Microbial Decomposition of Wood in Streams: Distribution of Microflora and Factors Affecting [ 14 C]Lignocellulose Mineralization

Abstract

The distribution and lignocellulolytic activity of the microbial community was determined on a large log of Douglas fir ( Pseudotsuga menziesii ) in a Pacific Northwest stream. Scanning electron microscopy, plate counts, and degradation of [ 14 C]lignocelluloses prepared from Douglas fir and incubated with samples of wood taken from the surface and within the log revealed that most of the microbial colonization and lignocellulose-degrading activity occurred on the surface. Labeled lignocellulose and surface wood samples were incubated in vitro with nutrient supplements to determine potential limiting factors of [ 14 C]lignocellulose degradation. Incubations carried out in a nitrogenless mineral salts and trace elements solution were no more favorable to degradation than those carried out in distilled water alone. Incubations supplemented with either (NH 4 ) 2 SO 4 or organic nitrogen sources showed large increases in the rates of mineralization over incubations with mineral salts and trace elements alone, with the greatest effect being observed from an addition of (NH 4 ) 2 SO 4 . Subsequent incubations with (NH 4 ) 2 SO 4 , KNO 3 , and NH 4 NO 3 revealed that KNO 3 was the most favorable for lignin degradation, whereas all three supplements were equally favorable for cellulose degradation. Supplementation with glucose repressed both lignin and cellulose mineralization. The results reported in this study indicate that nitrogen limitation of wood decomposition may exist in streams of the Pacific Northwest. The radiotracer technique was shown to be a sensitive and useful tool for assessing relative patterns of lignocellulose decay and microbial activity in wood, along with the importance of thoroughly characterizing the experimental system before its general acceptance.