- Email: [email protected]
Nutrient emissions from agriculture in the Netherlands, causes and remedies
e
Waf. Sci. Tech. Vol. 33. No. 4-5. pp. 183-189. 1996. Copynght C 1996 IAWQ. Published by Elsevier Science Ltd. Printed in Great BritalO. All rights reserved.
Pergamon
PU: S0273-1223(96)OO229-6
0273-1223/96 $15'00 + 0'00
NUTRIENT EMISSIONS FROM AGRICULTURE IN THE NETHERLANDS, CAUSES AND REMEDIES Paul C. M. Boers Institute/or Inland Water Management and Waste Water Treatment. P. O. Box J7. 8200 AA Lelystad. The Netherlands
ABSTRACT Agriculture causes 60% of the total nitrogen emissions and 40-50% of the total P emissions to the surface waters in the Netherlands. These high emissions are mainly caused by the large amounts of fenilizers used on Dutch farms. On average. 39 kg P ha· t and 340 kg N ha· 1 are given in excess to the uptake by the crop. The Netherlands follows a strategy of gradual reduction of the nutrient applications to crops. The firsts two phases were aimedat stabilisation and a gradual reduction of the use of organic manure. The goal of the third phase is to accomplish equilibrium fertilization in the year 2000. This means that the amount of fertilizer given may not exceed the crop uptake. considering an acceptable loss to the environment. The losses are based upon acceptable emissions. mainly to surface waters. With the average excess rainfall in the Netherlands. the water quality standards are met with a loss of 0.4 kg P ha· 1 and 6 kg N ha· l • much lower than the present excesses of minerals. The challenge for the future is to close the gap between environmentally acceptable and present losses. Equilibrium fenilization will not solve all problems at short notice. Therefore. additional techniques to reduce nutrient losses are in development. Examples are restoration of buffer strips and wetlands. lowering of groundwater levels. dosing of ferric and aluminum compounds in the soils and several additional measures in eutrophied lakes. Copyright © 1996 IA WQ. Published by Elsevier Science Ltd.
KEYWORDS Agriculture; diffuse pollution; eutrophication; policy making. INTRODUCTION Increasingly stagnant fresh waters and coastal waters around the world are suffering from enhanced nutrient input and the subsequent deterioration of water qUality. The sources of nutrients are generally distinguished in point sources and non-point or diffuse sources. The principal origins of nutrients on a global scale are the fertilizers. A variable fraction of these is converted into food and finally into human excreta and manure. Effluents from municipal treatments, untreated sewage discharged through a sewerage system and industrial effluents are the dominant point sources. Leaching from agricultural land and non-sewered urban areas are the most important diffuse sources. The Netherlands has solved most of its problems with point sources of nutrients. Non sewered urban areas are rare; almost all municipal sewage water is treated. Agreements with the fertilizer industries, the most important dischargers of industrial effluents containing phosphate, and the food industries, the most 183
P. C. M. BOERS
184
important dischargers of industrial effluents containing nitrogen compounds, have been made. Furthennore, techniques to enhance phosphorus removal and denitrification at the sewage treatment plants are being implemented. These measures together resulted in a reduction of the phosphate emissions to Dutch surface waters from point sources by over 50% between 1995 and 1993. In the same time, the average total P concentration in the River Rine decreased by 5~0%. The Rhine is an important source of nutrients for many Dutch surface waters. According to nationwide monitoring programmes, the total phosphorus concentrations decreased by about 30% during that same period; the total nitrogen concentrations remained essentially the same. However, the nutrient concentrations in the Dutch surface waters remain high. The national water quality standards for phosphorus (0.15 mg P I-I and 2.2 mg N 1-1) were still exceeded on about 75% of the 200 monitoring locations in 1993. At the same time, it has become increasingly clear that diffuse sources, notably the leaching from agricultural lands, are probably even more important than the point sources. Modelling exercises (Uunk, 1991) on national and regional scale and regional mass balance studies (Table 1) revealed that for nitrogen diffuse sources more important than point sources both at a national and a regional scale. Diffuse sources are also the most important sources for phosphate in several of the regions mentioned in Table I. Table 1. Some results of nutrient balance studies. total emission (tons y-')
area Netherlands lake WolderwijdlNuldemauw N-W of Holland Provo Friesland Rivers Mark and Dintel River Eem province South-Holland
agriculture (%)
P
N
P
N
25.000 35 680 1017 220 496
175.000 410
2S >SO 21 67 62 3S 46
69 48
1260
no data
11700 no data
3100 7900
no data
68 no data
63 40
CAUSES OF DIFFUSE EMISSIONS OF NUTRIENTS The principal cause of leaching of nutrients from agricultural areas is a dosing of fertilizers and dung in excess of the capacity of the crop to use them for growth. Possible transport routes are surface runoff, subsurface runoff and groundwater flow. The transport routes differ greatly in transport times. Usually, there is a certain capacity in the watershed to modify and to reduce the export of nutrients. There may be some irreversible fixation of phosphate on the soil matrix; in the presence of organic matter nitrogen can be removed by denitrification; there is some accumulation of organic nitrogen and organic phosphate compounds in the soil. However, the capacity of these processes to retain nutrients is not infinite. The sorption processes lead to a further reduction in transport rate. The long transport times and the sorption processes together lead to a delayed response to reductions of fertilizer applications. Notably, the strong adsorption of phosphate has become a chemical time-bomb from the point of view of eutrophication control (Reijerink and Breeuwsma, 1992). Insight in these processes is important for the general planning of nutrient control. Dosings of fertilizers in the Netherlands are much higher than the crop uptake and are among the highest of the world (Table 2). The most important reason is the superfluous availability of manure, caused by the high numbers of cattle. 100,000,000 chicken, 15,000,000 pigs and 6,000,000 cows together produce about ten times as many excreta as the 15,000,000 people in the Netherlands. Most of the dun~ ori~inates from fodder
Nutrient emissions from agriculture in The Netherlands
18S
