Substrate controls on nitrogen fixation and respiration in woody debris from the Pacific Northwest, USA

Abstract

We estimated the effect of wood species, tissue, and degree of decay on nitrogen fixation and respiration in woody debris from three sites in the Pacific Northwest. We also examined differences among sites and between actual and potential rates of nitrogen fixation and respiration where samples for potential measurements were amended with water and incubated for a week. We determined nitrogen fixation and respiration using acetylene reduction and CO 2 evolution, respectively. We also directly measured nitrogen fixation with 15 N 2 on a subset of samples to determine an average actylene reduced to 15 N 2 fixed (AR: 15 N 2 ) ratio of 4.4. Over the range of wood decay examined, actual and potential nitrogen fixation and respiration rates peaked in moderately decayed wood. Actual nitrogen fixation and respiration rates were significantly higher at a warmer, wetter coastal site when compared to two interior sites, but potential rates were not significantly different. There were no significant differences among Pseudotsuga menziesii (Mirb.) Franco, Tsuga heterophylla (Ref.) Sarg., or Picea sitchensis (Bong.) Carr. for nitrogen fixation or respiration. Nitrogen fixation and respiration rates were highest in bark, lower in sapwood, and lowest in heartwood. Patterns of microbial colonization and abundance, resource quality, and climate probably explain most of the patterns observed in our study.