Response of Soil Chemistry to Long-Term Occupancy by Introduced Pinus Species in Hardwood Forests

Abstract Conifers have been widely planted to stabilize soils and promote site recovery on former hardwood sites that were cleared for agriculture. Decades of confer occupancy may have lasting effects on nutrient availability, organic matter cycling, soil acidity, and soil buffering capacity. In many areas of eastern North America, conifer plantings consisted of non-native Pinus species planted on abandoned agricultural land that was once mesophytic hardwood forest. To investigate the long-term effect of Pinus occupancy on former hardwood soils, we compared the soil chemistry of P. echinata and P. strobus plantations planted on bedrock soils in southern Indiana to those of nearby naturally regenerated hardwood stands. To assess potential differences resulting from topographic position, Pinus plantations and hardwood stands were examined on both mesic ridges and bottoms. On average, mineral soils in Pinus plantations had lower pH and less organic matter (-21%), total carbon (-29%), total nitrogen (-30%), manganese (-37%), calcium (-24%), zinc (-13%), and boron (- 24%) compared to hardwood stands. Pinus stands also had 2–5 times greater O-horizon depth and 17% greater concentrations of aluminum compared to naturally regenerated hardwood stands. The difference between Pinus and hardwood stands was greater on ridges than bottoms, as Pinus trees planted in bottoms appeared to have had less impact on soil chemistry. This was likely due to greater buffering capacity in bottom soils and litter from codominant hardwood trees that had advanced into the overstory.