A Regional Assessment of Permanganate Oxidizable Carbon for Potential Use as a Soil Health Indicator in Managed Pine Plantations

Soil health assessments require the establishment of soil indicators that are easy to measure and sensitive to changes in management practices. Despite the global demand for wood products and the intensification of silvicultural practices, very few indicators are currently used to monitor changes in soil properties and processes in managed forest plantations. Labile pools of soil organic carbon (SOC) have been widely used in agriculture to detect early alterations in carbon (C) cycling processes but are largely untested in managed forests. Here, we investigate the potential use of permanganate oxidizable carbon (POXC) to monitor impacts of silvicultural practices on soil properties in loblolly pine plantations. Soil samples were collected from 77 managed forest sites and 297 experimental plots throughout the southeastern U.S. at four soil depths: 0–10, 10–20, 20–50, and 50–100 cm. Sites encompassed plantations with ages ranging from 2 to 29 yr that were distributed over five soil orders. Silvicultural practices included herbicide, fertilization at multiple rates, and thinning. Our study showed that POXC concentration decreases with soil depth, while POXC increased proportionally to SOC with depth. POXC positively correlates with SOC, total nitrogen, and mass of soil woody detritus other than live roots, and negatively correlates with soil pH across all soil depths. Impacts of silvicultural practices on POXC concentration were variable across sites. The mean relative response of POXC to silvicultural practices was negative in general but only significant (p < 0.10) in herbicide treatments; compared to control plots herbicide plots at 50–100 cm depth had on average 20% less POXC. The decade-long rotation lengths of southern pine plantations and the carbon cycling dynamics of forest soil O horizons may impact the sensitivity of POXC as an indicator in managed plantations. In contrast to this regional analysis, monitoring POXC in local assessments may be a more practical approach to investigate short-term impacts of silvicultural practices.