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Surface wood samples obtained from a Douglas fir log (Pseudotsuga menziesii ) in a Pacific Northwest stream were incubated in vitro with [14 C]lignocellulose in a defined mineral salts medium supplemented with 10 mg of N liter−1 of15 N-labeled NO3 − (50 atom%15 N). Evolution of14 CO2 , distribution and isotopic dilution of15 N, filtrate N concentrations, and the rates of denitrification, N2 fixation, and respiration were measured at 6, 12, and 18 days of incubation. The organic N content of the lignocellulose-wood sample mixture had increased from 132 μg of N to a maximum of 231 μg of N per treatment after 6 days of incubation. Rates of [14 C]lignocellulose decomposition were greatest during the first 6 days and then began to decline over the remaining 12 days. Total CO2 evolution was also highest at day 6 and declined steadily over the remaining duration of the incubation. Filtrate NH4 + -N increased from background levels to a final value of 57 μg of N per treatment. Filtrate NO3 − N completely disappeared by day 6, and organic N showed a slight decline between days 12 and 18. The majority of the15 N that could be recovered appeared in the particulate organic fraction by day 6 (41 μg of N), and the filtrate NH4 + N fraction contained 11 μg of15 N by day 18. The15 N enrichment values of the filtrate NH4 + and the inorganic N associated with the particulate fraction had increased to approximately 20 atom%15 N by 18 days of incubation, whereas the particulate organic fraction reached its highest enrichment by day 6. Measurements of N2 fixation and denitrification indicated an insignificant gain or loss of N from the experimental system by these processes. The data show that woody debris in stream ecosystems might function as a rapid and efficient sink for exogenous N, resulting in stimulation of wood decomposition and subsequent activation of other N cycling processes.

Nitrogen Dynamics in Stream Wood Samples Incubated with [ 14 C]Lignocellulose and Potassium [ 15 N]Nitrate