Background concentrations and loadings of nitrogen in Danish surface waters

Quantitative knowledge of background nitrogen (N) concentrations and loadings is essential for as it determines the minimally disturbed conditions for N in surface waters as a near reference condition. To determine background N concentrations, a total of 39 smaller Danish streams draining relatively undisturbed catchments in different regions were selected and screened for N concentrations and discharge during a 1-year period (2004-2005). Only 19 of the streams fulfilled the threshold set for least disturbed conditions (LDC) in their catchments (proportion of agricultural land <10%). Within the five sampled dominant landscape types in Denmark, the concentrations of ammonium N and total organic N in the LDC streams were found to be nearly constant: 0.05 +/- 0.01 mg N L-1 and 0.53 +/- 0.07 mg N L-1, respectively. In contrast, the concentration of nitrate N differed significantly (P < 0.05) between the five landscape types, with variations between 0.06 and 0.83 mg N L-1. An analysis of all the 39 streams sampled demonstrated significantly different relationships between the proportion of land use and the flow-weighted annual concentration of nitrate N for two dominant soil types with a Y-axis intercept equal to 0.12 mg N L-1 for streams draining coarse textural soils and 0.76 mg N L-1 for streams draining finer textural soils. A significant (P < 0.05) downward trend (18-41%) emerged for flow-weighted annual total N concentrations in four of the five streams that were sampled during the period 1990-2011. A 5 x 5 km grid map of Denmark showing nitrate N and total N background concentrations was produced and used for estimating background N losses to Danish surface waters. Based on the grid map, the average annual background loss of total N was calculated to be 13,000 tonnes N or 19% of the total N loading to Danish coastal waters during the period 2004-2005.