Risk of phosphorus leaching from low input grassland areas

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 20 cm in diameter, 20 cm high) were exposed to on average 64 ± 6 mm 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 0.4 mg L− 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 (0–5 cm) 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 (27–35 NTU) than in stubbled parts of the same field (62 NTU), and the turbidity of water percolating through the topsoil was lower (5–27 NTU).