SITE-SPECIFIC GROWTH AND NUTRITION OF PLANTED PICEA-MARIANA IN THE ONTARIO CLAY BELT .5. HUMUS NITROGEN AVAILABILITY

The relationship between forest humus chemistry and net N mineralization in three boreal ecosystems was investigated by measuring N release in samples during an 8-week aerobic laboratory incubation. Nitrogen mineralization was significantly greater in the upland Feathermoss humus, intermediate in the Ledum humus, and lowest in Alnus - Herb poor substrate, although differences between the last two lowland sites were not significant. Correlations of net mineralizable N at 8 weeks with humus chemistry variables were weak, although initial N release was correlated with total N. Field-fertilized (urea) and control substrates from the three ecosystems were similarly incubated to observe effects of N addition on mineralization and humus chemistry. Substrates were sampled 16 months after fertilization, and net N mineralization during incubation was most enhanced in Alnus humus and least affected in Feathermoss humus. Response was most strongly related to positive changes in humus total N. Growth response of outplanted, unfertilized seedlings was not correlated with N mineralization in the incubation, but was highly correlated with humus total N and N:P ratio, in both the 1st and 2nd years after outplanting.