Combined effect of water and organic matter on phosphorus availability in calcareous soils

Phosphorus removal from soil solution is mainly due to adsorption and precipitation. For calcareous soils, with a large reservoir of exchangeable calcium, precipitation of insoluble Ca-P phases is the predominant process that reduces P availability to plants. Soil water content positively affects P-precipitation, while the addition of organic matter (OM) has an opposite effect. Little information on the effect of soil organic matter on P-insolubilisation as a function of soil water contents has prompted this study of the variation of extractable P, after addition of mineral P fertiliser. Columns packed with a calcareous soil were enriched with different levels of OM, extracted from Irish peat, and subjected to different rainfall simulations. After 102 days of experimentation and 171 mm of accumulated rainfall, the Olsen-P was 53% of the initially applied amount in 6.2% OM-enriched soil, 37% in 4.1% OM-enriched soil, and 20% in untreated soil (1.9% of OM). While the curve describing Olsen-P decrease as a function of accumulated rainfall was clearly exponential for untreated soil, the curves for OM-enriched samples were flatter, evidence that OM addition modified P-insolubilisation. The P-insolubilisation, after P-fertilisation, at several constant values of soil moisture for (i) calcareous soil, (ii) calcareous soil after removing carbonates and saturating the exchange complex with Ca, and (iii) calcareous soil after addition of different levels of OM followed first-order kinetics. The K(obss) followed the order: Ca-saturated soil > untreated soil > OM-enriched samples. Results from rainfall simulation experiments and kinetics of Olsen-P decrease at several constant soil moisture contents indicated that the soil water amount was the main factor in reducing extractable P after P fertilisation and that the soil OM content was the main factor in keeping P in extractable forms. On the other hand, the addition of OM to calcareous soil increased the extractable P at each soil moisture regime, decreasing P-insolubilisation more effectively at lower soil water contents. P-sorption isotherms of calcareous soil after addition of different levels of OM showed that the presence of OM mainly influences P-insolubilisation, but not the adsorption process.