Soil quality assessment in domesticated forests – a southern pine example

Maintenance and enhancement of soil productivity is central to the long-term success of intensive forest management. A simple technique is required for monitoring the effects of different management practices on soils as an aid in developing codes of practice that foster maintenance and enhancement of soil productivity. The objective of our work was to determine if management impacts on soil productivity could be assessed using the soil-quality concepts developed in agriculture. A soil-quality index (SQI) model, that measures the effects of management practices on five key growth-determining attributes of forest soils, namely (1) promote root growth, (2) store, supply and cycle nutrients, (3) accept, hold, and supply water, (4) promote gas exchange, and (5) promote biological activity, was developed and tested as part of a controlled study in intensively-managed pine plantations on the Lower Coastal Plain of South Carolina. Three 20-ha, 20-year-old loblolly pine plantations were harvested under wet and dry conditions to create a broad gradient in soil disturbance. Within each harvested plantation, a subset of 3-ha plots were site prepared by either bedding or mole-plowing plus bedding, then all sites were established as third-rotation pine plantations. Field data were collected spatially for soil bulk density, net N mineralization, water table depth, soil aeration, and soil moisture. Literature-based sufficiency values were determined for each property and substituted into the SQI model as surrogate indicators for the five key attributes, thus obtaining spatial SQI predictions within each harvesting and site preparation treatment. Our study results demonstrate how SQI monitoring could be used to identify the effects of management practices on soil productivity, which should aid in developing codes of practice as a component of achieving long-term sustainability in domesticated forests.