Effects of Long-Term Prescribed Burning on Soil Physical and Chemical Properties in the Missouri Ozarks

Fire affects soil physical and chemical properties of soil. However, very few studies have examined the long-term effects of prescribed burning on soil properties. We investigated the effects of spring prescribed burning on soil porosity, bulk density, infiltration rates, pH, and mineral nutrient concentrations in oak-hickory forest soil collected from a total of six plots, two of which have been burned annually, periodically (every four years), and never, respectively, since 1949. Organic horizon and/or mineral soil up to 1 m depth was collected from three to eight locations per plot in June of 2008 and 2009. Preliminary analyses indicate that soil porosity (averaged across the 1 m soil profile) decreased due to annual burning by approximately 10 % relative to controls (0.36 and 0.4 in annual burn plots and control, respectively). Corresponding average soil dry bulk density increased in annual burn plots relative to unburned control plots (1.05 vs. 1.16 g/cm3, respectively). Surface soil infiltration rates (steady state) were on average 96, 37 and 10 cm hr-1 in control, periodic, and annual burn plots, respectively with associated standard deviations (SD) of 148, 40 and 8 cm hr-1 respectively (n=18 per treatment). Thus, soil infiltration rates ranged from approximately 2.5 times to an order of magnitude higher in the unburned control relative to the periodic and annual burn plots, respectively. Values of pH and PO4-, total C, and total N concentrations decreased with increasing soil depth (P < 0.05) with the sharpest decline occurring between the organic soil and top 10 cm of mineral soil, regardless of treatment. Preliminary analyses indicate a significant effect (P < 0.05) on pH and PO4- concentrations in the organic soil layer. Values were highest in the annual burns (pH = 5.2, PO4- = 260 mg P kg-1) and tended to be lowest in the controls (pH = 4.2, PO4- = 130 mg P kg-1). Analyses also indicate that prescribed burning may have a significant effect (P < 0.05) on C and N concentrations within the top 30 cm of mineral soil. C and N concentrations were lowest in annually burned soil (0.79 % and 0.05 %, respectively) but did not differ between periodically burned soil and controls (0.92 % and 0.07 %, respectively). The decrease in annually burned plots relative to the control can be explained by losses due to combustion. While more detailed analyses are ongoing, it is apparent that even low severity spring prescribed burning carried out repeatedly over long time periods can have a significant effect on soil physical and chemical properties. Potentially lower water and nutrient availability can impact forest productivity and should be taken into consideration in long-term planning by land managers.