Diffuse nutrient losses in Denmark

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Wat SCI Tech. Vol. 33, No. 4-5, pp. 81-88, 1996. Copyright «:> 1996 IA WQ. Pubhshed by Elsevier SCience Ltd. Pnnled In Great Britain. All nghts reserved.

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DIFFUSE NUTRIENT LOSSES IN DENMARK B. Kronvang, P. GrresbliSll, S. E. Larsen, L. M Svendsen and H. E. Andersen National Environmental Research Institute, Department of Freshwater Ecology, Vejls¢vej 25, DK-8600. Silkeborg, Denmark

ABSTRACf Since 1989, nutrient loading of the Danish aquatic environment has been monitored in 270 Danish streams draining catchment areas differing in climate, physico-geographic and land usage. Dtffuse nutrient loading from non-point sources (mainly agricultural) is now the main cause of eutrophication of the Danish aquatic environment; thus in 1993, diffuse sources accounled for 94% of riverine nitrogen loading and 52% of riverine phosphorus loading. Annual riverine total nitrogen (total-N) loading from diffuse sources during the period 1989-93 was on average \0 times greater in 66 small agricultural catchments (median 23.4 kg N ha- I) than in 9 natural catchments (median 2.2 kg N ha- I ). Correspondingly, annual riverine total phosphorus (total-P) loadmg from diffuse sources was on average 3.5 times greater in the agricultural catchments (0.29 kg P ha- I ) than in the natural catchments (0.07 kg P ha- I). The annual total-N and total-P load was found to increase with the proportion of agricultural land in the catchmenls. In 1993, intensive measurements of phosphorus load in 8 agricultural catchments showed that normal point sampling (fortmghtly) underestimates annual total-P loading by a median of 37% as compared to that estimated by frequent sampling. Moreover, estimates of monthly total-P loading are even more biased, especially in late summer and early autumn (50% to -65%). Copyright © 1996 IAWQ. Published by Elsevier Science Ltd.

KEYWORDS Agriculture; climate; diffuse pollution; monitoring; nutrients; retention; soils. INTRODUCfION Diffuse nutrient loading of the Danish aquatic environment from non-point sources (mainly agriculture) has attracted increasing attention over the last decade (e.g., Kronvang et aI., 1993), the reason being that despite the successful abatement of point-source pollution, many surface waters remain eutrophic (Danish Environmental Protection Agency, 1994). Thus, as is the case in many other European countries, diffuse nutrient loading is now one of the main threats to the ecological condition of the aquatic environment (Kristensen and Hansen, 1994), and can be expected to remain so in the decade to come. Agricultural chemical fertilizer consumption in Denmark amounted to 144 kg N ha -I and 15 kg P ha -I in the year 1989/90 (Danmarks Statistik, 1990), in case of nitrogen placing Denmark among the five EU countries with the highest consumption (Christensen et al., 1994). Moreover, the number of livestock units per hectare Danish agricultural land (which indicates the level of animal manure production) was 0.9 in 1989, thus placing Denmark among the five EU countries with the greatest livestock density (Christensen et aI., 1994). 81

82

B. KRONV ANG et al.

Diffuse nutrient loading of the Danish aquatic environment is therefore likely to be a major problem, and needs to be adequately monitored. The endeavour to reduce diffuse nutrient loading of the Danish aquatic environment started with the Danish Government's 1987 Action Plan on the Aquatic Environment. The main goal of the Action Plan with respect to agriculture was a 50% reduction in nitrogen loading of the aquatic environment within a five year period. This reduction target was reiterated in 1991 Action Plan on Sustainable Agricultural Development, although the time frame was extended until the end of the century. In order to ensure fulfilment of the reduction target, several binding requirements were imposed on the agricultural sector concerning improved storage and utilization of animal manure, the preparation of fertilization schedules and budgets, improved fertilizer application practices, increased green cover in autumn and winter, and various structural measures (Danish Environmental Protection Agency, 1994).

