Rygiewicz, P.T., D. Zabowski, and M.F. Skinner. 2004. Site disturbance effects on a clay soil under Pinus radiata – root biomass, mycorrhizal colonisation, 15ammonium uptake, and foliar nutrient levels. New Zealand J. of Forestry Science 34(3):238-254. WED-00-055
Timber harvesting can result in adverse physical, chemical, and biological alterations to soil. The objective of this study was to examine the effects of site disturbance to determine the extent and duration of possible harvesting impacts on soil chemical and biological properties, including fine roots and mycorrhizas of Pinus radiata D.Don. The disturbance study was located in the North Island of New Zealand and was examined 9 years after organic matter and compaction treatments were installed. Treatments included undisturbed control plots, O horizon removed with no compaction, and O and A horizons removed with heavy compaction. Soil was examined for soil solution nitrogen, extractable nitrogen, fine root biomass, mycorrhizal root tips, and specific mycorrhizal root tip 15ammonium uptake rates. Results showed that total fine root biomass was reduced with loss of both organic-rich soil horizons and compaction to approximately one-third of that in the control treatment, but that mycorrhizal infection rates were higher (averaging over 60%). With removal of only the O horizon, the largest effect was simply the loss of rooting volume and the roots that would normally occur in this horizon, with little reduction in root biomass or change in mycorrhizal infection rates in the mineral horizons. Specific mycorrhizal root uptake rates of 15ammonium did not appear to have been changed by the most severe disturbance treatment. However, the unaltered uptake rate may be due to the predominant mycorrhizal morphotype found in the most-severe treatment which was different from the dominant morphotype found in the two less-severe treatments. Reduced fine root biomass of the severe disturbance treatment correlated with reduced tree growth and foliar nitrogen.