Growth responses of Pinus radiata and soil changes following periodic fertilization

Low growth rates of Pinus radiata D. Don plantations on soils of low nutrient status in Tasmania require the development of nutrient management systems involving fertilization throughout the life of the stand. Periodic fertilization through 15 yr, in a Pinus radiata D. Don plantation in northeastern Tasmania was evaluated for effects on stand growth and soil. Substantial stem volume increases due to N and P fertilizer, applied as ammonium sulfate and single superphosphate were observed. Two applications of P fertilizer totaling 144 kg P ha(-1) produced substantial increases in stem volume growth from 67 to 192 ml ha(-1). With P plus 13 annual applications of N fertilizer at 100 kg N ha(-1) yr(-1), stem volume growth increased from 192 to 344 m(3) ha(-1) over the 15-yr period. Although fertilizing every second and fourth year produced less response, these treatments had better fertilizer-use efficiency. Long-term fertilization resulted in increases in concentrations of nutrients in the forest floor and the surface soil. The highest rates of fertilization were accompanied by significant reductions in soil pH throughout the soil profile, from 4.1 to 3.4 units. Reduction in pH occurred with both nitrogenous and phosphatic fertilizers. Substantial reductions in exchangeable Mg concentrations, from 235 to 88 mu g g(-1), were also measured throughout the soil profile. The highest rate of N application significantly increased the total O2 horizon biomass, with N mass doubling within the O2 horizon.