In 1989, a harmonized Nationwide Monitoring Programme was established in Denmark to provide information on both diffuse and point-source nutrient loading of the aquatic environment. This paper examines diffuse nutrient loading in Denmark, covering the findings of the Nationwide Monitoring Programme, as well as recent attempts to more precisely monitor diffuse phosphorus loading. The impact on diffuse nutrient loading of factors such as climate, soil type and land usage will also be considered. SOIL TYPE, CLIMATE AND LAND USAGE IN DENMARK Denmark has a total land area of 43,092 km 2 and is mainly covered by Pleistocene fluvioglacial sediment. The relief is low, and the country can be divided in two major regions according to soil type; the sandy soil areas of western Denmark (Jutland) and the loamy soil areas of eastern Denmark (Funen and Zealand). Land usage is similar throughout Denmark, 65% of the total area being intensively farmed and 12% being forest. Annual mean temperature, precipitation and runoff over the period 1989-93 averaged 8.7 oC, 702 mm and 299 mm, respectively (Table 1). Table 1. Annual mean temperature, precipitation and runoff in Denmark over the period 1989-93 Temperature (OC)

Precipitation (mm)

Runoff (mm)

1989

9.2

581

252

1990

9.3

812

327

1991

8.2

654

296

1992

9.0

706

294

1993

7.6

758

325

1989-93

8.7

702

299

Period

THE DANISH NATIONWIDE MONITORING PROGRAMME In 1989, a nationwide network of 130 stream monitoring stations was established to measure the riverine nutrient loading of Danish coastal areas (Danish Enviromental Protection Agency, 1993). The network covers about 60% of the Danish land mass. At the same time, monitoring was initiated of point-source nutrient discharges from sewage treatment plants, industrial plants, fISh farms, etc., the number of annual samples ranging from 4-26, the sampling rate being greatest at the large plants.

Diffuse nutrient losses in Denmark

83

Concomitantly, the level and trend in diffuse (non-point source) nutrient loading of the aquatic environment has been monitored in 9 small (average: 5.5 km 2) natural catchment areas and 66 small (average: 9.8 km 2) agricultural catchment areas devoid of point-source nutrient discharges. Monitoring of nutrient loading from about 40 small agricultural catchments with minor point-source nitrogen discharge « 0.5 kg N ha- I catchment area) is also undertaken. The nine natural catchment areas are mainly comprised of forest (average 93%) while the 66 agricultural catchment areas are comprised of a mixture of arable land (average 84%), forest (average 15%), rural areas (average 0.2%) and freshwater areas (average 0.2%). In six of the 66 agricultural catchments, it is possible to obtain a detailed nitrogen budget based on questionnaire surveys of agricultural practices at field level, measurements of nitrogen in soil water and the use of a leaching model to calculate nitrogen leaching from the root zone (Kronvang et al., 1995). Each stream monitoring station is outfitted with stage-discharge equipment to enable continuous determination of discharge. In addition, water samples are collected at either monthly or fortnightly intervals, the sampling frequency depending on the hydrological regime at each particular station and being highest in streams subject to marked seasonal variations in runoff (Kronvang and Bruhn, 1990). The water samples are analysed for several nutrient fractions (total nitrogen (total-N), nitrate-N, ammonium• N, total phosphorus (total-P) and dissolved reactive P) as previously described (Kronvang et al., 1993).

