Effects of forest management on soil nitrogen in Pinus radiata stands in the Australian Capital Territory

Abstract

Field ammonium and nitrate levels, and nitrogen mineralization and nitrification rates, were determined in the 0–20-cm soil layer in Pinus radiata (D. Don) stands (recently clearcut, young (5 years old) and mature (45 years old) near Canberra (A.C.T.) from 17 November 1981 to 3 May, 1982. Total nitrogen contents in the yellow podzolic soils were low (<0.14%). Field concentrations of NH 4-N were <11.5 μg/g, with the highest concentrations in the surface horizon (0–7 cm). Nitrate concentrations were much less, at <0.6 μg/g. Ammonium contents ranged from 2 to 11 kg/ha in the surface horizon and from 3 to 9.5 kg/ha in the lower (7–20 cm) horizon. Largest values occurred in the spring in the clearcut, with smallest values in summer. Largest N mineralization rates (21 kg/ha (0–20 cm) in 173 days) occurred in the clearcut. There was net immobilization of N during some periods in the young and mature stands. Very little nitrification occurred in the clearcut (0.6 kg/ha in 173 days) and net nitrate immobilization was observed in the young and mature stands. Clearcutting and N fertilization did not dramatically increase the potential for leaching of N loss from the low-N capital soils in this study.