NITROGEN AVAILABILITY IN RADIATA PINE PLANTATIONS ON FORMER PASTURE SITES IN SOUTHERN NEW-SOUTH-WALES

Studies of nitrogen availability were carried out in radiata pine (Pinus radiata D. Don) plantations on former pasture sites in N.S.W. in conjunction with studies of the effects of previous land use on tree form. Sites were selected on previously improved pastures (cleared with introduced legumes) and unimproved pastures (partially cleared without legumes) to form age sequences of stands which had been established for periods of up to fifteen years. Mineral-N pools in soils and forest floor samples were determined monthly for thirteen months and nitrification potentials were determined from periodic laboratory incubations. Nitrate and ammonium pools in 2-, 4-, 6-, 9- and 15-year-old radiata pine stands fluctuated seasonally, peaking in summer and autumn. Maximum total mineral-N concentrations of 20 to 40-mu-g g-1 soil occurred in the youngest, ex-improved pastures with nitrate-N concentrations of up to 25-mu-g g-1. In the 15-year-old stands, nitrate-N was only detected during autumn, at less than 5-mu-g g-1 soil. Net N-mineralization and nitrification potentials were consistently higher in the ex-improved pasture soils compared with the ex-unimproved pastures. N availability decreased with increasing stand age in the ex-improved pasture soils, but the pattern was less clear for the unimproved pasture sites. Suppression of clover by pines and the accumulation of nitrogen in the standing biomass are thought to be the major factors controlling the decline of available N during stand development.