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Available soil phosphorus in semi-natural grasslands: assessment methods and community tolerances

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Restoration of diverse semi-natural grasslands is potentially limited by high availability of soil phosphorus (P). Successful targeting of restoration effort requires a knowledge of plant community tolerances to soil P availability. Many extraction methods for P availability have been developed but most are calibrated against the growth and P uptake of crop species grown in monoculture. To test which methods are most suitable for measuring available P in soils of mesotrophic grasslands, a bioassay experiment was undertaken to compare seven extraction methods with the growth and P uptake of grassland species. Five species were grown together on a soil treated to create a range of conditions of pH, mycorrhizal infection and P availability. Olsen P and Bray P were found to be significantly correlated with P uptake in plant growth across the range of soil treatments whilst ion exchange membrane P and resin P were significantly correlated with P uptake in plant growth in all but the calcareous soils. The acid extractions of Truog, acetic acid and EDTA–ammonium acetate were found to be less correlated with P uptake in plant growth. All extraction methods correlated more strongly with P uptake in the sterilised treatments than in those inoculated with mycorrhizal spores. The method of Olsen was therefore selected to analyse P availability in soils supporting a range of mesotrophic grassland communities from eleven sites across England. At each sampling location, the species composition of the vegetation was assessed and classified using the British National Vegetation Classification (NVC). Species-rich hay meadows across a range of alliances were found to occur on soils with low phosphorus availability. Species-poor communities, such as inundation grassland, were found to occur on soils with higher phosphorus availability. Pasture communities, of intermediate species richness, tended to occur on soils of intermediate phosphorus availability. Olsen's method of P extraction is recommended for analysing soils of areas identified for habitat creation; values of less than 10 mg kg?1 will give the greatest potential for the restoration of species-rich mesotrophic grassland.

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