produced outside the Netherlands. This again causes an accumulation of nutrients in the Netherlands and a depletion in the fodder exporting countries (Table 2). Table 2. Surplus of phosphale and nitrogen per hectare of agricultural land in several countries in the year 1990 Country
P surplus tkg P ha·')
N surplus (kg N ha·')
Belgium Denmark
37 20 12 39 10 -I
208 160 129 340 87 -2
-2
-9
Great-Britain Netherlands Norway Angola Tanzania
GENERAL POUCY The problems with emissions of nutrients from agriculture became clear in the lale seventies. A policy decision to reduce the nutrient emissions from agricultural grounds followed in 1985. The most important targets to be achieved with general measures are: I) to reduce the emissions of nutrients to surface walers with 50% and on the long lerm with 7075%. as agreed in by the North Sea Minisler Conferences; 2) to meet the European quality standards for nitrale in ground waler (50 mg N031-1); 3) to meet the Dutch waler quality standards for nutrients in surface walers of 0.15 mg total P I-I and 2.2 mg total N I-I. A gradual solution of the problem was chosen. The fIrst phase. from 1987 to 1990. was aimed at stabilisation of the problems and at exploring possible solutions. The growth of the production of manure was stopped and the use of manure was set at such a level that all manure produced could be utilised on the fIelds. The second phase. from 1991 to 1994. was aimed at a gradual reduction of the problems. This was done by a gradual sharpening of the limits for permitled amount.. of manure used. At the same time factories for processing the excess manure were built. The third phase. from 1995 to 2000. is aimed at a further sharpening of the limits and to fInally reach "equilibrium fertilization" (Ministry of Agriculture. Nature Conservation and Fisheries. 1993). Equilibrium fertilization means that nutrients are supplied in an amount. equal to the needs of the crop (Sharpley et al.• 1994). This is a different approach than setting limits to the application of dung and fertilizers. However. several losses seem unavoidable. Examples are denitrification. irreversible sorption of phosphale and some leaching. Therefore eqUilibrium fertilization is defIned as supply of nutrients equal to the uptake by crop. considering acceptable losses. This implies that the acceptable losses have to be defIned. In two desk studies the actual. the environmentally acceptable, the present losses and the inevitable losses at good agricultural practice of both phosphale and nitrogen were identifIed (Oenema and Van Dijk. 1994; Van Eck. 1994). In a third study the economical concequences of several scenarios for the agricultural sector were explored (Sectie Agrarisch Management, 1995). For phosphale the environmentally acceptable loss was estimated from the average excess precipitation in the Netherlands (300 mm y-I) and the water Dutch quality standard set for phosphate in surface waters (0.15 mg P I-I). resulting in an acceptable loss of 0.45 kg P ha- l y-l. From model calculations it was concluded that the travelling time of the "phosphate front" in the soil is very variable. It depends on phosphate sorption capacity (mainly determined by the concentrations of amorphous aluminum and ferric hydroxides in the soil). by the fertilisation history of the soil and by the groundwater table. Furthermore it was concluded that. althou~h there is substantial sorption of phosphate in most soils, this process is a fInite sink for phosphate.
P. C. M. BOERS
186
All phosphate given in excess to the demands of the crop ultimately will be transported to groundwater andlor surface water. The present excesses of phosphate fertilization were estimated from experimental fields, demonstration farms and from farming practice data. All data sources indicated that, although there is considerable scatter, the agricultural losses are between 13 and 26 kg P ha-} y-I. Factors like the phosphorus level in the soil, the use of inorganic fertilizers, soil type and tilling practices determine the losses. The approach for nitrogen was somewhat different. Differences in soil characteristics and groundwater table are much more important for nitrogen than for phosphate and had to be taken into account. The groundwater table determines the distribution of the excess rainfall over groundwater and surface water. The quality standards for nitrate in ground water and for total nitrogen in surface water are different (11.3 and 2.2 mg N I-I, respectively), so the acceptable losses for soils discharging to groundwater and surface water are different. Furthermore, groundwater table, organic matter content and other soil characteristics cause a considerable variation in denitrification capacity of the soils. Denitrification is a often a major loss process of nitrate, but without negative environmental consequences. Considering these factors, environmentally acceptable nitrogen losses ranging from 10-150 kg N ha- I y-I were calculated. The actual nitrogen excesses were calculated in the same way as for phosphate and range from 50-390 kg N ha- I y-I. The study to the economic consequences indicated that the costs for the intensive livestock-farming are the dominant factor. These farms have no or only a limited amount of ground and they will have to find way to remove the dung produced by the animals. At high acceptable losses, the arable farming sector can take up most of the dung, be it that the costs of transport and application are higher than the costs of purchase of artificial fertilisers. At lower losses, this is no longer possible and the manure must be processed in manure processing factories. The costs of processing are about two times as high as for use in the arable farming sector These costs and the estimated decrease in crop yields together cause a sharp increase in the costs of equilibrium fertilization at surpluses of 200-300 kg N ha- t y-t and 10-15 kg P ha- I y-t. A summary of the costs is provided in Table 3. Table 3. Estimated costs of various scenarios of acceptable losses of nutrients in the year 2000, expressed as percentage of the proceeds of labour in 1990. Agricultural sector
scenario I: 17 kg P/ha; 350 kg N/ha
scenario 2: 13 kg P/ha; 250 kg N/ha
scenario 3: 9 kg P/ha; 150 kg N/ha
scenario 4: 4.5 kg P/ha; tOO kg N/ha
diary farming
+1
-4
-20
-48
pig-breeding
-9
-18
-27
-54
layer-breeding
0
0
-6
-8
arable fanning
+2
0
-IS
-37
