Phosphorus leaching as influenced by animal manure and catch crops

Sammanfattning: Leaching of phosphorus (P) constitutes an important part of P losses from Swedish agricultural soils. Phosphorus leaching is complex and is influenced by many factors, from source and mobilisation to transport pathways, as well as agricultural management practices. In order to design appropriate mitigation strategies to reduce P leaching, it is urgent to understand how different factors influence P leaching and to understand the methods for assessing P leaching. This thesis investigated the influence of two management practices, application of animal manure and use of catch crops, on P leaching under Swedish conditions and devised corresponding mitigation strategies. In the case of manure application, lysimeters were used to study P leaching from topsoil as influenced by soil type, manure application method and long-term manure application. In addition, P leaching from a field associated with long-term manure application was simulated with the ICECREAM model to clarify the processes dominating P leaching. In the case of catch crops, uptake of P by potential species and leaching of P from them after freezing and thawing were examined in a greenhouse and in a topsoil lysimeter study. Conclusions are: (1) Recent manure application to clay soils with macropore flow pathways generates a high risk of P leaching and thus such application of P during wet periods must be followed by incorporation of manure into the soil or avoided; (2) application of moderate rates of pig slurry to sandy soils with sufficient P sorption capacity does not increase the risk of P leaching, and P can be applied at a rate in balance with crop removal; (3) given the same P rate, pig slurry does not constitute a larger risk of P leaching than mineral P fertilisers; (4) descriptions of P sorption/desorption processes dominated by Fe and Al oxides must be included in P models such as ICECREAM; and (5) catch crops can become a source of P losses after exposure to freezing-thawing, which should be considered when using a catch crop. In future, more research is needed to investigate mitigation strategies to minimise P leaching from clay soils, to select or modify catch crop species to be efficient for P, and to develop methods for accurately assessing risk of P leaching.