Pw. Clinton et Dj. Mead, COMPETITION FOR NITROGEN BETWEEN PINUS-RADIATA AND PASTURE .2. TRENDSIN PLANT AND SOIL PROCESSES, Canadian journal of forest research, 24(5), 1994, pp. 889-896
N-15-labelled nitrogen fertilizer was used to examine the competition
for N between 4-year-old Pinus radiata D. Don and pasture grasses duri
ng one 8-month growing season. Availability and tree and pasture uptak
e of N-15-labelled fertilizer applied in spring were measured. Larger
quantities of (NO3-)-N-15-N than (NH4+)-N-15-N remained available in t
he soil throughout the growing season. However, P. radiata assimilated
similar quantities of N-15 when supplied as (NO3-)-N-15-N or (NH4+)-N
-15-N and uptake by pasture herbage was greater for (NO3-)-N-15-N than
for (NH4+)-N-15-N. Removing pasture by spraying with herbicide increa
sed the availability of soil N-15 soon after application and doubled t
he uptake of N-15 by P. radiata although the pattern of N uptake was n
ot greatly affected.