The desk studies indicate that there is a huge gap between environmentally acceptable losses and actual losses, be it that the uncertainties especially in the assessment of the present losses are large. This gap is by far the largest for phosphate. Furthermore, it is clear that at the moment the agricultural sector can not bear the costs of closing the gap between present and environmentally desired losses. this will be a long run. Based upon the results of the desk studies the Dutch government recently decides to set the standards for acceptable losses at 9 kg P ha- t and 180 kg N ha- t , to be reached in the year 2010. Research is being initiated to find ways for reduce the unavoidable losses and to close the gap between unavoidable and acceptable losses. Some of the possibilities are:
Nutrient emissions from agriculture in The Netherlands
187
- In some areas it may not be possible to meet the quality standards of nutrients in surface waters, because of a high "natural" loading, e.g. from nutrient rich brackish seepage or mineralisation of nutrient rich peat. In these cases higher losses from agriculture might be acceptable. - The present advises for fertilizer dosing are intended to assure maximal crop yields. Especially for phosphorus considerably lower dosing will have only minor effects on the yields for most products. So, possibly the advices can be reconsidered. - There are several possibilities to improve the uptake of nutrients by the crops. for example by row fertilisation, additions in agreement to the actual need of the crop and taking into account the stock of nutrients present in the soil. - The use of green fields or winter crops to improve the utilisation of nutrients that are still available at the end of the normal growing season. - Increasing the nutrient assimilation efficiency of animals decreases the amounts of nutrients in animal excreta and thereby the costs of producing it. Some possibilities. like addition of phosphatase are already explored. - To reduce the animal densities in dairy fanns. Simultaneously a discussion on methods to implement the principle of equilibrium fertilization was started. Mineral bookkeeping is now as the best tool available. Mineral bookkeeping means that every fanner has to keep a record of the amounts of both nitrogen and phosphate entering and leaving his fann. For fodder. inorganic fertilizer and to a lesser extent manure and milk this can be done with chemical analysis of the materials. For crop, cattle and meat this is not practicable. Standardized values have to be used for these products. A serious advantage of mineral bookkeeping is that the fanners become more aware of losses of nutrients to the environment and their own role in it. There are two serious disadvantages to mineral bookkeeping. One is that it is very difficult to trace the probably low acceptable losses of phosphate. The second is that the system is probably very sensitive for fraud. EXPLORATION OF EFFECTS OF EQUILIBRIUM FERTILIZATION In a pilot study the possible environmental effects of eqUilibrium fertilization on nitrogen emissions to surface waters were explored (Boers and De Vries, 1994). Two discriminating variants were analyzed in detail; the first variant consisting of the lowest fertilization rates suggested in the Dutch "third-phase fertilizer policyplan". i.e. a phosphate surplus of 5 kg P205 per hectare and effective nitrogen application rate of 150 kg N per hectare for grassland on high sandy ground. and effective nitrogen application rate of 250 kg N for grasslands on other sandy soil.This compares to an average nitrogen surplus of bout 200 kg N ha· l . In addition. a considerable reduction in livestock was assumed, namely a 50% reduction in the number of dairy cows and a 25% reduction in the number of pigs for slaughter, both in the year 2000 as compared with 1985. Furthermore. only a restricted transport of manure was assumed.
According to the calculations. the nitrogen application rate on the soil falls by over 40% in variant 1. This results in a reduction of nitrogen loads into surface water with 50%. This reduction will have been almost achieved - for approximately 90% - by the year 2000. The conclusion from this pilot study was that in this scenario an emission reduction of 50% (goal for 1995) will be met, but not the final goal of 70-75% reduction. A relatively high part of the emissions is from average moist to wet grassland on sandy soil. ADDITIONAL MEASURES It is recognized that a general policy aimed at a slow decrease will not solve all problems with diffuse pollution of nutrients timely. Therefore, several additional measures at the regional or local scale are under consideration. These measures are aimed at a) a further reduction of the emissions than the goals of the general policy; b) a sooner reduction of the emissions and c) to alleviate the water quality problems in the receiving waters, especially lakes.
188
P. C. M. BOERS
The measures to reach a greater and/or sooner reduction of emissions are usually taken in the watershed. Most of these are still under research. A very simple measure is to stop fertilizer and dung dosing along the watershed. This avoids accidental additions directly in the watershed and reduces some runoff. Probably the most promising is the restoration or creation of buffer strips or riparian ecotones along the water courses (Peterjohn and Correll, 1984; Vought et al., 1994). Such areas are known to have a high nitrogen removal capacity by denitrification and uptake by macrophytes, followed by harvesting, and by storage in the sediments. Probably, buffer strips are most efficient for nitrogen removal, because of the denitrification. Wetlands, possibly in the form of wet buffer strips along the watercourses, have the same potentials. Usually the denitrification capacity is higher than that of dry buffer strips. On the other hand, the potential to intercept surface runoff may be greater for dry strips. The use of buffer strips still has many problems. Drainage systems are commonly used in the Netherlands. Probably, they interfere with the proper functioning of buffer strips. Some studies indicate that buffer systems become less efficient with time (Vought et al., 1994). Uncertain is to what extent such problems can be solved with a proper management of the strips. This is still a matter of research. Other important problems are that the demands for an optimal removal of nitrogen differ from that of an optimal removal of phosphate. Furthermore, the amount of space required for buffer strips is rather high, especially in those parts of the Netherlands with a high density of ditches. Space is costly. Table 4. Estimates of costs of eutrophication control measures in lakes. It is assumed that the external loading was reduced with 3 g P m-2 y-1. (From Boers and Van Der Molen, 1993). Type of measure
costs (ECU/kg Pl
costs (ECU/ha)
P removal at treatment plant to I mg P I" in effluent
4
130 y"'
P removal at treatment plant to 0.2 mg P I·' in effluent
75
2,200 y.'