In 1993, thirteen of the stream monitoring stations in agricultural catchments were equipped with thermostatically heated (5°C) automatic samplers programmed to collect hourly water samples and pool batches of 8 individual samples in 1 litre polyethylene bottles. The automatic samples were emptied weekly and the 21 bottles representing each week of hourly sampling pooled to give a single weekly sample for analysis of phosphorus fractions. Complete annual data sets were successfully obtained from 8 of the stations. Daily nutrient concentration was calculated by linear interpolation between each water sampling date. Nutrient loading was then calculated by summing the product of daily discharge and daily nutrient concentration over the period in question. Evaluation of the accuracy of fortnightly sampling revealed the estimates of annual total-N loading to be accurate to within 5% (Kronvang and Bruhn, 1990). For the present pooled weekly samples, nutrient loading was calculated by summing the product of average weekly concentration and weekly runoff. RESULTS Source apportionment of nutrient loadin~ in Denmark Source apportionment of nutrient loading of Danish watercourses and lakes has been undertaken annually since the Nationwide MonitOring Programme was initiated in 1988. It is necessary to include the ongoing retention of nitrogen and phosphorus in freshwater (e.g. denitrification and sedimentation in lakes) in such calculations. Danish watercourses are relatively small and annual N retention is less than 5% of total annual loading. Total-N and total-p retention in Danish lakes is estimated annually on the basis of mass balances for 25 representative lakes. During the period 1989-93, annual retention varied from 12,000-19,000 tonnes in the case of total-N and 60-220 tonnes P in the case of total-P (Table 2). Annual nutrient retention in Danish lakes during that period thus amounted to 13-17% of the gross riverine loading for nitrogen and 2-7% for phosphorus (Table 2). Retention of both nitrogen and phosphorus was highest in the dry year of 1989. Source apportionment of nutrient loading of Danish rivers and lakes over the period 1989-93 is illustrated in Fig. 1. Of total inputs to freshwater, diffuse sources accounted for 87% of total-N and 31 % of total-P in 1989, as compared with 94% of total-N and 52% of total-P in 1993. The apparent increase in diffuse nutrient loading over the period 1989-93 is to some extent attributable to the reduction in point-source discharges.

84

B. KRONV ANG el al.

Table 2. Annual retention of nitrogen and phosphorus in Danish lakes expressed in absolute terms and as a percentage of annual gross riverine loading Retention (tonnes)

Period

Retention

(% gross riverine loading)

Total N

Total P

Total N

Total P

1989

13,000

220

17%

7.1%

1990

19,000

60

16%

1.7%

1991

12,000

170

13%

6.8%

1992

13,200

120

13%

5.8%

1993

16,800

140

15%

6.4%

A

B

100% 80% 60% 40% ~ Agricultural areas

20% 0%

D

Natural areas

•

Point sources

~ Scattered dwellings

1989

1990

1991

1992

1993

1989

1990

1991

1992 1993

Figure I. Source apportionment of annual total-N (A) and total-P (B) loading to the Danish freshwater environment during the period 1989-93. The figure above each bar indicates the total load in tonnes.

Diffuse nitro~en IQadin~ from representative natural and

a~ricu!tura1

catchments

Annual median total-N concentration in the streams draining the 9 natural catchments was 1.2 mg N 1-1 in 1993. Inorganic N fractions (nitrate, nitrite and ammonium) comprised 50% of total-N, the annual median concentration being 0.63 mg N 1-1. In comparison, annual median total-N concentration in the streams draining the 66 agricultural catchments was 6.5 mg N 1-1 in 1993, with inorganic N accounting for 92% of the total-N (6.0 mg N 1-1). Annual total-N loading from the agricultural catchments was much higher than that from the natural catch• ments (Fig. 2). The observed variation in annual total-N loss (especially with the agricultural catchments) was attributable to annual variation in runoff, the latter being highest in 1990 and lowest in 1989 (Table 1). Besides climate, soil type and land usage also had considerable impact on total-N loading (Fig. 3), the latter increasing with increasing proportion of agricultural land. In 1993, for example, total-N loss ranged from 2.2 kg N ha- I in catchments with less than 20% arable land to 23.4 kg N ha- I in catchments with more than 80% arable land.

85

Diffuse nutrient losses in Denmark

50 til

~

Z

40

CI

0~ .2 c: II>

~ :t:: c:

~

30

A

~ 75% 95%

B

Median 25% 5%

20 10 0

i::ii

::E

Eli

89

90

91

~

92

~

93

90

89

91

92

93

Figure 2. Annual total-N loss from 9 natural catchments (A) and from 66 agricultural catchments (B) during the period 1989-93.

Besides climatic conditions, land usage also had considerable impact on annual total-p loading (Fig. 5), as did the discharge of phosphorus from scattered dwellings in the catchments. However, it was not possible to establish a significant relationship between the number of inhabitants in the individual catchments and the level of diffuse P loading.

