Assessment of agricultural nitrogen pressures and legacies in Denmar

The links between nitrate-nitrogen (N) leaching from agricultural fields and N measured in streams can in general terms be divided in two pathways: groundwater and a more surface-near transport (e.g. tile drains).  The former with a typical slower hydrological response than the latter.  Therefore, catchments with quick hydrologic pathways respond also quickly on programme of measures for N. On the other hand, catchments having bot high N-attenuation or longer N time lags makes it complicated for managers and policy makers as the response of the implemented programme of measures might both be dampened and delayed. As River Basin Management Plans (RBMPs) under the Water Framework Directive (WFD) runs in 6 years periods – such time lags might end up as an overdosing of measures. Therefore, attenuation and time lags needs to be mapped as they have major effects on the expected effects of RBMPs and its legacy for water quality. The aim of this study is to improve our understanding of N lags mapped based on 30 years of data from 160 Danish stream monitoring stations.

A national wide screening for trends in annual flow-weighted total nitrogen (TN) concentrations at 163 river monitoring stations shows in most cases a downward trend (average: 30% ± 17%) during the last 30 years 1990-2019). The N-surplus has been reduced (farm gate: -44%; field:  -45%) during the same period. Before 1990, the N-surplus in agriculture was increasing and started at first levelling off in the mid 1980ies. Diffuse N-sources and mostly agriculture contributed the most to TN in streams (93% ±8%) during the period 1990-2019). The reduction in the diffuse N loadings are paralleling the development of the N surplus for most Danish streams. However, in certain parts of Denmark several river monitoring stations shows a much different response, which in some cases is no response at all. Such a pattern can only be explained by N-flows in the catchments to be delayed in groundwater aquifers. Using long term data for national N-surplus a simple lag-time analysis shows that the time lags for N are long for 21 catchments (up to 20 years), medium long for N in 62 catchments and with nearly no delay for N in 80 catchments (Fig. 1). Moreover, all the stream stations experiencing long time lags are situated in the chalk and partly karstic landscapes of Denmark from the Danien period. The catchments having long delays for N shows in most cases also a very low attenuation of N in groundwater as measured N-concentrations are substantially higher than found in the streams having nearly no time lags. Therefore, we conclude that incorporation of biogeochemical and hydrologic time lag principles into water quality regulations will be necessary for providing managers and regulators with realistic expectations when implementing new policies for N.

Figure 1: Map of Denmark showing catchments with short (< 20% older than 10 years), medium (20-40 % older than 10 years) and long (> 40% older than 10 years) for nitrogen in groundwater.