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Abstract: Concentrations of dissolved reactive phosphorus (CDRP) and particle-bound phosphorus (CPP) were investigated in soil columns taken from different parts of three selected agricultural fields. The columns (mini-lysimeters 20cm in diameter, 20cm high) were exposed to on average 64±6mm of simulated rainfall. Total phosphorus concentrations (CTP) in water percolating from the lysimeters were similar to the CTP observed in drainage water leaving the fields, but the CDRP/CTP ratio was significantly (p <0.001) higher in the percolate. The high CDRP (up to 0.4mgL−1) measured in the percolate may have partly derived from decomposition of accumulated senescing vegetation and litter material, since CDRP in percolating water was related to total organic carbon concentration (TOC) in the shallow topsoil (0–5cm) and to phosphorus concentration in soil extract of ammonium lactate (P-AL) from the same soil layer. Another easily identified factor clearly related to CDRP in percolate from the lysimeters was the degree of phosphorus saturation (DPS) in the same soil P-AL extract. The factors giving best prediction of CPP and leaching were the amount of percolate passing through the soil columns and total phosphorus concentration in the topsoil determined after oxidation with nitric acid (TP-HNO3). One field under grass ley, including the slope and depression close to the field outlet, comprised 24% of total field area. In the other two fields these areas (11 and 17% of total area respectively) were under long-term permanent green fallow as an internal buffer zone (IBZ). The lysimeter studies indicated that these areas had a higher risk of DRP losses than the rest of the fields. In addition, the P content of the particles in water from the depression was unusually high. Aggregate stability, measured indirectly as readily dispersible clay and expressed as nephelometric turbidity units (NTU), was significantly stronger at the lowest point of one IBZ (27–35NTU) than in stubbled parts of the same field (62NTU), and the turbidity of water percolating through the topsoil was lower (5–27NTU). [Copyright &y& Elsevier]

Abstract Efficient phosphorus (P) fertilization strategies are essential for intensive crop production with minimal negative environmental impacts. A key factor in sustainable P use is assessment of the plant available soil P pool using soil P tests. This study determined isotopically exchangeable P after six days of reaction with 33PO 4 (P- E (6 d)) to determine how accurately two commonly used P tests, Olsen and AL (acid ammonium acetate lactate) can quantify the amount of labile P. Soil samples were taken from both highly P-amended and unamended plots at six sites within the Swedish long-term soil fertility experiments. According to P K-edge XANES spectroscopy, the P speciation was dominated by Al-bound P and organic P, with additional contributions from Fe-bound P and Ca phosphates in most soils. The results showed that the AL test overestimated P- E (6 d) by a factor of 1.70 on average. In contrast, the Olsen test underestimated P- E (6 d), with the mean ratios of P-Olsen to P- E (6 d) being 0.52 for high-P and 0.19 for low-P soils. The 33P/31P ratio in the Olsen extract of a 33PO 4 spiked soil was closer to that of a 0.005 mol L−1 CaCl 2 soil extract than the corresponding ratio in the AL extract, suggesting that AL extraction solubilized more non-labile P. In conclusion, the AL and Olsen methods are not suitable for direct quantification of the isotopically exchangeable soil P pool after 6 days of equilibration. However, based on the results, Olsen may be superior to AL for classification of soil P status, due to its even performance for calcareous and non-calcareous soils and lower extraction of non-labile P. Highlights • Extraction with sodium bicarbonate (Olsen) underestimated isotopically exchangeable P. • P fertilization increased the ratio of Olsen-P to isotopically exchangeable P. • AL extraction solubilized a large fraction of non-isotopically exchangeable P. • For classification purposes, the Olsen test is probably superior to AL. [ABSTRACT FROM AUTHOR]