The effect of nitrogen mitigation measures evaluated by monitoring of nitrogen concentrations and loadings in Danish mini-catchments – 1990 – 2015

Monitoring of agricultural mini-catchments has been part of the Danish national monitoring programme (NOVANA) since 1989. Thus, nitrogen (N) concentrations and loads have been monitored in soil water, tile drains, and streams within ﬁ ve agricultural mini-catchments. Moreover, extensive monitoring of N concentrations and loads in streams draining 46 mini-catchments has been conducted every year since 1989. This has resulted in two national datasets on trends in ﬂ ow-weighted N concentrations relative to factors such as groundwater age and management history. We analyzed these datasets and found that the intensively monitored micro-catchments generally showed a strong signal with signi ﬁ cant downward trends in ﬂ ow-weighted N concentrations in monitored soil water ( (cid:1) 22% to (cid:1) 68%), tile drains ( (cid:1) 38% to (cid:1) 59%), and streams ( (cid:1) 19% to (cid:1) 53%). The 46 micro-catchments monitored for N in streams also exhibited downward trends in ﬂ ow-weighted N concentrations, which can mainly be ascribed to the introduction of mandatory national regulation of N in agriculture in Denmark in the mid-1980s. However, classi ﬁ cation of the mini-catchments according to the age of the oxidized groundwater revealed signi ﬁ cant differences in N trends between the groups of mini-catchments. Thus, the strongest downward trend in ﬂ ow-weighted N concentrations was as follows: < 1 year ( (cid:1) 52%), 1 – 3 years ( (cid:1) 44%) and > 3 year ( (cid:1) 38%).


Study of mini-catchments
The 46 studied agricultural mini-catchments (ACMs) are situated in different georegions of Denmark and have been part of the NOVANA monitoring programme since 1990 ( Figure 1). The age of oxidised groundwater percolating to the 46 streams was modeled using the MIKE-SHE model with a coupled particle tracer as described in Højbjerg et al. (2015). Five of the 46 ACMs are intensively monitored (IAMCs) and the remaining 41 are extensively monitored (EAMCs). The IAMCs are grouped according to dominant soil type and the entire group of AMCs (IAMCs & EAMCs) has been categorized relative to the age of oxidized groundwater, the age at which 95% of the percolating water with dosed particles is recaptured at the outlet stream monitoring station (see Table 1). 95% age was chosen as the grouping parameter for the IAMCs and   (Table 1). A detailed description of the monitoring stations within each of the five catchments can be found in Table 2

Extensively monitored agricultural mini-catchments
Each mini-catchment has an established stream monitoring station instrumented with equipment for continuous recording of water stage, and discharge is measured at fortnightly to monthly intervals to enable establishment of a stage/discharge relationship. The N concentrations in streams are measured in water samples collected at fortnightly to monthly intervals, the interval being longest in baseflow-dominated streams (Kronvang and Bruhn, 1996). At all monitoring stations, the daily N transport is calculated using linearly interpolated daily N concentrations multiplied with daily discharge (recommended in Kronvang and Bruhn (1996) as the most robust transport estimator).  (2)), which are assumed to be a more robust method for these time-series.
If we seek to identify the total change in nutrient inputs over the whole time series expressed as a percentage, we can use the two methods below. Estimated linear slope: where n is the length of the series,α is the estimated input at start year minus one year, andβ is the estimated slope. Formula 1 is based on the Theil-Sen slope estimator, and α is estimated using the estimator suggested by Conover (). When using start and end values we have the formula: 100 Á (end-start)=start (2)

RESULTS AND DISCUSSION
Intensively monitored agricultural mini-catchments and 2(f)) showed a much higher difference between the concentration of N in soil water and stream water N than the three loamy catchments (Figure 2(a)-2(c) (Table 3) The monitored tile drains also revealed a downward trend in the same order as that found in soil water (Table 3). In the loamy catchments, Lillebaek, only two of the six fields with soil water monitoring were monitored. Therefore, the different trend signals observed in soil and tile drain water could potentially be ascribed to the different number of paired observations.

Extensively monitored agricultural mini-catchments
The three groups of AMCs show no major differences in catchment size, mean proportion of agriculture, and mean annual runoff (Table 1). The grouping according to age when 95% of the oxidized groundwater is discharged to the stream monitoring station at the catchment outlet shows relatively large differences in mean age, ranging from 0.5 years to more than 5 years (Table 1)

CONCLUSIONS
Our analysis of nitrogen data from soil water, tile drains, and streams within 46 agricultural mini-catchments in Denmark Soil water) À22 (À53 to 19) À32 (À67 to À2) À67 (À118 to À28) À68 (À100 to À28) À51 (À136 to 72) Tile drainage water À38 (À29 to À51) À51 (À44 to À57) No tiles À59 No tiles Streams À19 À42 À53 À35 À32 showed the importance of scale, pressure of L.U., and groundwater age for measuring trends in nitrogen concentrations and loads over a relatively long monitoring period . Especially the time lag of oxidized groundwater was found to markedly influence the downward trend in nitrogen concentrations, and this knowledge is of high importance for catchment managers and policy makers.  <1 year 9 À52 ( ± 3) a 1-3 years 22 À44 ( ± 2) b >3 years 15 À38 ( ± 4) c