Cm. Preston et Dj. Mead, GROWTH-RESPONSE AND RECOVERY OF N-15-FERTILIZER ONE AND 8 GROWING SEASONS AFTER APPLICATION TO LODGEPOLE PINE IN BRITISH-COLUMBIA, Forest ecology and management, 65(2-3), 1994, pp. 219-229
This paper reports plant and soil distribution of N-15 1 and 8 years a
fter fertilizer application near Spillimacheen in the British Columbia
interior. The experiment was originally established to test the effic
acy of fertilization on snow; N-15-urea, (NH4NO3)-N-15 and (NH4NO3)-N-
15 were applied at 100 kg N ha-1 to 11-year-old lodgepole pine (Pinus
contorta Dougl. var. latifolia Engel M .) in January 1981. After one g
rowing season (October 1981), eight of the 16 plots were destructively
sampled. Total recovery of N-15 in plot trees was low, from 1.9 to 10
.1%. Recovery in understorey was comparable (2.4-3.4%), and 30.6-73.2%
of N-15 was retained in the soil in organic form. The remaining eight
plots were sampled in August 1988. There was a significant growth res
ponse to fertilization, amounting to a 34% increase in stem volume for
fertilized (ammonium nitrate or urea) versus control trees after eigh
t growing seasons. Approximately two-thirds of the N-15 recovered in 1
981 could still be accounted for in plant biomass and soil. There had
been little additional N-15 uptake by plot trees, but more continuing
uptake by understorey. About one-fifth of N-15 recovered in 1988 was f
ound outside the plot boundaries. The results are consistent with the
hypotheses that (i) tree response to fertilization is largely the resu
lt of the increase in photosynthetic capacity generated by the first y
ear of uptake, (ii) fertilizer N, once immobilized in the soil, has lo
w availability to crop trees, and (iii) the N mineralized is subject t
o losses, presumably by leaching and denitrification. Strategies are n
eeded for maximizing the uptake of N in the first growing season. Furt
her research is recommended to determine what factors limit the uptake
of available N by trees, and to quantify natural levels of leakage of
mineral N from the ecosystem due to denitrification and leaching.