Are Histosols a Major Contributor of P to Surface Waters? Results from a Short and Long Soil Column Study.

Leaching of phosphorus (P) from arable mineral soils is often known to result in significant amounts of P entering surface waters causing eutrophication. However, P leaching from organic soils and their contribution to the eutrophication problem are less documented and understood. In an attempt to identify potential concentrations, loads and types of P that leach from organic soils, two column studies were carried out utilising the same two organic soils. The aim of the short columns was to identify maximum potential P loadings and from which soil horizons P was mainly released. Columns, 20cm in diameter by 20cm deep were extracted from both soils at four depths: 0-20, 20-40, 40-60 and 60-80cm. Four 50mm rain simulations were applied at 5mm/hr over 10 hours with two days in between each event. This resulted in 85-92% of leached total-P originating from the 0-20cm depth. The total-P from the 0-20cm depth was comprised of 76-100% orthophosphate with smaller amounts of particulate and organic-P. Combined mean leaching loads from the four depths of soil 1 resulted in 5.1kg ha^-1 compared to 17.7kg ha^-1 from the second soil. These loadings related well to plant available P (ammonium lactate P) values from both soils.

 In the second study 90cm intact outdoor lysimeters utilising the same two soils were subjected to more natural climatic conditions. Between 0.4 and 0.66kg ha^-1 of total-P was leached from soil 1 over a one year period (575mm rainfall) compared to 0.6-1.49kg ha^-1 from soil 2. These results suggest that although these soils can leach very high loads of P, the organic subsoil acts as an efficient filter by removing a large portion of mobile soil solution P.