~

50~------------------~~----~-,

95%

z 40

g

~ 30

~

20

""c:

10

68

~n'~lan ~2~~

12

13

~ O+-~~~~~-r~~~~~~~~~ 20-40 40-60 60-80 80-100 Proportion of agriculfuralland (%)

Figure 3. Annual total-N loss as related to proportion of agricultural land within catchments in 1993. The figure above each bar indicates the number of catchments.

In the six of the 66 small agricultural catchments in which the nitrogen budget was detennined, the average

annual input of nitrogen in the fonn of chemical and animal fertilizer during the period 1990-93 was 235 kg N ha- I , 134 kg N ha- l of which was removed in the harvested crop. During the same period, average annual nitrogen leaching from the root zone was 106 kg N ha- I , while nitrogen loading of freshwater was only 21 kg N ha- I. Diffuse phosphorus loadin~ from representative natural and

a~ricultural

catchments

Annual median total-P concentration in the streams draining the 9 natural catchments was 0.044 mg P 1-1 in 1993. Dissolved reactive P comprised 48% of total-P, annual median concentration being 0.021 mg P I-I. In comparison, annual median total-P concentration in the streams draining the 66 agricultural catchments was 0.120 mg P I-I in 1993, with dissolved reactive P accounting for 42% of the total-P (0.050 mg PI-I).

B. KRONV ANG et al.

86

Annual total-P loading from the agricultural catchments was much higher than that from the natural catchments (Fig. 4). The observed variation in annual total-P loading (especially with the agricultural catchments) was attributable to annual variation in runoff, the latter being highest in 1990 and lowest in 1989 (Table I).

~

'" Il.

1.0

.:=

~ r/) r/)

9.

0.8 0.6

A

~ 95% 75%

B

Median 25% 5%

r/)

2 0

.:=

0.

0.4

I/)

~

0.2

~

0.0

0.

~ 89

a

2 91

90

8

fj 89

93

92

90

91

92

93

Figure 4. Annual total-P loss from 9 natural catchments (A) and from 66 agriculturaI catchments (B) during the period 1989-93.

1.0..----------------:-::--. 12 68 ~ 0.8 ~

.:=

C

13

~ 0.6

~ ~

0.4 4

~

0.2

I-~

0.0 +---:--:-:----.--"-,-,---,r---,,..,,--.---:--::-:--,---,---~ 40-60 60-80 80-100

c..

Proportion of agricultural land (%)

Figure 5. Annual total-P loss as related to proportion of agricultural land within catchments in 1993. The figure above each bar indicates the number of catchments.

Comparison of annual total-P loading based on discrete (fortnightly) and continuous (hourly) sampling at 8 monitoring stations in agricultural catchments in 1993 revealed that discrete sampling underestimated total-p loading but overestimated that of dissolved reactive P (Table 3). Underestimation of monthly total-P loading determined on the basis of discrete (fortnightly) sampling was most pronounced in autumn and winter (-50% to -65%).

Diffuse nutrient losses in Denmark

87

Table 3. Bias in annual loading estimates for total phosphorus (total-P) and dissolved reactive phosphorus (dissolved-P). Bias is estimated from the calculated loading based on either discrete sampling (T diS> or intensive (hourly) sampling (Tint): (Tdis - Tint) / Tdis' N

