Soil Constituent Facilitated Transport of Phosphorus from a High-P Surface Soil

Mobile soil constituents such as soil colloidal matter and dissolved organic matter may affect the mobility of phosphorus (P) in soils. Phosphorus leaching of two fractions (colloidal and dissolved phosphorus) was studied in soil column experiments, using a surface loam soil from Higashi-hiroshima, Japan, with a high Olsen-P content (93 mg-P/kg). Repacked soil columns with or without artificial macropores were prepared. Artificial irrigation solution (0.085mM NaCl+0.015mM CaCl2 solution) was applied for 6 hrs at an intensity of 10 mm/hr or for 2 hrs at 30 mm/hr. Six irrigations were applied to each column with 42 hr intervals between irrigations. Colloidal phosphorus (CP) leaching showed a minor increase with time. Dissolved phosphorus (DP) leaching was almost constant. DP dominated total phosphorus leaching with a contribution of 81-86%. Higher CP and DP leaching at lower irrigation intensity and from soil without macropores were observed. A high correlation between DP leaching and dissolved organic matter (DOM) leaching was seen (R2=0.82~0.86). Batch experiments were conducted to investigate P sorption and results showed that phosphorus was sorbed to or formed complexes with dissolved organic matter. This study shows that DOM-facilitated transport is likely to be a dominating process for phosphorus leaching from high-P surface soils.