P removal from inlet water of lake
220 - 300
6.500 - 9,000
y-'
costs depend of inlet water quality and annual variation in flow rate
sedimentation basin in inlet water of lake
220 - 450
6,500 \3,500
costs depend of inlet water quality and annual variation in flow rate
P removal using AI-salts in deep lake
400
average of 9 cases in the USA around 1980
P removal using Fe-salts in shallow lake
6,000
pilot experiment in 1989
dredging
14.000 40.000
average of \ 0 cases
biomanipulation
900
shallow Lake Wolderwijd, The Netherlands
hypolimnetic aeration
400
experiments around 1980
artificial circulation
250
example from only one case
Remarks
hypolimnetic "ithdrawal
very cheap
flushing
very variable
Because the soils are saturated with phosphate in a large area of the Netherlands (Reijerink and Breeuwsma, 1992) and because of the chemical time bomb character of this phenomenon several measures specifically aimed at reducing the leaching of phosphate are under study. Examples are: - addition of ferric com pounds in the soil near the watershed to increase the sorption capacity; - lowering of the groundwater table to lengthen the travelling time and to create new sorption capacity:
Nutrient emissions from agriculture in The Netherlands
189
- to keep the nutrients in the agricultural areas by reducing the discharge of local water excess to the receiving waters; - use and if necessary develop crop with a high phosphate uptake capacity, combined with an adequate dosing of nitrogen fertilizer. In this way, the phosphate is extracted from the soil. Another type of measure is aimed at alleviating the water quality problems in the receiving lakes. Typical problems are massive growth of algae or macrophytes, oxygen depletion, fish kills, loss of water depth by sedimentation. Most of the measures are end-of-the- pipe techniques. They relieve the eutrophication symptoms, but they do not solve the structural problem, namely an overloading with nutrients. Table 4 summarizes rough estimates of costs of several control measures. CONCLUSIONS Diffuse pollution from agriculture is an important source of both nitrogen and phosphorus for the Dutch surface waters. This pollution is caused by the high surpluses for both elements in agriculture. The Dutch policy is aimed at reduction of the pollution by decreasing the nutrient surpluses in agriculture. According to desk studies, it is at present not possible to decrease these surpluses enough to meet the Dutch quality standards for surface waters. This is too expensive for the agricultural sector. Therefore a path of gradual reduction of the nutrient surpluses will be taken. This must be combined with additional measures, both in agriculture and in the aquatic systems, to combat eutrophication problems sucessfully. REFERENCES Boers, P. C. M. and Van der Molen D. T. (1993). Control of eutrophication in lakes: the state of the art in Europe. Journ. EWPC, 3, 19-25. Boers, P. C. M. and De Vries I. (eds) (1994). Evaluatie van de gevolgen van het mestheleid derde fase voor de eutrofi!!ring van de Noordzee. [Evaluation of the effects of the third phase fertilize policyplan on the eutrophication of the North Sea; in Dutcb]. Report 94.041, RIZA, Lelystad, the Netherlands. Lijklema, L. et aL, (1993). Non-point nutrient sources and their control. Mem. 1st. ital. IdrobioL 52,207-221. Ministry of Agriculture, Nature Conservation and Fisheries, 1993. Mest- en ammomakbeleid derde fase [third-pbase fertilizer policyplan; in Dutch]. Oenema, O. and Van Dijk T. A. (eds) (1994). Fosfaatverliezen en fosfaatoverscbotten in de Nederlandse landbouw [Pbosphate losses and phosphate excesses in Dutcb agriculture; in Dutcb]. Report ministries of Agricullure, Public Health & Environment, Transport, Public Works & Water Management. Peleljohn. W. T. and CorreU D. L. (1984). Nutrient dynamics in an agricultural watershed: observations on the role of a riparian forest. Ecology. 65, 1466-1475. Reijerink, J. G. A. and Breeuwsma A. (1992). Ruimtelijk beeld van de fosfaatverzadiging in mestoverschotgebieden [Phosphate saturation of soils in manure excess areas; in Dutch). Report 222. Staring Centre. Wageningen. the Netherlands. Sectie Agrariscb Management (1995). Verkenning van de sociaal-economische gevolgen van diverse rekenvarianten voor fosfaaten stikstofverliesnormen [Exploration of the socio-economic consequences of several scenarios of acceptable losses of phosphate and nitrogen; in Dutch]. Report ministries of Agriculture. Public Heallh & Environment, Transport. Public Works & Water Management. Sharpley, A. N. et aI., (1994). Managing agriCUltural phosphorus for protection of surface waters: issues and options. J. Environ. Qual. 23,437-451. Uunk. E. J. B. (1991). Eutrophication of surface waters and the contribution of agriculture. The Fertilizer Society, London. Van Eck, G. (ed) (1994). Stikstofverliezen en stikstofverschotten in de Nederlandse Iandbouw [Nitrogen losses and nitrogen excesses in Dutch agriCUlture; in Dutch]. Report Ministries of Agriculture. Public Health & Environment, Transport, Public Works & Water Management. Vought, L. B.-M. et al. (1994). Nutrient retention in riparian ecotones. Ambio. 23, 342-348.
