Impact of Nitrogen and Phosphorus on [14C]Lignocellulose Decomposition by Stream Wood Microflora †

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RESUMO

Nutritional and physical factors affecting the decomposition of [14C]lignocellulose prepared from Douglas fir (Pseudotsuga menziesii) were examined by incubating the labeled substrate with homogenized surface wood scrapings obtained from a Douglas fir log in a Pacific Northwest stream. Incubations were conducted in distilled water, in stream water collected from four different sources, or in a defined mineral salts solution with or without supplemental N (KNO3). Decomposition rates of [14C]lignocellulose, as measured by 14CO2 evolution, were greater in each of the four filter-sterilized sources of stream water than in distilled water alone. Decomposition experiments conducted in stream water media with the addition of defined mineral salts demonstrated that [14C]cellulose decomposition was stimulated 50% by the addition of either KNO3 or KH2PO4/K2HPO4 and further enhanced (167%) by a combination of both. In contrast, [14C]lignin decomposition was stimulated (65%) only by the addition of both N and P. Decomposition of [14C]lignocellulose was greatest when supplemental KNO3 was supplied in concentrations of at least 10.0 mg of N liter−1 but not increased further by higher concentrations. The decomposition of [14C]lignocellulose increased as the incubation temperature was raised and NO3−1-N supplementation further increased these rates between three-and sevenfold over the range of temperatures examined (5 to 22°C). Accumulation of NH4+ (2 to 4 mg of N liter−1) was always observed in culture filtrates of incubations which had been supplemented with KNO3, the quantity being independent of NO3− concentrations ≥ 10 mg of N liter−1. The role of supplemental NO3− in the decomposition of [14C]lignocellulose is discussed in relation to wood decomposition and the low concentrations of N found in stream ecosystems of the Pacific Northwest.

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