NITROGEN AND PHOSPHORUS RELEASE FROM HUMUS AND MINERAL SOIL UNDER BLACK SPRUCE FORESTS IN CENTRAL QUEBEC

In the black spruce (Picea mariana (Mill.) BSP) forests which span Nor th America, low annual temperatures, high nitrogen and phosphorus abso rption by feathermosses and small litter inputs contribute to reduced annual N and P transformation rates in soils of these ecosystems. In p ast studies of nutrient dynamics in these systems, concentrations of d issolved organic N (DON) and P (DOP) in soil extracts have equaled or exceeded those of mineral N and P; therefore, organic forms of N and P may be an important source of nutrition to plants growing in this reg ion. Our objective was to determine if DON and DOP were important cons tituents in repeated extractions of laboratory incubated organic mater ial and mineral soils taken from recently burned, recently harvested a nd fully stocked black spruce stands in central Quebec. Cumulative con centrations of DON ranged from 7 to 17% and 31 to 45% of total N extra cted from the organic material and mineral soils, respectively. Cumula tive concentrations of DOP ranged from 35 to 44% and 37 to 48% of tota l P extracted from the organic material and mineral soils, respectivel y. We detected a pulse of CO2-C release from the organic material afte r thawing, and weekly CO2-C release was related. to NH4+-N release (R- 2 = 0.36, P = 0.0001). These results suggest that increases in tempera ture after a winter freeze result in a pulse of microbial activity and NH4+-N mineralization in organic layers and that DON and DOP are an i mportant part of N and P cycling in these boreal systems. (C) 1998 Els evier Science Ltd. All rights reserved.