SITE DISTURBANCE EFFECTS ON A CLAY SOIL UNDER RADIATA PINE .1. SOIL SOLUTIONS AND CLAY MINERAL STABILITY

Timber harvesting of forested lands can cause impacts which reduce the long-term productivity of the soil. This study examined long-term eff ects of timber harvesting on soil morphology, soil solutions and clay mineral stability. A disturbance study established in 1981 an Ultisol located in the North Island of New Zealand was examined in 1990. Distu rbance treatments were installed following cable logging of radiata pi ne (Pinus radiata D. Don); treatments consisted of no disturbance (UN) , O horizon removed (OR), and O and A horizons removed with compaction of the Bt1/A horizon (OARHC). The morphology of the A and Bt1/A horiz ons of the OR treatment showed little difference from the UN treatment . Soil solutions were collected using centrifugation and soil mineralo gy determined. Soil solutions of the O horizon had nutrient concentrat ions that were approximately 10 times greater than that of the mineral horizons, indicating that nutrient availability would be reduced by r emoval of this horizon during harvesting. Soil solutions of the Bt1/A horizon showed substantially lower nutrient concentrations in the OARH C treatment compared to the UN and OR treatment, at 9-years after trea tment. Stability diagrams of soil solutions for clay minerals of the s oil showed that smectite was unstable and weatherable with the highest disturbance treatment, but was stable in the no and low disturbance t reatment. No disturbance effects were evident in the stability of iron minerals. Results suggest that this soil is capable of returning to p re-disturbance conditions well-within a rotation period when disturban ce is limited, but that recovery with the highest disturbance treatmen t could take substantially longer.