Nitrogen and Phosphorus Inputs in Water Bodies: New Estimates for Agriculture

Agroscope researchers calculated the nitrogen and phosphorus inputs into Swiss water bodies for the year 2020. Although these inputs decreased in comparison to 2010, the ‘agriculture’ environmental target for nitrogen inputs was missed by a significant margin.

Many of our water bodies are polluted with nitrogen and phosphorus. This can jeopardise water and drinking-water quality. Inputs from point sources such as sewage treatment plants and combined sewer overflows have fallen significantly over recent decades, but inputs from so-called diffuse sources, e.g. from runoff, erosion, tile drainage or leaching, remain high.

On behalf of the Federal Office for the Environment (FOEN) and with the support of the Federal Office for Agriculture (FOAG), Agroscope researchers have now calculated these diffuse nitrogen and phosphorus inputs into water bodies throughout the whole of Switzerland for the year 2020. Using an updated version of the MODIFFUS model, water and substance flows were calculated based on field trials, literature research and expert knowledge. The results are used by the FOEN for reporting to international commissions and are also relevant in the current agricultural policy context. They are available as maps in hectare grid on the Swiss Confederation’s geoportal (map.geo.admin.ch). Results are also available for all the Cantons as well as for various hydrological catchments.

70,000 tonnes of nitrogen entered Swiss waters in 2020

According to the model calculations a total of around 70,000 t of nitrogen entered Swiss water bodies in 2020, with around 47,500 t from diffuse sources and around 22,800 t from point sources. Almost half (46% or around 32,500 t) of the total nitrogen input was associated with farming. The main diffuse input pathway was leaching (70%), and the main diffuse input sources were arable land (33%), followed by grassland (24%) and forest (19%).  

Compared to 2010, the sum of diffuse total nitrogen inputs to water bodies for 2020 was about 4000 t or 8 % lower. The uncertainty for these figures is around 25%, i.e. three-quarters of the reduction in nitrogen inputs compared to 2010 is to be considered real, and is in particular due to the decrease in land used for agriculture (especially arable land), the (climate-related) reduction in the amount of seepage water, and lower nitrogen deposition (the positive outcome of air-pollution control measures).  

The ‘agriculture’ environmental target that obliges the Swiss agricultural sector to reduce agriculture-related nitrogen inputs into water bodies by 50% compared to 1985 was not met in 2020; a target gap of just under 8000 t remains.

Dissolved phosphorus inputs: only 27% are related to agriculture

In 2020, dissolved (i.e. eutrophication-impacting) phosphorus inputs into Swiss water bodies were at around 1400 t. Only around 380 t or 27% of all dissolved inputs were caused by agriculture – the remainder come from sewage treatment plants, combined sewer overflows, forest, and unproductive areas (natural background load).

Of the total 1400 t, around 740 t or 53% come from diffuse sources and around 660 t from point sources. The largest share of diffuse inputs was caused by run-off (54%). Forty-five per cent of the diffuse inputs were from grassland and 20% from arable land.

Compared to 2010, diffuse inputs of dissolved phosphorus fell by around 165 t or 18%. The greatest absolute change is to be found in farmland, with a decrease of 49 t. The main reason for this were changes of a predominantly methodological nature, as well as the decrease in arable land.