Waf. Sci. Tech. Vol. 33. No. 4-5. pp. 183-189. 1996. Copynght C 1996 IAWQ. Published by Elsevier Science Ltd. Printed in Great BritalO. All rights reserved.
Pergamon
PU: S0273-1223(96)OO229-6
0273-1223/96 $15'00 + 0'00
NUTRIENT EMISSIONS FROM AGRICULTURE IN THE NETHERLANDS, CAUSES AND REMEDIES Paul C. M. Boers Institute/or Inland Water Management and Waste Water Treatment. P. O. Box J7. 8200 AA Lelystad. The Netherlands
ABSTRACT Agriculture causes 60% of the total nitrogen emissions and 40-50% of the total P emissions to the surface waters in the Netherlands. These high emissions are mainly caused by the large amounts of fenilizers used on Dutch farms. On average. 39 kg P ha· t and 340 kg N ha· 1 are given in excess to the uptake by the crop. The Netherlands follows a strategy of gradual reduction of the nutrient applications to crops. The firsts two phases were aimedat stabilisation and a gradual reduction of the use of organic manure. The goal of the third phase is to accomplish equilibrium fertilization in the year 2000. This means that the amount of fertilizer given may not exceed the crop uptake. considering an acceptable loss to the environment. The losses are based upon acceptable emissions. mainly to surface waters. With the average excess rainfall in the Netherlands. the water quality standards are met with a loss of 0.4 kg P ha· 1 and 6 kg N ha· l • much lower than the present excesses of minerals. The challenge for the future is to close the gap between environmentally acceptable and present losses. Equilibrium fenilization will not solve all problems at short notice. Therefore. additional techniques to reduce nutrient losses are in development. Examples are restoration of buffer strips and wetlands. lowering of groundwater levels. dosing of ferric and aluminum compounds in the soils and several additional measures in eutrophied lakes. Copyright © 1996 IA WQ. Published by Elsevier Science Ltd.
KEYWORDS Agriculture; diffuse pollution; eutrophication; policy making. INTRODUCTION Increasingly stagnant fresh waters and coastal waters around the world are suffering from enhanced nutrient input and the subsequent deterioration of water qUality. The sources of nutrients are generally distinguished in point sources and non-point or diffuse sources. The principal origins of nutrients on a global scale are the fertilizers. A variable fraction of these is converted into food and finally into human excreta and manure. Effluents from municipal treatments, untreated sewage discharged through a sewerage system and industrial effluents are the dominant point sources. Leaching from agricultural land and non-sewered urban areas are the most important diffuse sources. The Netherlands has solved most of its problems with point sources of nutrients. Non sewered urban areas are rare; almost all municipal sewage water is treated. Agreements with the fertilizer industries, the most important dischargers of industrial effluents containing phosphate, and the food industries, the most 183
P. C. M. BOERS
184
important dischargers of industrial effluents containing nitrogen compounds, have been made. Furthennore, techniques to enhance phosphorus removal and denitrification at the sewage treatment plants are being implemented. These measures together resulted in a reduction of the phosphate emissions to Dutch surface waters from point sources by over 50% between 1995 and 1993. In the same time, the average total P concentration in the River Rine decreased by 5~0%. The Rhine is an important source of nutrients for many Dutch surface waters. According to nationwide monitoring programmes, the total phosphorus concentrations decreased by about 30% during that same period; the total nitrogen concentrations remained essentially the same. However, the nutrient concentrations in the Dutch surface waters remain high. The national water quality standards for phosphorus (0.15 mg P I-I and 2.2 mg N 1-1) were still exceeded on about 75% of the 200 monitoring locations in 1993. At the same time, it has become increasingly clear that diffuse sources, notably the leaching from agricultural lands, are probably even more important than the point sources. Modelling exercises (Uunk, 1991) on national and regional scale and regional mass balance studies (Table 1) revealed that for nitrogen diffuse sources more important than point sources both at a national and a regional scale. Diffuse sources are also the most important sources for phosphate in several of the regions mentioned in Table I. Table 1. Some results of nutrient balance studies. total emission (tons y-')
area Netherlands lake WolderwijdlNuldemauw N-W of Holland Provo Friesland Rivers Mark and Dintel River Eem province South-Holland
agriculture (%)
P
N
P
N
25.000 35 680 1017 220 496
175.000 410
2S >SO 21 67 62 3S 46
69 48
1260
no data
11700 no data
3100 7900
no data
68 no data
63 40
CAUSES OF DIFFUSE EMISSIONS OF NUTRIENTS The principal cause of leaching of nutrients from agricultural areas is a dosing of fertilizers and dung in excess of the capacity of the crop to use them for growth. Possible transport routes are surface runoff, subsurface runoff and groundwater flow. The transport routes differ greatly in transport times. Usually, there is a certain capacity in the watershed to modify and to reduce the export of nutrients. There may be some irreversible fixation of phosphate on the soil matrix; in the presence of organic matter nitrogen can be removed by denitrification; there is some accumulation of organic nitrogen and organic phosphate compounds in the soil. However, the capacity of these processes to retain nutrients is not infinite. The sorption processes lead to a further reduction in transport rate. The long transport times and the sorption processes together lead to a delayed response to reductions of fertilizer applications. Notably, the strong adsorption of phosphate has become a chemical time-bomb from the point of view of eutrophication control (Reijerink and Breeuwsma, 1992). Insight in these processes is important for the general planning of nutrient control. Dosings of fertilizers in the Netherlands are much higher than the crop uptake and are among the highest of the world (Table 2). The most important reason is the superfluous availability of manure, caused by the high numbers of cattle. 100,000,000 chicken, 15,000,000 pigs and 6,000,000 cows together produce about ten times as many excreta as the 15,000,000 people in the Netherlands. Most of the dun~ ori~inates from fodder
Nutrient emissions from agriculture in The Netherlands
18S
