Soil phosphorus dynamics in a temperate silvopastoral system

Abstract

Silvopastoral systems comprise part of the continued expansion of conifer plantings on grassland in New Zealand. Previous studies of afforestation and agroforestry in New Zealand have not adequately addressed the effects of interactions between over and understorey species on soil phosphorus (P) dynamics and availability. Accordingly, the main objective of this study was to investigate soil P dynamics and availability under a temperate conifer-based silvopastoral system in New Zealand. This involved field and glasshouse experiments which examined the effects of combinations of Pinus radiata, lucerne (Medicago sativa L.) and ryegrass (Lolium perenne L.) on soil P. Thus spatial and temporal aspects were examined under field conditions, while the influence of soil properties was investigated under controlled conditions.

The main silvopastoral system examined in this study was established at Lincoln University in 1990 on soil that had previously been managed under intensive arable cropping. The investigation of spatial variation found that the more labile inorganic P fractions increased while organic P fractions decreased, under trees compared to forages, close to the ripline in the tree row. These changes tentatively indicated that trees had enhanced mineralisation of soil organic P. However, organic P fractions appeared to be greater in the ripline under trees grown without understorey vegetation. This apparent contradiction was attributed to the competing effects of enhanced mineralisation of organic P by trees, compared to forages, and increased accumulation of organic P under trees and forages as a consequence of conversion to perennial vegetation from intensive cropping. Although soil samples were collected to a depth of 60 cm, the major changes in soil P occurred in the surface 20 cm.

Changes in soil P over the first 10 years of the Lincoln silvopastoral system were determined on samples taken in 1991, 1994, 1996 and 2000. The findings of other workers that plant available inorganic P was enhanced and organic P reduced under conifers were not confirmed in this study. This was mainly attributed to the fact that soil organic carbon and P increased with time at the Lincoln site following conversion to perennial vegetation from intensive cropping.

A study of the rhizosphere was carried out on the Lincoln agroforestry trial over a one year period. Lower levels of labile organic P, and higher levels of labile inorganic P and phosphatise enzyme activity were determined in tree and forage rhizosphere soils in spring compared with autumn. This data confirmed that overall rates of soil organic P mineralisation were greater in spring. In addition, concentrations of labile inorganic P were consistently greater in tree rhizosphere soil compared to the companion understorey. Lucerne-tree rhizosphere soil and lucerne rhizosphere soil contained the lowest concentrations of total inorganic and organic P compared with ryegrass-tree and ryegrass rhizosphere soils. This was partly attributed to higher levels of phosphatase enzyme activity in the lucerne rhizosphere soils.

It became apparent from the spatial and temporal studies that initial soil properties would likely influence soil P changes. Accordingly a detailed glasshouse experiment was carried out to examine the short-term effects of different soil properties on combinations of Pinus radiata, lucerne and ryegrass. This included an investigation of the effect of tree ectomycorrhizal hyphae on soil P. Although P uptake was greatest under radiata pine, trees tended to deplete inorganic P to a lesser extent than the forages. Isotopic exchange kinetics analysis revealed that trees and lucerne combined caused a greater decline in the most recalcitrant forms of soil inorganic P. This enhanced depletion of recalcitrant inorganic P under trees and lucerne may have been related to increased N availability. There was also evidence of significant redistribution of soil P from less labile to more labile forms. Where levels of P and carbon were very low, trees and lucerne were able to deplete most P forms, and when lucerne was combined with trees, depletion of recalcitrant inorganic P increased. Results indicated that in low P soils redistributed P was mainly taken up by plants, whereas concentrations of available inorganic P increased in high P soils. In high P soils labile organic P was not depleted, suggesting that redistribution of moderately available and recalcitrant P to available inorganic and organic forms had occurred. An important aspect of the effect of Pinus radiata on soil P was the ability of ectomycorrhizal hyphae to substantially deplete most P fractions, thus confirming the important role that ectomycorrhizal fungi play in determining P uptake by trees. The findings of this study confirm that changes in soil P forms are strongly influenced by interactions between plant species and soil properties which, in turn, are influenced by land use and management.