Extracellular enzyme activities in decaying wood across a natural subtropical forest diversity gradient
Coarse woody debris (CWD) plays an important role in forest ecosystems and is the source of up to 20% of stored carbon in these systems. The main decomposers of CWD are saproxylic insects and wood decaying fungi, the latter relying on extracellular enzymes for decomposition. A majority of knowledge on CWD decomposition has been derived from temperate and boreal forests, and from lignin and cellulose degradation enzymes. How do forest succession and diversity alter CWD decomposition, and how might multiple enzymes be involved in CWD decomposition processes? To address these questions we conducted a study of CWD decay and extracellular enzyme production in a species rich subtropical Chinese forest as part of the Biodiversity Ecosystem Function (BEF)-China experiment. We placed small logs of two wood species, Pinus massoniana (gymnosperm) and Schima superba (evergreen), in 27 permanent study plots varying in successional age and tree species diversity. The small logs were also covered in either a fine or coarse mesh designed to evaluate the influence of macro invertebrates. Decay rates and microbial enzyme activities of six enzymes (acid phosphomonoesterase (AP), leucine aminopeptidase (LAP), β-1,4-N-acetyl-glucosaminidase (NAG), β-1,4-glucosidase (BG), phenol oxidase (PO) and peroxidase (PER) were measured after two years of decomposition.
In general we found relatively high activities of NAG and AP, the products of which could be important for balancing nutrient stoichiometry during microbial decomposition in of wood, which mostly contains carbon. Between CWD species the activities of BG, PO, PER and NAG were higher in the angiosperm wood of S. superba. CWD mass loss was directly positively related to the activity of both lignocellulase degradation and nutrient acquisition (nitrogen or phosphorus) enzyme activities. This suggests that microbial enzyme production is closely tied to CWD decomposition rates and that angiosperm wood, which generally decomposes more quickly than gymnosperm wood, will not persist as long in a given stand. Across our forest plots, we found no direct effect of forest diversity or successional age, suggesting that forest diversity or successional age may only play indirect effects on wood decomposition through determining which species of decomposing wood is present. We also found no differences in enzyme activity between control and insect exclusion treatments. In conclusion, multiple extracellular enzymes may be important for wood decomposition processes, but this enzyme production seems independent of the presence of macro invertebrates or on the surrounding forest composition or age.