produced outside the Netherlands. This again causes an accumulation of nutrients in the Netherlands and a depletion in the fodder exporting countries (Table 2). Table 2. Surplus of phosphale and nitrogen per hectare of agricultural land in several countries in the year 1990 Country
P surplus tkg P ha·')
N surplus (kg N ha·')
Belgium Denmark
37 20 12 39 10 -I
208 160 129 340 87 -2
-2
-9
Great-Britain Netherlands Norway Angola Tanzania
GENERAL POUCY The problems with emissions of nutrients from agriculture became clear in the lale seventies. A policy decision to reduce the nutrient emissions from agricultural grounds followed in 1985. The most important targets to be achieved with general measures are: I) to reduce the emissions of nutrients to surface walers with 50% and on the long lerm with 7075%. as agreed in by the North Sea Minisler Conferences; 2) to meet the European quality standards for nitrale in ground waler (50 mg N031-1); 3) to meet the Dutch waler quality standards for nutrients in surface walers of 0.15 mg total P I-I and 2.2 mg total N I-I. A gradual solution of the problem was chosen. The fIrst phase. from 1987 to 1990. was aimed at stabilisation of the problems and at exploring possible solutions. The growth of the production of manure was stopped and the use of manure was set at such a level that all manure produced could be utilised on the fIelds. The second phase. from 1991 to 1994. was aimed at a gradual reduction of the problems. This was done by a gradual sharpening of the limits for permitled amount.. of manure used. At the same time factories for processing the excess manure were built. The third phase. from 1995 to 2000. is aimed at a further sharpening of the limits and to fInally reach "equilibrium fertilization" (Ministry of Agriculture. Nature Conservation and Fisheries. 1993). Equilibrium fertilization means that nutrients are supplied in an amount. equal to the needs of the crop (Sharpley et al.• 1994). This is a different approach than setting limits to the application of dung and fertilizers. However. several losses seem unavoidable. Examples are denitrification. irreversible sorption of phosphale and some leaching. Therefore eqUilibrium fertilization is defIned as supply of nutrients equal to the uptake by crop. considering acceptable losses. This implies that the acceptable losses have to be defIned. In two desk studies the actual. the environmentally acceptable, the present losses and the inevitable losses at good agricultural practice of both phosphale and nitrogen were identifIed (Oenema and Van Dijk. 1994; Van Eck. 1994). In a third study the economical concequences of several scenarios for the agricultural sector were explored (Sectie Agrarisch Management, 1995). For phosphale the environmentally acceptable loss was estimated from the average excess precipitation in the Netherlands (300 mm y-I) and the water Dutch quality standard set for phosphate in surface waters (0.15 mg P I-I). resulting in an acceptable loss of 0.45 kg P ha- l y-l. From model calculations it was concluded that the travelling time of the "phosphate front" in the soil is very variable. It depends on phosphate sorption capacity (mainly determined by the concentrations of amorphous aluminum and ferric hydroxides in the soil). by the fertilisation history of the soil and by the groundwater table. Furthermore it was concluded that. althou~h there is substantial sorption of phosphate in most soils, this process is a fInite sink for phosphate.
P. C. M. BOERS
186
All phosphate given in excess to the demands of the crop ultimately will be transported to groundwater andlor surface water. The present excesses of phosphate fertilization were estimated from experimental fields, demonstration farms and from farming practice data. All data sources indicated that, although there is considerable scatter, the agricultural losses are between 13 and 26 kg P ha-} y-I. Factors like the phosphorus level in the soil, the use of inorganic fertilizers, soil type and tilling practices determine the losses. The approach for nitrogen was somewhat different. Differences in soil characteristics and groundwater table are much more important for nitrogen than for phosphate and had to be taken into account. The groundwater table determines the distribution of the excess rainfall over groundwater and surface water. The quality standards for nitrate in ground water and for total nitrogen in surface water are different (11.3 and 2.2 mg N I-I, respectively), so the acceptable losses for soils discharging to groundwater and surface water are different. Furthermore, groundwater table, organic matter content and other soil characteristics cause a considerable variation in denitrification capacity of the soils. Denitrification is a often a major loss process of nitrate, but without negative environmental consequences. Considering these factors, environmentally acceptable nitrogen losses ranging from 10-150 kg N ha- I y-I were calculated. The actual nitrogen excesses were calculated in the same way as for phosphate and range from 50-390 kg N ha- I y-I. The study to the economic consequences indicated that the costs for the intensive livestock-farming are the dominant factor. These farms have no or only a limited amount of ground and they will have to find way to remove the dung produced by the animals. At high acceptable losses, the arable farming sector can take up most of the dung, be it that the costs of transport and application are higher than the costs of purchase of artificial fertilisers. At lower losses, this is no longer possible and the manure must be processed in manure processing factories. The costs of processing are about two times as high as for use in the arable farming sector These costs and the estimated decrease in crop yields together cause a sharp increase in the costs of equilibrium fertilization at surpluses of 200-300 kg N ha- t y-t and 10-15 kg P ha- I y-t. A summary of the costs is provided in Table 3. Table 3. Estimated costs of various scenarios of acceptable losses of nutrients in the year 2000, expressed as percentage of the proceeds of labour in 1990. Agricultural sector
scenario I: 17 kg P/ha; 350 kg N/ha
scenario 2: 13 kg P/ha; 250 kg N/ha
scenario 3: 9 kg P/ha; 150 kg N/ha
scenario 4: 4.5 kg P/ha; tOO kg N/ha
diary farming
+1
-4
-20
-48
pig-breeding
-9
-18
-27
-54
layer-breeding
0
0
-6
-8
arable fanning
+2
0
-IS
-37
