Frigörelse av fosfor från färskt, fryst och torkat växtmaterial

Phosphorus is the single most potent contributor to eutrophication of freshwater and has also been shown to contribute to the toxic algae blooms in the Baltic sea. The dynamics and paths of losses of phosphorus from arable lands are not fully understood. It is known that phosphorus can be lost directly from plant material to water. In studies where plant material has been subjected to several freeze-thaw cycles, large amounts of phosphorus have been lost. Most studies have been conducted under field conditions, with many factors effecting the measured losses of phosphorus, such as weather and type of soil. This study was performed in a controlled environment without the presence of soil. Instead of soil, small glass beads were mixed with the plant materials in vessels, to make possible homogeneous leaching with water through the mixture of glass beads and plant materials. In this study, clover, ryegrass and dry straw were cut to pieces, approximately 2 cm long. Fresh, frozen and dried plant material from clover and ryegrass were mixed with the glass beads and incubated though 43 days. Straw was only studied dried. At 7 occasions, the samples were leached with water. The leached water was then analyzed for inorganic and total phosphorus released from the plant materials. The results showed that the plant materials released 212 to 1216 mg phosphorus per kg dry matter during the first day. Dried material released the largest and fresh material the least amounts of phosphorus. After two days, 45-57 % of phosphorus in the dried materials had been released, 34-35 % in frozen materials and 9-13 % in fresh materials. The release of phosphorus declined after the first two days in dried and frozen materials, while the release continued or increased from the fresh materials. After 43 days, 57-69 % of phosphorus in the dried materials had been released, 49-51 % from the frozen materials and 42-50 % from the fresh materials. Differences between frozen and fresh materials were not significant at the end of the experiment. In the beginning, primarily inorganic phosphorus was released, while later in the experiment, more organic phosphorus was released, especially from clover. Fresh grass on the other hand increased the release of inorganic phosphorus almost until the end of the experiment. Calculations showed that if the results were scaled up to field conditions, the release of phosphorus from plant material during winter and spring in cold climate could contribute to significant losses of phosphorus from arable lands. I a worst case scenario, about 3 kg of phosphorus per hectare could be released from a ley with frozen grass and clover during the winter. If surface runoff would occur from 10 % of the surface, leaching from plant material could contribute with more than 0.2 kg of phosphorus loss per hectare of farmland.

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