in late April 1978. High rainfall followed the fall fertilizer treatment and low rainfall occurred after the spring treatment. Differences in variability in fertilizer recovery, fertilizer movement, and tree response to fertilization appeared to be related to differences in rainfall patterns shortly after fertilization. On the average, fall fertilization gave better volume growth response than spring fertilization, but substantial variation in response was observed between sites and season of application. Fertilizer N did not appear in substantial concentrations in foliage until after 6 weeks, with high levels of ¹⁵N still evident in foliage after 2 years. With the spring treatment, fertilizer N could have appeared earlier in the newly formed foliage, but this foliage was not sampled until August. The increase in total percent N in foliage lasted only about 1 year following spring treatment and 15 months following fall treatment. Peak concentrations of ammonium N in soil occurred in samples collected 9 d to 3 weeks after fertilization. Ammonium content in the soil declined rapidly and approached control levels at 24 weeks. However, at one site, nitrification was active and relatively high nitrate content was present in the soil at 24 weeks. Recovery of fertilizer N from analysis of soil was variable, averaging 59 ± 10% of the application at 3 d and about 100% between 9 d to 3 weeks. After two growing seasons, recovery from analysis of soil averaged only 38 ± 6%. Substantial increases in mineral N levels were observed in the soil beyond the quantity applied as fertilizer (the priming effect). Data from early samples of foliage showed increased concentration of N possibly resulting from this phenomenon. However, after the middle of the first growing season following treatment, no increased concentration of N in foliage from soil organic matter was evident.