The use of different soil nitrogen sources by young Norway spruce plants

Three-year-old Norway spruce trees were planted into a low-nitrogen mineral forest soil and supplied either with two different levels of mineral nitro gen (NH4NO3) or with a slow-release form of organic nitrogen (keratin). Sup ply of mineral nitrogen increased the concentrations of ammonium and nitrat e in the soil solution and in CaCl2-extracts of the rhizosphere and bulk so il. In the soil solution, in all treatments nitrate concentrations were hig her than ammonium concentrations, while in the soil extracts ammonium conce ntrations were often higher than nitrate concentrations. After 7 months of growth, N-15 labelled ammonium or nitrate was added to the soil. Plants wer e harvested 2 weeks later. Keratin supply to the soil did not affect growth and nitrogen accumulation of the trees. In contrast, supply of mineral nit rogen increased shoot growth and increased the ratio of above-ground to bel ow-ground growth. The proportion of needle biomass to total above-ground bi omass was not increased by mineral N supply. The atom-% N-15 was higher in younger needles than in older needles, and in younger needles higher in pla nts supplied with N-15-nitrate than in plants supplied with N-15-ammonium. The present data show that young Norway spruce plants take up nitrate even under conditions of high plant internal N levels.