Effect of air-drying on phosphorus fractions in clay soil

Despite the publication of a number of papers dealing with the effect of drying on the soil labile P pool, less attention has been paid to the possible drying-evoked changes in the more stable P pools. We applied Hedley's sequential fractionation procedure that aims at quantifying soil P reserves according to their decreasing plant availability to examine the effects of drying on soil P fractions in clayey soil samples of different cultivation history. To further investigate the contribution of organic matter disruption to the solubility of soil P, the P extracted in each fractionation step was divided into two size classes by filtering the suspension through a 0.2 mu m membrane filter. There were no air-drying-induced changes in the total amount of P extracted in each fractionation step. However, air-drying changed the distribution of water-extractable P in size fractions; increase in the small-sized P took place at the expense of large-sized P. Air-drying increased also small-sized molybdate-unreactive P (MUP) in the NaOH fraction giving evidence that drying-induced alterations take place also in less labile P forms. The results revealed that air-drying alters the extractability and distribution of P in various pools rather than the total amount of extracted P and that a large proportion of H2O- and NaOH-extractable large-sized MUP may remain undetected if only filtered samples are analyzed.