The desk studies indicate that there is a huge gap between environmentally acceptable losses and actual losses, be it that the uncertainties especially in the assessment of the present losses are large. This gap is by far the largest for phosphate. Furthermore, it is clear that at the moment the agricultural sector can not bear the costs of closing the gap between present and environmentally desired losses. this will be a long run. Based upon the results of the desk studies the Dutch government recently decides to set the standards for acceptable losses at 9 kg P ha- t and 180 kg N ha- t , to be reached in the year 2010. Research is being initiated to find ways for reduce the unavoidable losses and to close the gap between unavoidable and acceptable losses. Some of the possibilities are:
Nutrient emissions from agriculture in The Netherlands
187
- In some areas it may not be possible to meet the quality standards of nutrients in surface waters, because of a high "natural" loading, e.g. from nutrient rich brackish seepage or mineralisation of nutrient rich peat. In these cases higher losses from agriculture might be acceptable. - The present advises for fertilizer dosing are intended to assure maximal crop yields. Especially for phosphorus considerably lower dosing will have only minor effects on the yields for most products. So, possibly the advices can be reconsidered. - There are several possibilities to improve the uptake of nutrients by the crops. for example by row fertilisation, additions in agreement to the actual need of the crop and taking into account the stock of nutrients present in the soil. - The use of green fields or winter crops to improve the utilisation of nutrients that are still available at the end of the normal growing season. - Increasing the nutrient assimilation efficiency of animals decreases the amounts of nutrients in animal excreta and thereby the costs of producing it. Some possibilities. like addition of phosphatase are already explored. - To reduce the animal densities in dairy fanns. Simultaneously a discussion on methods to implement the principle of equilibrium fertilization was started. Mineral bookkeeping is now as the best tool available. Mineral bookkeeping means that every fanner has to keep a record of the amounts of both nitrogen and phosphate entering and leaving his fann. For fodder. inorganic fertilizer and to a lesser extent manure and milk this can be done with chemical analysis of the materials. For crop, cattle and meat this is not practicable. Standardized values have to be used for these products. A serious advantage of mineral bookkeeping is that the fanners become more aware of losses of nutrients to the environment and their own role in it. There are two serious disadvantages to mineral bookkeeping. One is that it is very difficult to trace the probably low acceptable losses of phosphate. The second is that the system is probably very sensitive for fraud. EXPLORATION OF EFFECTS OF EQUILIBRIUM FERTILIZATION In a pilot study the possible environmental effects of eqUilibrium fertilization on nitrogen emissions to surface waters were explored (Boers and De Vries, 1994). Two discriminating variants were analyzed in detail; the first variant consisting of the lowest fertilization rates suggested in the Dutch "third-phase fertilizer policyplan". i.e. a phosphate surplus of 5 kg P205 per hectare and effective nitrogen application rate of 150 kg N per hectare for grassland on high sandy ground. and effective nitrogen application rate of 250 kg N for grasslands on other sandy soil.This compares to an average nitrogen surplus of bout 200 kg N ha· l . In addition. a considerable reduction in livestock was assumed, namely a 50% reduction in the number of dairy cows and a 25% reduction in the number of pigs for slaughter, both in the year 2000 as compared with 1985. Furthermore. only a restricted transport of manure was assumed.
According to the calculations. the nitrogen application rate on the soil falls by over 40% in variant 1. This results in a reduction of nitrogen loads into surface water with 50%. This reduction will have been almost achieved - for approximately 90% - by the year 2000. The conclusion from this pilot study was that in this scenario an emission reduction of 50% (goal for 1995) will be met, but not the final goal of 70-75% reduction. A relatively high part of the emissions is from average moist to wet grassland on sandy soil. ADDITIONAL MEASURES It is recognized that a general policy aimed at a slow decrease will not solve all problems with diffuse pollution of nutrients timely. Therefore, several additional measures at the regional or local scale are under consideration. These measures are aimed at a) a further reduction of the emissions than the goals of the general policy; b) a sooner reduction of the emissions and c) to alleviate the water quality problems in the receiving waters, especially lakes.
188
P. C. M. BOERS
The measures to reach a greater and/or sooner reduction of emissions are usually taken in the watershed. Most of these are still under research. A very simple measure is to stop fertilizer and dung dosing along the watershed. This avoids accidental additions directly in the watershed and reduces some runoff. Probably the most promising is the restoration or creation of buffer strips or riparian ecotones along the water courses (Peterjohn and Correll, 1984; Vought et al., 1994). Such areas are known to have a high nitrogen removal capacity by denitrification and uptake by macrophytes, followed by harvesting, and by storage in the sediments. Probably, buffer strips are most efficient for nitrogen removal, because of the denitrification. Wetlands, possibly in the form of wet buffer strips along the watercourses, have the same potentials. Usually the denitrification capacity is higher than that of dry buffer strips. On the other hand, the potential to intercept surface runoff may be greater for dry strips. The use of buffer strips still has many problems. Drainage systems are commonly used in the Netherlands. Probably, they interfere with the proper functioning of buffer strips. Some studies indicate that buffer systems become less efficient with time (Vought et al., 1994). Uncertain is to what extent such problems can be solved with a proper management of the strips. This is still a matter of research. Other important problems are that the demands for an optimal removal of nitrogen differ from that of an optimal removal of phosphate. Furthermore, the amount of space required for buffer strips is rather high, especially in those parts of the Netherlands with a high density of ditches. Space is costly. Table 4. Estimates of costs of eutrophication control measures in lakes. It is assumed that the external loading was reduced with 3 g P m-2 y-1. (From Boers and Van Der Molen, 1993). Type of measure
costs (ECU/kg Pl
costs (ECU/ha)
P removal at treatment plant to I mg P I" in effluent
4
130 y"'
P removal at treatment plant to 0.2 mg P I·' in effluent
75
2,200 y.'
