Agronomic measures of P, Q/I parameters and lysimeter-collectable P in subsurface soil horizons of a long-term slurry experiment

Soils from a long-term slurry experiment established in 1970 at Hillsboroug h, Northern Ireland, were used in the experiment. The site has a clay loam soil overlying Silurian shale. Seven treatments were used with three replic ate plots per treatment under the following manurial regimes: (1) mineral f ertiliser supplying 200 kg N, 32 kg P and 160 kg K ha(-1) yr(-1); (2)-(4) p ig slurry applied at 50, 100 or 200 m(3) ha(-1) yr(-1); (5)-(7) cow slurry applied at 50, 100 or 200 m(3) ha(-1) yr(-1). Agronomic measures of P deter mined on subsurface layers down to 90 cm were compared with sorption isothe rm data and rates of desorption. Adsorption isotherms were fitted using a s tandard Langmuir model. Data were compared with soluble (molybdate-reactive ) P levels in soil water collected at 35 and 90 cm using PTFE suction cup l ysimeters. Agronomically available P was concentrated in the top 30 cm of s oil in all treatments. The accumulation of P in surface layers of the plots was significantly greater in the pig slurry treatments compared to the cow slurry, reflecting the history of P amendments. Nevertheless, over a perio d of a year, molybdate-reactive phosphorus (MRP) concentrations in lysimete r collections was consistently higher at 35 cm depth in the highest cow slu rry treatment (7) compared to the equivalent pig slurry treatment (4). Eith er the movement of soluble P down the profile is facilitated by the higher organic content of cow slurry or P movement is not directly related to P ac cumulation in the soils. In addition, it is hypothesised that P movement do wn the soil profile depends upon two separate mechanisms. First, a 'break' point above which the accumulated P in the surface horizons is less strongl y held and therefore amenable to dissolution and movement down the profile. Second, a mechanism by which some solute P from the surface horizons can t ravel rapidly through horizons of low P status to greater depth in the soil , i.e., by preferential flow. (C) 2000 Elsevier Science Ltd. All rights res erved.