Thinning and Prescribed Burn Seasonality and Return Interval Effects on Soil Properties in a Mixed Red Pine Stand in Northern Wisconsin.

The objectives of this study were to determine the influence of thinning along with recurrent prescribed fire on soil nutrient cycling, physical properties, and microorganisms in a mixed conifer-hardwood ecosystem. The study was conducted on the 17-ha Treehaven Fire Plot study area (45°29’22”N, 89°34’28”W). Prescribed burns were performed in May 2014, September 2014, and April 2015. Water holding capacity, bulk density, soil horizon depths, infiltration, pH, OM, N, C, Mg, Mn, K, Ca, S, Al, Na, P, and soil respiration were all measured in response to thinning and fire frequency and seasonality of fire treatments. Prescribed burning resulted in lower OM (-20 mg kg^-1 ) and increased pH (+0.3 units). Thinning lowered soil pH in the top 5 cm of soil. Bulk density only changed in the units that were burned twice, where it increased by 0.12 g cm ^-3. Soil C did not decrease alongside OM, suggesting that incomplete combustion due to low fire intensity led to the presence of char in the soil. Thinning increased soil S, as the soil became more acidic. P increased as the intensity of a given fire increased, as extractable P was leached into the soil from ash. Ca was 642 mg kg^-1 higher in units that experienced two spring burns compared to those that had one fall burn. Total N was not affected by burning. Spring burns lowered soil respiration immediately, but respiration rates had returned to pre-burn levels within a month after fire. Soil wettability was not affected by the prescribed burns.