Activities of Extracellular Enzymes in Soils Following Woody Plant Invasion of Grassland

Extracellular enzymes produced by microbes and immobilize in the soil environment are the principle means by which complex plant and microbial compounds are degraded. The concentration of these enzymes and their ability to interact with litter and soil organic matter contributes both to the stabilization and destabilization of soil carbon. We quantified the activities of three extracellular enzymes, B-glucosidase, B- glucosaminidase, polyphenol oxidase (PPO), and a general marker for hydrolytic activity through fluorescein diacetate (FDA) hydrolysis activity, in a subtropical savanna parkland in southern Texas where woody plants have invaded a once open grassland. Previous research has demonstrated that areas which have shifted to woody vegetation are accruing soil carbon, undergoing a dramatic shift in the chemistry of plant input, and increasing in hyphal biomass. Soils were obtained along a successional chronosequence from grassland dominated by C4 grasses to woody patches dominated by C3 trees/shrubs in Oct 2006 and stored immediately frozen until thawing for enzyme assay. Most enzymes, with the exception of PPO, show distinct behavior when comparing grassland and clusters in that grasslands exhibit far lower mass normalized activity than clusters and no activity trend with respect to age of the adjacent cluster. Both FDA and B- glucosaminidase activities are positively correlated with the age of the woody clusters and increase their activity by as much as 10-fold across the age gradient from 14 yr to 86 yr old clusters. The cellulose degrading enzyme, B-glucosidase, always exhibited greater activity (1.5 -4 fold) in woody clusters than in grasslands, but did not exhibit a trend with increasing cluster age. The PPO activity is anomalous in that there is no quantitative difference in mass normalized activity between grassland and cluster and no trend with cluster age. The results for the FDA and B-glucosaminidase assays are consistent with concurrent studies showing that there is a nearly 4 fold greater hyphal biomass, which is abundant in chitin, in wooded areas compared to remnant grasslands and that hyphal productivity rates increased linearly with woody plant stand age. These results have important implications for understanding how this globally important vegetation change will influence soil carbon cycling storage and release.