Mitigation of soil phosphorus transfer to waters - Improved fertilization and chemical/structural soil conditions

Eutrophication of surface waters is a serious problem in a large part of the world. It has been shown that nutrient levels in lakes and rivers are higher in areas with intense agriculture compared to uncultivated or pristine land. Inputs of nutrients to arable land are needed to sustain a good crop production, but losses of phosphorus (P) and nitrogen (N) to water needs to be substantially reduced if environmental goals on water quality are going to be reached. In the soil, most of the P compounds are not available for plant uptake because they have been more or less fixed and many farmers have applied more P, as fertilizers and manure, than is being removed with the crop. Repeated over several years this leads to an accumulation of P in the soil and more P is lost to the water.

The main objective of the thesis is to carry out a scientific evaluation of how the chemical and structural conditions of soils, governed by agricultural management practices, influence the P transport from arable land to surface waters. Focus will be on the P loss that occurs with water that moves through the soil, and may reach drain tiles. The hypotheses are:

• P leaching from the topsoil increases with increasing levels of  plant available P (P-AL) in the soil;
• More P is leached if the same amount of P is added as manure, compared to mineral fertilizer P;
• P leaching increases with increasing amounts of applied manure (pig slurry) on sandy soils;
• Soil test P and P leaching can decrease through P “mining” by grass; and
• Management practices like broadcast application of mineral fertilizer compared to placement of fertilizer in rows, conventional plowing compared to shallow cultivation, and structure liming compared to not limed, have a major impact on P leaching.

To test these hypotheses, experiments will be carried out using: small lysimeters (20cm deep) placed in a laboratory where rainfall can be simulated, large lysimeters (118 cm deep) outdoors under normal climate conditions and a new experimental field with separately drained plots.