Molybdenum limitation of asymbiotic nitrogen fixation in forests of Pacific Northwest America

Abstract

Nitrogenase activity as measured by acetylene reduction and nitrogen fixation as measured by 15N 2 uptake is widespread in forest litter, decaying wood and the lichen Lobaria in Pacific Northwest forests. In all cases the activity of nitrogenase is enhanced by addition of plant nutrient solutions and this is shown to be a specific molybdenum effect. A wide range of forest species have litter which supports nitrogen fixation during decay and the majority show molybdenum stimulation. The effect is most prominent in the acid soils situated between the Coast Range and the Cascade Range and extends from Oregon through Washington to British Columbia. Nitrogenase is increased quantitatively by Mo additions and the effect, though best seen in the laboratory, is also measurable in the field. The results are discussed on the basis of geographical distribution of the effect, plant species and the likely role of asymbiotic nitrogen fixation in contributing to long-term nutrient cycling.