Temporal and spatial distribution of the nifH gene of N2 fixing bacteria in forests and clearcuts in western Oregon

Decomposition of plant litter is a primary mechanism of nutrient recycling and redistribution in most terrestrial ecosystems. Previously we demonstrated by a nested PCR protocol that 20 distinctive nifH (the gene encoding nitrogenase reductase) HaeIII restriction fragment length polymorphism (RFLP) patterns were derived from bulk DNA associated with samples of plant litter and soil collected at one Douglas Fir (DF) forest [33]. Five of the nifH DNA patterns (II-VI) were dominant types in DF litter with characteristic fragments of 237-303 bp length, whereas samples from soil contained primarily seven other patterns 131-188 bp length (IX-XV). Here we report that the 237-303 bp fragments characteristic for forest litter could generally not be detected in plant litter or soil samples collected in clearcuts that adjoin the forest sites. The same fragments (237-303 bp) were also found in the litter at this DF forest site over 16 months and were consistently found in litter at 12 other DF forest or recent (<2 yrs) clearcut sites. However, trace to none of these fragments were detected in 6 clearcut (5-10 yrs) or different forest types (oak, alder) collected over a 200 km east-west direction in western Oregon, USA. Data suggest that the logging practice in DF forests that creates a clearcut removes a unique gene pool of nitrogen-fixing microorganisms. These organisms could potentially contribute more to nitrogen fixation in forest litter than litter from natural or invasive plants that grow in clearcuts [26].