Risks of Phosphorus Leaching Associated With Long-Term Repeated and Single Applications of Animal Manure.

Phosphorus (P) leaching from arable lands contributes to eutrophication of adjacent water bodies. To assess risks of P leaching from a sandy soil associated with long-term manure applications, patterns of total-P leaching from separately tile-drained field plots during 1989-2003 were analyzed and simulated with the ICECREAM model. The plots were annually applied with pig slurry at high (56 kg P ha^-1 yr^-1) and low (28 kg P ha^-1 yr^-1) rates, and only mineral P (24 kg P ha^-1 yr^-1). Intact topsoil columns from the plots were used to study potential of P leaching resulting from such long-term slurry applications and after incorporation of single dose of slurry (22 kg P ha^-1). In addition, effects of different slurry application methods, i.e. surface application and incorporation of 30 kg P ha^-1, on potential of P leaching were studied with columns from a similar sandy soil with relatively low degree of P saturation (DPS) and from a clay soil. We found that moderate rate of pig slurry (20-30 kg P ha^-1) could be applied to low-DPS sandy soils without increasing potential of total-P leaching. However, long-term repeated applications of slurry at high rates increased the leaching potential. Concentration of dissolved reactive P, the dominant form in total-P, was positively correlated with soil DPS (R²=0.95, p<0.0001). Surprisingly, measured total-P leaching in drain tiles was low (0.17 kg ha^-1 yr^-1), due to a high sorption of P especially in the subsoil. In contrast to a sandy soil, moderate slurry application to the clay soil constituted a high risk of total-P leaching, most likely due to presence of macropore flow pathways. This risk was reduced by 50% when the slurry was incorporated, but it was still high. Therefore, manure applications to this type of soil during wet periods should be followed by incorporation or avoided.