Trend Reversal of Nitrate in Danish Groundwater - a Reflection of Agricultural Practices and Nitrogen Surpluses since 1950

Abstract

This paper assesses the long-term development in the oxic groundwater nitrate concentration and nitrogen (N) loss due to intensive farming in Denmark. First, up to 20-year time-series from the national groundwater monitoring network enable a statistically systematic analysis of distribution, trends, and trend reversals in the groundwater nitrate concentration. Second, knowledge about the N surplus in Danish agriculture since 1950 is used as an indicator of the potential loss of N. Third, groundwater recharge CFC (chlorofluorocarbon) age determination allows linking of the first two data sets. The development in the nitrate concentration of oxic groundwater clearly mirrors the development in the national agricultural N surplus, and a corresponding trend reversal is found in groundwater. Regulation and technical improvements in the intensive farming in Denmark have succeeded in decreasing the N surplus by 40% since the mid 1980s, while at the same time maintaining crop yields and increasing the animal production of especially pigs. Trend analyses prove that the youngest (0-15 years old) oxic groundwater shows more pronounced significant downward nitrate trends (44%) than the oldest (25-50 years old) oxic groundwater (9%). This amounts to clear evidence of the effect of reduced nitrate leaching on groundwater nitrate concentrations in Denmark.

Regional analysis of groundwater nitrate concentrations and trends in Denmark in regard to agricultural influence

“…The present paper continues the analysis initially presented and published in Hansen et al (2011). The focus is still on nitrate in the oxic zone of the Danish groundwater, because we are examining the effect of nitrate leaching on groundwater nitrate concentrations.…”

“…Since the 1980s, agriculture in Western Europe has managed to reduce its nitrogen surpluses, owing to stringent national and European community policies (Vitousek et al, 2009;Grizzetti et al, 2011;Hansen et al, 2011;Dalgaard et al, 2012). However, Vitousek et al (2009) reckon that regions in Africa continue to extract the nutrient capital of what were once highly fertile soils with low yields, while in contrast intensive agricultural production in Northern China has a very high input of N to agricultural fields and high yields, but also a very high N loss to the environment.…”

Spatial distribution of soils determines export of nitrogen and dissolved organic carbon from an intensively managed agricultural landscape

“…As pointed out in many studies (Böhlke and Denver, 1995;Ruiz et al, 2002a;Worrall and Burt, 2001), soils are able to store N, leading to poor correlations between N losses from agricultural soils and headwater quality. Consequently, N leaching is affected by past land use and management practices (Hansen et al, 2011(Hansen et al, , 2012, which varied a lot in this area during the past 300 yr (Caspersen and Fritzbøger, 2002), but a further study of this aspect was out the scope of the present study. Furthermore, the NO − 3 stored in soils can be released into sub-surface and groundwater after some time.…”

Management, regulation and environmental impacts of nitrogen fertilization in northwestern Europe under the Nitrates Directive; a benchmark study

“…Analysis of lag times required for improvements of groundwater nitrate levels in Ireland showed that the achievement of good water quality status for some water bodies may be too optimistic but improvements are predicted within subsequent 6-and 12-yr cycles (Fenton et al, 2011). Analyzing a 50 yr time series of SNB and nitrate concentration in groundwater in Denmark, Hansen et al (2011) found that nitrate concentrations have been decreasing since 1980. They found that the frequency of downward nitrate trends in groundwater samples clearly increased with lower recharge age, providing proof that younger groundwater responds fastest to decreasing trends of SNB.…”