Potential microbial nitrogen and phosphorus availability in forest floors

The potential availability of nitrogen and phosphorus to microorganisms in forest floors was studied by means of a bioassay. Microbial N and P availability was assessed by analyzing the respiration rate response to addition of different amounts of N and P when glucose and other nutrients were added in excess. Forest floors of Norway spruce, Sitka spruce, Douglas-fir, beech, and oak from three sites of different nutrient status were studied. Oak forest floors had higher microbial N and P availability than forest floors of the other species, and P availability was lowest in Norway spruce forest floors. Sites differed only slightly in microbial P availability. The site with the most P rich soil also had the highest P availability in forest floors. The microbially-available proportion of total P was very high, and much higher than the available proportion of total N. Microbially-available N was not significantly related to KCl-extractable N, total N concentrations or C-to-N ratios, nor was microbially-available P related to concentrations of total P or C-to-P ratios. Basal respiration rates were positively related to microbial N and P availability. The bioassay assessed simple organic N compounds fairly well when these were added to forest floor material in low amounts. Microbial N and P availability in forest floors may be more dependent on other quality variables than total N and P concentrations, e.g. the organic forms of N and P.