Jensen, P.N. (Ed.) 2017. Estimation of Nitrogen Concentrations from root zone to marine areas around the year 1900. Aarhus University, DCE – Danish Centre for Environment and Energy, 126 pp. Scientific Report from DCE – Danish Centre for Environment and Energy No. 241. http://dce2.au.dk/pub/SR241.pdf

Summary

Determining the reference conditions for the various water types (rivers, lakes and coastal areas) is a central element in the Water Framework Directive (WFD), as it sets the starting point for determining the boundaries between the quality classes, especially the boundary between good and moderate quality, as this line in general defines, whether or not a certain water body fulfils the objective.

Aarhus University has decided to undertake an in-depth analysis of the main factors, that may have influenced the entire nitrogen cycle and to investigate the factors that may have influenced the annual mean nitrogen (N) concentrations from source to sea around the year 1900 in Denmark in order to estimate the N-concentration at that time. Fulfilling such an aim require, that a range of parameters known to influence the N cycle, such as climate, hydrology, land use, agricultural practices, drainage, landscape, etc. , are described for the period around the year 1900, as very few measurements of N concentrations in streams and rivers are available from that time.

A number of different methods, that could assist in estimating and/or finding indications of the N concentrations around the year 1900 in soil water, groundwater and surface waters, are discussed – such as agricultural statistics, trends in climate and hydrology, review of historical measurements found in the international and Danish literature  or use of model estimates. The aim of the analysis is to conduct an assessment of, what this range of indicators in combination shows about, what might have been the N-concentration in the water discharging into marine areas around the year 1900.

The main findings of the report are listed below.

• The runoff from the Danish area was app. 25% lower in the year 1900 than today.
• The temperature has increased  app. 1.5 °C since the year 1900
• Around the year 1900, 67% of the land was cropped, 8% was bare fallow and 25% was nature areas.
• For cropped land it has been estimated, that the nitrate concentration in the leachate of the root zone would have been 12 mg N/l around the year 1900, which is in line with today’s organic cash crop farming.
• Approximately 22% of the area used for agriculture was tile drained around the year 1900 (16% of the total area), as compared with today where app. 50% of the agricultural area is tile drained
• The atmospheric deposition on land has been estimated to 4 kg N/ha in the year 1900 – about one third of today’s deposition level.
• Around the year 1900 point sources (cities, industry) may have had an impact locally, but the point source load of nitrogen was too low to have influenced the overall national nitrogen budget.
• Model calculations have shown, that the total retention (from root zone to the coast) was considerably higher than today – in the range of 76 – 87% around the year 1900.
• Routing of the estimated nitrogen concentration in the root zone combined with the estimated nitrogen removal in ground and surface water around the year 1900 results in a concentration range of 1-2 mg N/l.
• Empirical models, measurements from the time around the year 1900 and similar international analyses all show N-concentrations close to or within the range of 1-2 mg N/l some without accounting for full retention in surface water.

In this attempt to estimate an N-concentration in water running to the sea around the year 1900, many assumptions have been made for nearly all elements included in the analysis. This means that the result of the individual element is probably associated with relatively high uncertainty, but nearly all elements point in the same direction (same range). 

Overall conclusion

From the indications etc. described in the chapters and synthesis above, the best estimate of the nitrogen concentration in the water running into Danish coastal areas at the time around the year 1900 is within the range of 1-2 mg N/l.