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

Abstract

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 31P 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 be ectomycorrhizal 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.