Response of surface horizons in an oak forest to prescribed burning

The reduction of forest floor ground cover and litter layers by prescribed fires may alter the morphology (field and micro) and physical properties of surface horizons. This study determined long-term (35 yr) changes in surfa ce horizon bulk density, organic matter concentration and content, and morp hology in response to periodic (5 yr) and annual (1 yr) prescribed fires. S oils were fine-silty, siliceous, thermic Glossic Fragiuldults, supporting m ixed oak vegetation in middle Tennessee. Upper mineral soils (0- to 2-cm an d 0- to 7.6-cm depths) were sampled and detailed field descriptions made. P eriodic and control plots had a thin layer of Oi, Oe, and Oa horizons 5 yr after the 1993 burn, whereas on annual burn plots a 1- to 2-cm charred laye r was present. Significant reductions in organic matter concentration and m ean thickness of the A horizon were found from burning (A horizons thicknes ses were 6.4, 4.6, and 2.9 cm in control, periodic, and annual plots, respe ctively), Periodic burns did not significantly alter the organic matter and bulk density of the upper 7.6 cm of mineral soil; however, annual burns di d result in significantly higher bulk densities (1.01, 1.07, and 1.29 Mg m( -3) in control, periodic, and annual plots, respectively) and lower organic matter concentrations and contents. Microscopic investigations confirmed t hat compaction was increased from annual burning. Thin sections also reveal ed that the granular structure of the A horizons in control and periodic pl ots resulted from bioterbation of macro and mesofauna, fungi, and roots. Lo ng-term annual burning greatly affected surface soil properties, whereas pe riodic burning on a 5-yr cycle had only limited effects.