The effect of biocidal treatments on respiration and enzymatic activities of douglas-fir needle litter

Douglas fir (Pseudotsuga menziesii (Mirb.) Franco) needle-litter was given treatments of varying biocidal severity including air-drying, oven-drying, fumigation with methyl bromide plus chloropicrin. autoclaving, and a control. The litter was inoculated with a small amount of fresh litter homogenate and monitored for CO₂ evolution rate and extractable enzymatic activity during a 77-day incubation. Enzymatic activities assayed included cellulase. xylanase, mannase, amylase. invertase, β-glucosidase, protease, polyphenoloxidase and peroxidase. The concentrations of extractable ninhydrin-reactive compounds, glucose, and reducing substances were also determined.

Air-drying produced a small, initial stimulation in respiration but had no effect on any enzymatic activity throughout the 77-day incubation except for an initial decrease in polyphenoloxidase and peroxidase activities. Oven-drying produced a substantial loss of all enzymatic activities initially but all carbohydrase and protease activities recovered to greater than control levels by 7 days; after a 1-day lag, oven-drying produced a pronounced stimulation in the rate of CO₂ evolution. Autoclaving produced a 2-day lag in a less pronounced (compared to oven-drying) respiratory stimulation: initially all enzymatic activities were destroyed and only protease and β-glucosidase activities recovered to control levels. Fumigation produced a respiratory stimulation after a 9-day lag (probably due to residual chloropicrin) which coincided with a pronounced increase in cellulase. xylanase, mannase and amylase activities: initially, no activities except polyphenoloxidase and peroxidase were adversely affected by fumigation.

During the entire incubation, the activities of cellulase. xylanase. mannase. amylase and invertase were highly correlated with each other regardless of treatment. After 77 days, the rates of CO₂ evolution of all samples were correlated with the level of amylase and cellulase activity (r = 0.86 and 0.83, respectively). This observation points to the value of enzymatic assays for following the processes which release substrate for microbial assimilation from plant litter. The close association of polysaccharidase and protease activities with the flush of CO₂ evolution following biocidal treatment, particularly oven-drying, indicates that the decomposition of carbohydrates and proteins released from microbial biomass or plant litter is a mechanism for this often-observed respiratory stimulation.