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 North America, low annual temperatures, high nitrogen and phosphorus absorption by feathermosses and small litter inputs contribute to reduced annual N and P transformation rates in soils of these ecosystems. In past studies of nutrient dynamics in these systems, concentrations of dissolved 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 region. Our objective was to determine if DON and DOP were important constituents in repeated extractions of laboratory incubated organic material and mineral soils taken from recently burned, recently harvested and fully stocked black spruce stands in central Quebec. Cumulative concentrations of DON ranged from 7 to 17% and 31 to 45% of total N extracted from the organic material and mineral soils, respectively. Cumulative concentrations of DOP ranged from 35 to 44% and 37 to 48% of total P extracted from the organic material and mineral soils, respectively. We detected a pulse of CO2-C release from the organic material after thawing, and weekly CO2-C release was related to NH4+-N release (R2=0.36, P=0.0001). These results suggest that increases in temperature 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 important part of N and P cycling in these boreal systems.