Does Phosphorous Limit the Growth of Pine Trees in Volcanic Ash Soils?.

Phosphorous is a major plant nutrient; however its availability in soil is often small, since it is easily immobilized. We analysed different P fractions in volcanic ash soils of different age in Central Mexico following the P fractionation method of Tiessen and Moir (1993). We did so to investigate how the different P fractions change along a chronosequence (60 to 100,000 years BP), and to discover if P availability limits growth at particular development stage. We analysed top soil and subsoil samples at field moisture content, air dried and burned at 300 ° C in order to evaluate the effects of desiccation and of forest fire on P availability. At each site we also collected pine needles and determined their P contents.

The fractionation extracted 39% of total P. Burning the samples increased P extraction in all fractions, while air drying had no effect compared with field moist samples. The dominant P fraction was the one of primary P minerals (P Ca), followed by non-occluded P in Regosols and Andosols, while occluded P was constant along the chronosequence. Apparently the method does not extract exhaustively neither occluded P, nor organic P and fails to dissolve P Ca, since in the volcanic ashes the dominant P Ca is fluorapatite. Nevertheless the P extracted by ion exchange resins is present sufficiently in all soils along the chronosequence and correlates well with foliar P contents. The latter indicates that P is not limiting tree growth in young and moderate developed volcanic ash soils (Regosols and Andosols). To elucidate the evolution of P fractions during soil development in volcanic soils, the extraction of organic and occluded fractions needs to be improved, as well as the dissolution of primary P minerals.