Soil Organic Carbon Fractions and Carbohydrate Hydrolase Activities in Forest Ecosystem Following Prescribed Burning and Thinning.

The southern pine beetle (Dendroctonus frontalis Z) epidemic wreaked havoc on the loblolly pine (Pinus taeda L) stands of Bankhead National Forest.  In an effort to restore and regenerate the damaged forest, the United States Department of Agriculture Forest Service (USDA FS) applied prescribed thinning and burning treatments.  Fire regimes were imposed as: frequent low intensity understory burn (every 3 years) and unburned control.  Stands density were manipulated by three levels of thinning, no thin, thin to 17.22 m² ha^-1 (75ft² acre^-1) basal area, and thin to 11.46 m² ha^-1 (50ft² acre^-1) basal area.  Here the impact of prescribed burning and thinning on: (I) labile organic carbon fractions (microbial biomass, light fraction and particulate organic carbon), (II) carbohydrate hydrolases activities (amylase, cellulase and N-acetyl -β-glucosaminidase), and (III) potential carbon mineralization and components of dissolved organic matter (hexose and free amino acids) were evaluated. These soil carbon fractions and enzymes play essential roles in carbon cycling and soil fertility.  Soils were collected from 0 to 10 cm depth in triplicate.  Labile organic carbon was isolated using the density method, and carbon enzyme activities were determined by p-nitrophenol and Somogyi-Nelson colorimetric assays. The concentration of light fraction carbon was 63.26% higher in the light thinned plot compared to the reference plot.  Results show that light thin + burn plot (115 ± 42 µmol g^-1 24hrs^-1) had the highest amylase activity compared to reference, burn only, and heavy thinned plots (17 ± 24, 12 ± 13, 18 ± 8 µmol g^-1 24hrs^-1 respectively).  An appreciable amount of dissolved amino acids were detected in the heavily thinned plots (4085 ± 357 µmol L^-1 leucine equivalent) compared to other plots. Relative to the reference plot, stands treated by light thinning alone or in combination with burning (triennial, low intensity) consistently revealed high amylase activity, elevated concentrations of dissolved hexose and light carbon fraction.