INJECTION OF N-15 INTO TREES TO STUDY NITROGEN CYCLING IN SOIL

Most N-15 dilution techniques disturb either the soil or N-pool size. The objective of this study was to develop a method of labeling the roots of Populus trees with N-15 without physically disturbing the soil. Such a method would enable the direct measurement of the flux of N-15 from dead roots into the soil organic matter. Leaf and root biomass were labeled by injection of N-15 directly into the vessel elements of hybrid Populus trees during their second growing season. The N-15 was uniformly distributed throughout the canopy and root system. The rate and amount of N-15 turnover from plant tissue can be determined by pool transfer or through differences in plant N-15 concentrations. The N-15 was detected in the dead-root pool 8 wk after injection, indicating root turnover. Results demonstrate the ability to measure the contribution of fine-root litter to N-cycling processes without disturbing the soil environment.