Markku Ollikainen1*, Petri Ekholm2, Eliisa Punttila1
1 University of Helsinki, P.O.Box 27,00014 Helsingin yliopisto, Finland
2 Finnish Environment Institute, P.O.Box 140, 00251 Helsinki, Finland
The Baltic Sea Action Plan aims to reduce the annual load of phosphorus and nitrogen entering the sea by 14 400 and 89 300 tons, respectively. This reduction should be achieved mostly by measures within wastewater treatment and agriculture. Wastewater treatment plants can still considerably reduce nitrogen loads at low costs when compared to those of agriculture. Possibilities to reduce phosphorus are, however, more limited in many countries increasing the need to reduce phosphorus in agriculture.
Current measures to reduce phosphorus runoff from arable fields are quite ineffective and limited. Gypsum provides a new measure that has been found to effectively cut down phosphorus runoff from fields. Gypsum suits well to clay soils and the average gypsum dose per hectare is four tonnes. Once spread on fields, gypsum increases the ionic strength of soil. It creates larger aggregates of soil particles and affects phosphorus binding, which decreases the phosphorus losses to waterways. The soil structure improves, erosion decreases, and phosphorus remains available to plants. These beneficial effects occur immediately after the dissolution of gypsum, last for several years and are achieved without any loss of crop yields. Gypsum treatment can be easily combined to the ordinary farming practices, as shown by the recent gypsum pilot project in the Savijoki catchment.
Previous research suggests that gypsum treatment of fields reduces dissolved reactive phosphorus by about 30% and particulate phosphorus by 60%. In southwestern Finland, the average P runoff may rise up to 1.3 kg/ha/y entailing 0.4 kg dissolved phosphorus. Gypsum amendment reduces this runoff by 0.63 kg, that is, almost by a half. We estimate that gypsum may reduce P loading into the Archipelago Sea as much as by 100 tons. For comparison, increasing the phosphorus removal rate to 98% in all the Finnish wastewater treatment plants (larger than 10 000 PE) would result in only 30 tons reduction in loads.
Gypsum has yet another advantage: it is much cheaper than the measures currently included in the Finnish agri-environmental payment scheme (buffer strips, fertilization limits and wetlands). Previous work suggests that the marginal costs of reducing 30% of P runoff from fields with these measures are about 230 €/kg. Drawing on the cost data from the gypsum pilot we conclude that gypsum reduces loading by more than 40% with a cost of 70 €/kg.
Gypsum treatment can provide an exciting solution to agricultural phosphorus loading in the entire Baltic Sea. Gypsum treatment may suite e.g. for Sweden, Denmark and Poland. Clay soils are dominant in Sweden and Denmark, while soils in Poland more coarse. Together with Finland, the agricultural phosphorus runoff to the Baltic Sea from these four countries amounts to 8000 tonnes annually. By rough estimates gypsum treatment of fields could reduce the load by up to 1500 tonnes from these countries alone.
The gypsum pilot contributes to the NutriTrade project lead by John Nurminen Foundation and funded by the European Union Central Baltic Program.