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

Abstract

Surface wood samples obtained from a Douglas fir log (Pseudotsuga menziesii) in a Pacific Northwest stream were incubated in vitro with [C]lignocellulose in a defined mineral salts medium supplemented with 10 mg of N liter of N-labeled NO(3) (50 atom% N). Evolution of CO(2), distribution and isotopic dilution of N, filtrate N concentrations, and the rates of denitrification, N(2) 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 mug of N to a maximum of 231 mug of N per treatment after 6 days of incubation. Rates of [C]lignocellulose decomposition were greatest during the first 6 days and then began to decline over the remaining 12 days. Total CO(2) evolution was also highest at day 6 and declined steadily over the remaining duration of the incubation. Filtrate NH(4)-N increased from background levels to a final value of 57 mug of N per treatment. Filtrate NO(3) N completely disappeared by day 6, and organic N showed a slight decline between days 12 and 18. The majority of the N that could be recovered appeared in the particulate organic fraction by day 6 (41 mug of N), and the filtrate NH(4) N fraction contained 11 mug of N by day 18. The N enrichment values of the filtrate NH(4) and the inorganic N associated with the particulate fraction had increased to approximately 20 atom% N by 18 days of incubation, whereas the particulate organic fraction reached its highest enrichment by day 6. Measurements of N(2) 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.