Background concentrations and loadings of nitrogen in Danish surface waters

Abstract

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 × 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.