Extracellular enzyme activities link wood traits with decomposition rates
Wood-rotting fungi are important agents of lignocellulose decay. Fungal enzymes catalyze the degradation of carbon fractions in wood and facilitate nutrient uptake to support fungal growth. We are exploring the effects of plant traits, environmental conditions, and fungal community structure and function on the decay rates of 21 species of woody plants that are common in our field site in the Ozark Highlands, MO, USA. To assess the role of lignocellulolytic enzymes in decay, we conducted assays for four hydrolytic and two oxidative enzymes on sawdust collected from experimental logs that were left to decay for one or three years. Replicate logs were assayed from upland and lowland locations in two watersheds. We tested the following hypotheses: 1) enzyme activity correlates positively with wood mass loss, 2) logs left to decay for three years lose more mass than logs left for one year, 3) due to higher soil moisture, logs in lowland plots lose more mass and have higher enzyme activities than logs in upland plots, and 4) the plant species in the experiment differ in levels of overall enzyme activity and types of enzyme activity due to variation in chemical compositions and anatomical constructions of the woody species.
Activities of all enzymes were positively correlated with mass loss across species and plot locations. Logs assayed after 3 years of decay lost more mass and had higher enzyme activities than logs assayed after one year of decay. Hydrolase enzymes showed similar patterns of activity across plant species that did not match with patterns of oxidase activity. Logs placed in lowland watersheds showed higher hydrolytic enzyme activity than logs of the same species placed in upland watersheds. In all plots, hydrolase enzyme activities were highly positively correlated with wood cellulose, hemicellulose, and protein content, a variety of micronutrients, and wood density. Activities of these enzymes were negatively correlated with lignin content. Oxidase enzyme activities were low across all species and watersheds with the exception of a few high-activity logs that showed no watershed or species pattern. This suggests that wood-decay fungi may have been actively deploying oxidase enzymes in only a few logs at the time of sampling. Our results demonstrate that the lignocellulolytic functions of the microbial community in decaying wood are mediated by plant traits and environmental conditions.