Modeling nitrogen fixation in dead wood

A mechanistic simulation model of nitrogen fixation in dead wood was developedto help synthesize knowledge, develop hypotheses, and estimate rates of nitrogen fixationin the Pacific Northwest. In this model nitrogen fixation is directly controlled by logsubstrate, temperature, moisture, and oxygen content. Respiration and diffusion ofoxygen indirectly affect nitrogen fixation and respiration by regulating log oxygencontent.
The relationships of abiotic and biotic variables on nitrogen fixation andrespiration and the relationships of wood moisture and density were determined inlaboratory experiments to parameterize the model. Nitrogen fixation and respiration hadsimilar responses to temperature, with nitrogen fixation being optimum near 30°C and
respiration being optimum over a broader range from 30°C to 50°C. Nitrogen fixationand respiration responded similarly to wood moisture with little activity below 50%, andoptimal activity above 175% to 100% moisture content for nitrogen fixation andrespiration, respectively. Nitrogen fixation was optimized at 2% 02. In contrast,respiration rates were optimal when 02 exceeded 1%. Nitrogen fixation and respiration in woody debris were significantly influenced by the degree of decay of the wood, andthe woody tissue type, but not by the species of dead wood. In both the radial andlongitudinal directions, the oxygen diffusion coefficient (Do2) in wood increasedexponentially as the fraction of pore space in air (FPSA) increased and as densitydecreased. Do2 in the longitudinal direction was 1.4 to 34 times greater than for theradial direction at zero and one FPSA, respectively.
In comparison to independent data, the model of nitrogen fixation reasonablyestimated seasonal patterns of log temperature, moisture, oxygen content, and respirationrate. The model estimates an annual nitrogen fixation rate of 0.7 kg 1\1-ha-1-yr-1 for an old-growth stand at the H. J. Andrews, which is reasonably close to an independent estimateof 1.0 kg N-ha-1-yr-1 made for the same stand.
Despite low annual rates of asymbiotic nitrogen fixation in wood, soil, and litter,this process can contribute 9% to 42% of a stands nitrogen inputs over succession whensymbiotic fixers such as Alnus rubra and Lobaria oregana are present and absent,respectively. Managed stands with reduced levels of woody debris and litter maytherefore be losing a significant nitrogen input.