How to reduce nutrient load from agriculture?

The mitigation measures decreasing the runoff of solid matter and nutrients from field cultivation can be divided into measures taken on the field, at the edge of the field and outside the field. As for the measures taken on the fields, fertilization levels of phosphorus and nitrogen have decreased significantly. At the same time the total area plowed in autumn has decreased by half as it has been replaced by reduced tillage and direct sowing systems. This has been shown to reduce especially erosion and particulate phosphorus runoff but also the runoff of total nitrogen. The problems related to dissolved phosphorus runoff, especially from fields with soils rich in phosphorus, still call for separate solutions, e.g. reducing the overall P content of fields.

Water sampling in the Aurajoki river.

The reduction in nutrient load achieved by buffer zones depends not only on the extent of the measure itself but also on the other, on-field measures implemented on the portion of the field remaining in cultivation. The efficiency of wetlands depends on how much field area is included in the upstream catchment area and what is the area of the wetland in relation to the catchment area. The total effect of wetlands and buffer zones is less than that of the measures taken on the fields.

The estimations of changes in agricultural nutrient loads in water bodies involve much uncertainty and not all of the changes can be measured with direct monitoring methods. In reality the nutrient load from agriculture will start to decrease noticeably only after several different measures are implemented on a wide range. The smaller the proportion of cultivated fields in the catchment area, the harder it is to perceive the change in load levels. Automatic nutrient load measuring systems have shown that the agricultural runoff happens mostly during peak periods and almost 100% of the total yearly load occurs outside the growing season. In addition, the yearly load levels vary significantly from one hydrological year to another which in its part makes perceiving changes in load levels achieved by mitigation measures more difficult.

The change in vegetation cover during different seasons.

Targeting of measures to fields with higher risks

Environmental measures should mainly be timed to the period when most of the nutrient runoff occurs. Efficient targeting requires the identification of hot spots that can be field parcels, individual catchments or complete river basins. This means that targeting of measures can mean implementation of measures in fields with the highest loading potential, simultaneous implementation of effective measures in catchments, or implementation of wide-ranging water protection programs in whole water systems. There are clear viewpoint differences between these and they also differ when it comes to coordinating the planning and implementation of these measures. Examples include the farm-level implementation of agri-environmental mitigation measures, recycling of nutrients in the catchments discharging into the Archipelago Sea, and WFD water protection programs in large river basins.

Key areas in solid matter transport and nutrient runoff are steep-sloped fields and fields that flood repeatedly. Fields with high P content and peat soil are also risk areas. The distance of the field parcel from a water body, the soil type of the field and the level of vegetative cover are also risk factors. Steep fields create especially solid matter and particulate phosphorus transport, whereas flooding-vulnerable fields and peat land fields create dissolved phosphorus and nitrate leaching. A more accurate elevation model and more source information about the P content of the soil, the manure spreading areas and the vegetative cover outside the growing season would make risk assessment more reliable.

Risk area maps could be used when targeting water protection measures such as reduced tillage systems, wintertime vegetative cover and buffer zones. These maps should be clear and objective so that they could be used to support decision making.

Sirkka Tattari and Markku Puustinen, Finnish Environment Institute

Below are some slides from the presentation (click to open in a new window)