Microbial and organic matter patterns in a prescribed burned and thinned managed forest ecosystem

Prescribed burning and thinning were implemented in an Alabama forest (Bankhead National Forest) as a management strategy to control pest and disease outbreaks and also to increase forest productivity. However, using fire as a control mechanism in this forest may alter soil nutrient cycling, soil organic matter (SOM), and soil microbial populations. There is the need for continuous research on forest ecosystems to bridge knowledge gaps in our understanding of SOM transformations and microbial processes in a repeatedly burned forest ecosystems. The objectives of this study were to assess and document the impact of prescribed burning and thinning on soil labile organic matter fractions, microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN), potential carbon mineralized (PCM), enzymatic activity, and energy (ATP) potentials. Labile organic carbon was isolated using the density method, whereas enzyme activities were determined as described in Bottomley et al. (Methods of soil analysis: part 2-Microbiological and biochemical properties, Madison, Wisconsin, USA: Soil Science Society of America, 1994). Microbial biomass C and N (MBC and MBN) were determined using the fumigation-incubation method. Treatment applications had some effects on MBC and MBN although these effects were not statistically significant (P <= 0.05). Light fraction carbon (LFC) and light fraction nitrogen (LFN) were significantly (P <= 0.05) affected by treatments. Irrespective of treatment, xylanase activity was the highest (3244 +/- 327-5223 +/- 567 mu mol/g 24 h(-1)), whereas amylase activity was the lowest (12.57 +/- 8.9-116 +/- 42.8 mu mol/g 24 h(-1)). Correlation analysis revealed that amylase, beta-glucosidase, and NAGase correlated with particulate organic carbon (POC), particulate organic nitrogen (PON), and LFC, whereas cellulase, xylanase, and invertase had no correlation with the labile organic matter fractions. Compared to the referenced treatment plot, burned and thinned plots had reduced MBC, MBN, and increased PCM although this was not statistically significant. Although enzyme activities within plots were significant, no significant enzyme activities between plots were observed except with amylase. The lack of statistical significance on microbial activities, MBC, and MBN support our hypothesis that prescribed burning and thinning temporarily affected microbial indices, and these parameters are expected to return to pretreatment levels after a period of time.