Nitrogen dynamics in stream wood samples incubated with (C-14) lignocellulose and potassium (N-15) nitrate

Surface wood samples obtained from a Douglas fir log (Pseudotsuga menziesit) in a Pacific Northwest streamwere incubated in vitro with ['4C]lignocellulose in a defined mineral salts medium supplemented with 10 mg ofN liter-1 of 15N-labeled NO3- (50 atom % 15N). Evolution of 14CO2, distribution and isotopic dilution of 15N,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 from132 gg of N to a maximum of 231 p.g of N per treatment after 6 days of incubation. Rates of [14C]lignocellulosedecomposition were greatest during the first 6 days and then began to decline over the remaining 12 days. TotalCO2 evolution was also highest at day 6 and declined steadily over the remaining duration of the incubation.Filtrate NH,s+-N increased from background levels to a final value of 57 p.g of N per treatment. FiltrateNO3 N completely disappeared by day 6, and organic N showed a slight decline between days 12 and 18. Themajority of the 15N that could be recovered appeared in the particulate organic fraction by day 6 (41 of N),and the filtrate NI14+ N fraction contained 11 1,4 of 15N by day 18. The 15N enrichment values of the filtrateNH4+ and the inorganic N associated with the particulate fraction had increased to approximately 20 atom %15N by 18 days of incubation, whereas the particulate organic fraction reached its highest enrichment by day6. Measurements of N2 fixation and denitrification indicated an insignificant gain or loss of N from theexperimental system by these processes. The data show that woody debris in stream ecosystems might functionas a rapid and efficient sink for exogenous N, resulting in stimulation of wood decomposition and subsequentactivation of other N cycling processes.