There is increasing evidence that sorbed phosphorus may be vertically transported in the suspended form through macropores in subsoil. Phosphorus discharge through this pathway is of particular interest in Japan, where a considerable acreage of former paddy fields is currently used for non-rice crop production, and the fields with clayey subsoil have shrinkage cracks formed upon drying. The objective of this study was to elucidate discharge process of suspended particles and associated phosphorus to tile drains in a clayey "ex-paddy" field. Surface runoff and tile drainage as well as the soil water condition at different depths were continuously measured in the experimental field, where the topsoil (0-20 cm depth) was underlain by a clayey subsoil, with tile drains installed at a 65 cm depth. Water in the surface runoff and the tile drainage was collected to determine the suspended particle (> 0.1 μm) and phosphorus concentrations. Discharge of water to the tiles was initiated soon after the pressure potential at the topsoil-plowsole boundary became positive. Once the tile drainage started, the discharge responded quickly to changes in the rainfall intensity, suggesting the water flow was preferentially through the cracks. The concentrations of suspended particles and phosphorus in the tile drainage were highest at the beginning of discharge, with the maximum concentrations of 0.60 g/L and 1.34 mg-P/L, respectively. Total discharge of phosphorus to the tiles and the surface runoff during the monitoring period (June-October, 2004) were estimated at 0.57 kg-P/ha and 0.27 kg-P/ha, respectively. Ninety-two percent of the phosphorus found in the tile drainage was in the suspended form, whereas in the surface runoff the suspended form constituted 78 %. These results demonstrate that subsoil cracks can be a major pathway for discharge of suspended soil particles and associated phosphorus from clayey "ex-paddy" fields.
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