Soil phosphorus dynamics and bioavailability in New Zealand forest ecosystems

Abstract

Phosphorus (P) is an essential element for all living organisms and the productivity of natural and managed ecosystems is generally limited by the bioavailability of P in soil. Previous studies showed that significant changes in soil P occurred following a land-use change from grassland to short-rotation plantation forestry in hill and high country areas of New Zealand. However, most of these studies involved paired-site comparison at a single point in time after forest planting (commonly 10-20 years), and our understanding of when the changes in soil P occurred and the mechanisms involved is limited. The main objective of this study was to investigate temporal changes in the nature and bioavailability of soil P associated with forest development, including the effect of different tree species. This involved conducting five major experiments over a range of timescales from seasonal to millennia at four sites in New Zealand (Orton Bradley Park, Glendhu, Lincoln, Haast) using a variety of techniques to quantify temporal changes in the nature and bioavailability of soil P. The results of this research clearly demonstrated that dramatic changes in the nature and bioavailability of soil P occurred during the first 5 years following tree planting, which resulted in significant mineralisation of soil organic P. This in turn was mainly attributed to a combination of factors including P acquisition by trees and reduced quantities of organic matter and P returned to soil as a consequence of the cessation of grazing. Surprisingly, the initial changes in soil P were similar under three contrasting tree species (P. radiata, C. macrocarpa, E. nitens), which indicated that these tree species accessed similar forms of P in soil despite differences in mycorrhizal associations and growth. This was confirmed in results from the seasonal study carried out on the same site, while the relative short-term bioavailability of P in soils taken from the Haast native forest chronosequence was similar for P. radiata pine and C. macrocarpa. It was also shown that net mineralisation of organic P occurred when trees were planted in high organic matter/low P fertility soils, while net immobilisation of P (i.e. increases in organic P) occurred when trees and pasture were established simultaneously in a low organic matter/high P fertility soil. Increases in recalcitrant soil P indicated a shift towards more stable P forms with forest development, which was associated with the increased inputs of more recalcitrant organic matter and P with time.