Mean

Median

1st Quartile

3rd Quartile

Total-P

8

-70%

-37%

-55%

-27%

Dissolved-P

8

6%

9%

-3%

16%

DISCUSSION AND CONCLUSION The importance of diffuse nutrient loading in large catchments and on a national scale is often underestimated due to ongoing nutrient retention in the aquatic environment. Thus in determining source apportionment of nutrient loading in Denmark, diffuse loading is estimated as the difference between measured riverine nutrient loading and the sum of the nutrient discharge from all point sources upstream of the monitoring station(s), plus the estimated retention in the freshwater environment. If retention is not included when determining source apportionment, the results could be erroneous, especially as regards diffuse loading. Diffuse sources account for a considerable part of riverine nitrogen loading in Denmark (87%), much more than in, for example, Sweden (25%) and Germany (45%) (Kristensen and Hansen, 1994). This is also reflected in the very high total-N concentration and loading in Danish watercourses, the levels being among the highest documented in Europe (Kristensen and Hansen, 1994; Kronvang et aI., 1995). With total-P, in contrast, diffuse riverine loading is supposedly considerably less (0.2-0.4 kg P ha- l) than that reported for agricultural areas of, for example, Finland (0.9-1.8 kg P ha- I ) and Norway (0.7-1.4 kg P ha• l) (Ulen et al., 1991), this being explicable by the lower levels of soil erosion and delivery of P via surface runoff in Denmark. However, the values for total-P loading in Denmark are based on normal discrete sampling (fortnightly), and as the present intensive measurement study shows, discrete sampling under• estimates diffuse P loading. The true value for diffuse P loading is therefore greater than hitherto believed. It will be necessary to undertake further intensive measurements of riverine loading under different climatic conditions before the true level of diffuse P loading can be determined for Denmark. The influence of agriculture on diffuse nutrient loading has been documented for many countries by establishing empirical relationships between riverine nutrient loading and the percentage of agricultural land in the catchment (e.g. Rekolainen, 1989). Based on results from 101 stream monitoring stations in 1993, it has been shown that diffuse nitrogen and phosphorus loading in Denmark also increases in proportion to the percentage of agricultural land in the catchment. The nitrogen budget study showed, however, that of the 106 kg N ha- I that leached from the root zone of the agricultural catchments, only 20% (21 kg N ha- l), entered the watercourses. The measures adopted in Denmark aimed at reducing diffuse nitrogen loading of the aquatic environment have not yet had any major impact. New strategies and measures are therefore under consideration, including a levy on chemical fertilizers and the re-establishment of wetlands (lakes, wet meadows) in catchments to restore their natural capacity for nitrogen removal and phosphorus retention. Measures of the latter kind necessitate systems analysis at the regional level. This requires a more detailed knowledge of the level of diffuse nutrient pollution and the effect of important catchment parameters such as soil type, climate and land usage.

88

B. KRONV ANG tl aL

ACKNOWLEDGEMENTS The funding for this study was partly provided by the Danish Environmental Research Programme, Centre for Root Zone Processes and the Centre for Freshwater Environmental Research. We gratefully acknowledge the Danish county authorities whose responsibility it is to run the stream monitoring stations for the Danish Nationwide Monitoring Programme. REFERENCES Christensen, N., Paaby, H. and 1. Holten-Andersen (1994). Environment and Society - a review of environmental development in Denmark, National Environmental Research Institute, Technical Reporll08, 164 pp. Danish Environmental Protection Agency (1993). Aquatic Environment Nationwide Monitoring Programme 1993-1997. Rtdeg_relse Nr. 3, 171 p. Danish Environmental Protection Agency (1994). Aquatic Environment 1994. Redeg_relse Nr. 2, 137 p. Danmarks Statistik (1990). Landbrugsslalisli/( 1990, 256 pp. Kristensen, P. and H.O. Hansen (1994). European Rivers and Lakes - Assessment of their Environmental State. European Environment Agency, EEA Environmental Monographs I, 122 pp. Kronvang, B. and A. I. Bruhn (1990). Stoftransport i vandl~b. Beregningsmetodlk og pr~vetagningsfrekvens. Danmarks Milj_unders_gelser, 62 pp. (In Danish). Kronvang, B., Alrtebjerg, G., Grant, R., Kristensen, P., Hovmand, M. and I. Kirkegaard (1993). Nationwide Monitoring of Nutrients and Their Ecological Effects: State of the Danish Aquatic Environment AMBIO, 22, 176-187. Kronvang, B., Grant, R., Larsen, S. E., Svendsen, L. M. and P. Kristensen (1995). Non point-source nutrient losses to the aquatic environemt in Denmark: Impact of agriculture. Marine and Freshwater Research. 46, (In print). Rekolainen, S. (1989). Phosphorus and nitrogen load from forest and agriCUltural areas in Fmland. Aqua Fennica, 19,95-107. U\~n, B., Kronvang, B. and L. M. Svendsen (1991). Loss of phosphorus from Woodland, natural land and agricultural land. In: Phosphorus in the Nordic Countries. Svendsen, L. M. and B. Kronvang (Eds.), NORD 1991: 47, 83-100.