P removal from inlet water of lake
220 - 300
6.500 - 9,000
y-'
costs depend of inlet water quality and annual variation in flow rate
sedimentation basin in inlet water of lake
220 - 450
6,500 \3,500
costs depend of inlet water quality and annual variation in flow rate
P removal using AI-salts in deep lake
400
average of 9 cases in the USA around 1980
P removal using Fe-salts in shallow lake
6,000
pilot experiment in 1989
dredging
14.000 40.000
average of \ 0 cases
biomanipulation
900
shallow Lake Wolderwijd, The Netherlands
hypolimnetic aeration
400
experiments around 1980
artificial circulation
250
example from only one case
Remarks
hypolimnetic "ithdrawal
very cheap
flushing
very variable
Because the soils are saturated with phosphate in a large area of the Netherlands (Reijerink and Breeuwsma, 1992) and because of the chemical time bomb character of this phenomenon several measures specifically aimed at reducing the leaching of phosphate are under study. Examples are: - addition of ferric com pounds in the soil near the watershed to increase the sorption capacity; - lowering of the groundwater table to lengthen the travelling time and to create new sorption capacity:
Nutrient emissions from agriculture in The Netherlands
189
- to keep the nutrients in the agricultural areas by reducing the discharge of local water excess to the receiving waters; - use and if necessary develop crop with a high phosphate uptake capacity, combined with an adequate dosing of nitrogen fertilizer. In this way, the phosphate is extracted from the soil. Another type of measure is aimed at alleviating the water quality problems in the receiving lakes. Typical problems are massive growth of algae or macrophytes, oxygen depletion, fish kills, loss of water depth by sedimentation. Most of the measures are end-of-the- pipe techniques. They relieve the eutrophication symptoms, but they do not solve the structural problem, namely an overloading with nutrients. Table 4 summarizes rough estimates of costs of several control measures. CONCLUSIONS Diffuse pollution from agriculture is an important source of both nitrogen and phosphorus for the Dutch surface waters. This pollution is caused by the high surpluses for both elements in agriculture. The Dutch policy is aimed at reduction of the pollution by decreasing the nutrient surpluses in agriculture. According to desk studies, it is at present not possible to decrease these surpluses enough to meet the Dutch quality standards for surface waters. This is too expensive for the agricultural sector. Therefore a path of gradual reduction of the nutrient surpluses will be taken. This must be combined with additional measures, both in agriculture and in the aquatic systems, to combat eutrophication problems sucessfully. REFERENCES Boers, P. C. M. and Van der Molen D. T. (1993). Control of eutrophication in lakes: the state of the art in Europe. Journ. EWPC, 3, 19-25. Boers, P. C. M. and De Vries I. (eds) (1994). Evaluatie van de gevolgen van het mestheleid derde fase voor de eutrofi!!ring van de Noordzee. [Evaluation of the effects of the third phase fertilize policyplan on the eutrophication of the North Sea; in Dutcb]. Report 94.041, RIZA, Lelystad, the Netherlands. Lijklema, L. et aL, (1993). Non-point nutrient sources and their control. Mem. 1st. ital. IdrobioL 52,207-221. Ministry of Agriculture, Nature Conservation and Fisheries, 1993. Mest- en ammomakbeleid derde fase [third-pbase fertilizer policyplan; in Dutch]. Oenema, O. and Van Dijk T. A. (eds) (1994). Fosfaatverliezen en fosfaatoverscbotten in de Nederlandse landbouw [Pbosphate losses and phosphate excesses in Dutcb agriculture; in Dutcb]. Report ministries of Agricullure, Public Health & Environment, Transport, Public Works & Water Management. Peleljohn. W. T. and CorreU D. L. (1984). Nutrient dynamics in an agricultural watershed: observations on the role of a riparian forest. Ecology. 65, 1466-1475. Reijerink, J. G. A. and Breeuwsma A. (1992). Ruimtelijk beeld van de fosfaatverzadiging in mestoverschotgebieden [Phosphate saturation of soils in manure excess areas; in Dutch). Report 222. Staring Centre. Wageningen. the Netherlands. Sectie Agrariscb Management (1995). Verkenning van de sociaal-economische gevolgen van diverse rekenvarianten voor fosfaaten stikstofverliesnormen [Exploration of the socio-economic consequences of several scenarios of acceptable losses of phosphate and nitrogen; in Dutch]. Report ministries of Agriculture. Public Heallh & Environment, Transport. Public Works & Water Management. Sharpley, A. N. et aI., (1994). Managing agriCUltural phosphorus for protection of surface waters: issues and options. J. Environ. Qual. 23,437-451. Uunk. E. J. B. (1991). Eutrophication of surface waters and the contribution of agriculture. The Fertilizer Society, London. Van Eck, G. (ed) (1994). Stikstofverliezen en stikstofverschotten in de Nederlandse Iandbouw [Nitrogen losses and nitrogen excesses in Dutch agriCUlture; in Dutch]. Report Ministries of Agriculture. Public Health & Environment, Transport, Public Works & Water Management. Vought, L. B.-M. et al. (1994). Nutrient retention in riparian ecotones. Ambio. 23, 342-348.