The importance of subsoil properties for P leaching and selection of effective mitigation strategies

Agriculture is the main diffuse phosphorus (P) source in the Baltic Sea catchment area and the gross load from Swedish agriculture is estimated to be 37% of the total load to the surrounding coastal areas. P can be lost either as a result of erosion and surface runoff, or by leaching through the soil with drainage water. Surface runoff was previously considered the main pathway for P losses from agricultural land, but the importance of P leaching through soil has been re-evaluated in recent years and this is now known to be a major P loss pathway. Such losses are mainly a problem in sandy soils or in soils with extensive flow through large pores or cracks.

Many studies have been carried out in the topsoil or the entire soil profile and countermeasures used today to reduce P losses from agricultural land are often based on topsoil features. However, the P content in the topsoil is not always correlated with the P concentration in drainage water. When water is transported down through the soil, P can either be released from the soil into percolating water or be sorbed to soil particle surfaces. Hence, the subsoil can function either as a source or a sink for P losses. It is therefore important to account for subsoil features in applying appropriate mitigation strategies to reduce nutrient loads from the field.

The main objective of this PhD project is to study how the subsoil affects P losses from different agricultural soils. The work will take the form of lysimeter studies, where subsoil features will be studied with and without the influence of topsoil. A second objective of this project is to investigate the effects of liming between the topsoil and subsoil as a management strategy to reduce P losses. The reason for placing lime between the two layers is that P thereby will stay available for plant uptake in the topsoil without risking high P content in the subsoil, which can increase leaching. This will be tested on soils where the subsoil is not expected to act as a sink.