Impact of different tree species on soil phosphorus immediately following grassland afforestation

Previous studies have revealed that significant changes in soil phosphorus (P) occurred as a consequence of grassland afforestation, although when these changes occurred and the influence of different tree species remains largely unknown. This study involved assessing changes in soil phosphorus (P) over a 10 year period following the afforestation of grazed pasture with 3 contrasting tree species (Pinus radiata, Cupressus macrocarpa, Eucalyptus nitens) in a replicated field trial at Orton Bradley Park, New Zealand. A combination of techniques (sequential fractionation, alkaline phosphatase hydrolysable P, solution P-31 nuclear magnetic resonance spectroscopy) was used to quantify changes in the nature and bioavailability of soil P. Results revealed that the establishment and growth of trees caused a significant decrease in soil organic P within 10 years, indicating net organic P mineralisation. Surprisingly this trend was similar under all three tree species, which suggested similar soil P acquisition despite differences in the type of mycorrhizae associated with each species: P. radiata is ectomycorrhizal, C. macrocarpa is arbuscular mycorrhizal, and E. nitens can beectomycorrhizal or arbuscular mycorrhizal. The observed changes in soil P dynamics were attributed a combination of tree growth and P uptake irrespective of species and changes in P inputs and organic P turnover associated with the cessation of grazing following tree planting. Changes in the nature of organic P determined 10 years after establishment indicated that organic matter inputs associated with tree growth were having an increasing influence on soil P dynamics with time.