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

Abstract

Nutritional and physical factors affecting the decomposition of [ 14 C]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 (KNO 3 ). Decomposition rates of [ 14 C]lignocellulose, as measured by 14 CO 2 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 [ 14 C]cellulose decomposition was stimulated 50% by the addition of either KNO 3 or KH 2 PO 4 /K 2 HPO 4 and further enhanced (167%) by a combination of both. In contrast, [ 14 C]lignin decomposition was stimulated (65%) only by the addition of both N and P. Decomposition of [ 14 C]lignocellulose was greatest when supplemental KNO 3 was supplied in concentrations of at least 10.0 mg of N liter −1 but not increased further by higher concentrations. The decomposition of [ 14 C]lignocellulose increased as the incubation temperature was raised and NO 3 −1 -N supplementation further increased these rates between three-and sevenfold over the range of temperatures examined (5 to 22°C). Accumulation of NH 4 + (2 to 4 mg of N liter −1 ) was always observed in culture filtrates of incubations which had been supplemented with KNO 3 , the quantity being independent of NO 3 − concentrations ≥ 10 mg of N liter −1 . The role of supplemental NO 3 − in the decomposition of [ 14 C]lignocellulose is discussed in relation to wood decomposition and the low concentrations of N found in stream ecosystems of the Pacific Northwest.