Comparison of Fall and Spring Applications of 15N-Labeled Urea to Douglas-Fir: II. Fertilizer Nitrogen Recovery in Trees and Soil after 2 years

Thirteen separate tree components were sampled for estimation of dry weight and for analysis for total N and ¹⁵N on relatively high site quality plots that had been treated with 224 kg N/ha as ¹⁵N-labeled urea. Although the increase in volume growth from fertilization was significant, we were unable to demonstrate a significant increase in weight of any tree component including foliage and boles. Specific gravity of bole wood was significantly reduced by urea fertilization. Fertilization significantly increased concentration of N in the 1977 and older foliage, in bole wood, and in bole bark, but not in other biomass components including roots. The only component in which total weight of N was significantly increased by fertilizer was bole wood. Fertilizer N in individual components as a percent of the total N in those components averaged 22.3% in 1979 foliage, 27.5% in 1978 foliage, 17.1% in 1977 and older foliage, 13 to 17% in twigs and branches, 19% in bole wood produced since fertilization, 19% in bole bark, and 16% in roots. Foliage contained an average of 55% of the total fertilizer taken up by the trees. Fertilizer N in the total tree varied from 25 to 36% of the application. Volume growth response directly corresponded to the quantity of fertilizer N recovered in trees. Differences both in N fertilizer in trees and in volume growth response appeared related to pattern of rainfall soon after treatment. Fertilizer accounted for in soils and trees at the end of two growing seasons averaged 68% of the application. Lowest recovery (52%) corresponded to the treatment which had lowest rainfall after application. Real and apparent N loss averaged 32 ± 6% of the application. Causes of apparent losses likely include litter falling outside of the plots, transport of N beyond the plots by roots passing below the plot barriers, and leaching to deeper soil depths than sampled but still in the rooting zone. Relative magnitudes of real and apparent losses of N were not determined.