Characterization of lignocellulose decomposition in stream wood samples using 14C and 15N techniques

A study was conducted on the distribution and lignocellulolyticactivity of the microbial community on a Douglas fir log(Pseudotsuga menziesii) in a Pacific Northwest stream. Scanningelectron microscopy, plate counts, and [14C]lignocellulose degra-dation experiments show that microbial colonization and lignocel-lulolytic is mainly a surface phenomenon. Incubations of [14C]-lignocelluloses and wood samples in a nitrogenless mineral saltsmedium showed no increase in 14CO2 evolution as compared to incu-bations in distilled water. (NH4)2SO4 or organic nitrogen stimu-lated lignocellulose decay, with the greatest effect observed from(NH4)2SO4 addition. Subsequent incubations revealed that KNO3stimulated lignin decomposition more than (NH4)2SO4 or NH4NO3,whereas all three were equally favorable to cellulose decomposi-tion. Glucose repressed both lignin and cellulose decay. Decomp-osition of [14C1lignocelluloses was greatest when incubated instream water collected from four different sources than in dis-tilled water. Phosphate and nitrate additions alone and in
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combination to a stream water medium yielded increases in[ [14c/-cellulose decay, whereas 14c.1 lignin decomposition responded only
to both in combination. Decomposition of 4C]lignocelluloses wasgreatest when KNO3 was added in concentrations >10 mg N•1-I to amineral salts medium. Decomposition rates increased three- toseven-fold over a range of temperatures from 5 to 22°C. Accumula-tion of filtrate NH+4 N (2-4 mg N.1-1) was always observed in incu-bations with KNO3 addition, and was independent of NO3 concentra-tions )10 mg N-1-1. An incubation was conducted of stream woodsamples with [14C]lignocelluloses in a mineral salts medium with10 mg N•1-1 K15NO3 (50% 15N). Evolution of 14CO2, distribution of15N, filtrate nitrogen, denitrification, nitrogen fixation, andrespiration were measured at 6, 12, and 18 days. The organic ni-trogen in the lignocellulose/wood sample mixture increased two-
[
fold over the first 6 days. 14c] lignocellulose decompositionrates were greatest during the first 6 days, then diminished overthe remaining 12 days, as did the rate of overall respiration.
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Filtrate NH4 N increased from background levels to a final valueof 57 ug N per treatment. Filtrate NO3 disappeared by day 6, andorganic nitrogen showed a slight decline between 12 and 18 days.The majority of the 15N that was accounted for (53%) was in theparticulate organic fraction. By 18 days, the 15N enrichments ofthe filtrate NH4' the organic fraction, and the inorganic nitrogenassociated with the organic fraction had all increased to 20% 15N.Nitrogen fixation and denitrification measurements indicated in-significant gain/or loss of nitrogen from the incubations by
these processes. The role of wood in streams is discussed from ageomorphic and biological standpoint, along with the effect of ni-trogen on decomposition of lignocelluloses.