278 M. Hoffmann et al. Agriculture, Ecosystems and Environment 80 2000 277–290
as lakes, where nitrate concentrations generally are low. However, locally in agricultural areas in southern
Sweden, nitrate in groundwater is a concern and it is estimated that about 100 000 people have private
wells with nitrate concentrations well above 50 mg l
− 1
Thoms and Joelsson, 1982. For these reasons, the fo- cus in Sweden is mostly on agricultural N load to the
Baltic Sea instead of nitrate pollution of groundwater. Water quality data from major European rivers, such
as the Rhine and the Daugava, indicate that a major increase in riverine loads of N coincided with the in-
creased use of commercial fertilisers during the first decades of the post-war period Tsirkunov et al., 1992;
Grimvall et al., 2000. However, the relationship be- tween N input to agriculture and export of N from
land to sea is complex. Firstly, the turnover of N in soil involves a variety of different processes operating
on a wide range of time-scales. Secondly, long-term changes in riverine loads of N may, at least in part,
be due to drainage of agricultural land, ditching of wetlands and other measures that influence the reten-
tion of N during transport from field to river mouth. Thirdly, the classic experiments at Rothamsted indi-
cate that, due to a less efficient N uptake in crops, losses of this element from arable land to water were
already considerable in the 19th century Lawes et al., 1881. Another reason for taking a historic view of N
leaching from arable land is that most of the arable land in Sweden is situated within the catchment of
the Baltic Sea. The Baltic is often considered to have been in a more healthy condition in the first half of the
20th century. Since agriculture today is considered to be the largest single source of N outlet to the Baltic
Sea, it is important to study change in N load from agriculture over a longer time perspective. This will
increase overall understanding of the causes of dete- riorating water quality in the Baltic Sea.
Hence, there is a great need for a closer examination of long-term changes in the leaching of N from arable
soils. No systematic study of this has previously been done for Swedish conditions.
Only a few monitoring data are available before the 1970s. Sondén 1912 was one of the first researchers
in Sweden to compile water quality analyses for a large number of lakes and rivers, but without pointing
out any particular source such as agriculture. Arrhe- nius 1954 investigated nitrate concentrations in dug
wells and suggested that there was an influence of nitrate originating from arable land. One of the first
measurements of nitrate leaching from arable land was conducted in 19491950 by Wiklander and Wall-
gren 1960, who estimated nitrate leaching of 4 kg NO
3
–N ha
− 1
with 123 mm discharge on a clay soil in central Sweden. Wiklander and Wallgren 1970
collected 358 samples of drainage water throughout Sweden in 1965 and concluded from their own cal-
culations that minimum and maximum N leaching varied between 4 and 8 kg ha
− 1
on average for the whole country, and between 8 and 17 kg ha
− 1
in the county of Skåne, south of Sweden. From these few
and scattered measurements, it is difficult to draw any conclusions concerning N leaching in the past.
The current systematic monitoring programmes were started at the beginning of the 1970s.
For the purpose of this study, past N leaching was estimated by combining agricultural statistics with a
process-oriented model of the turnover of N in arable soils. An internationally unique collection of agricul-
tural statistics, going as far back as 1865, makes Swe- den an excellent case for such a study. Data regarding
land use, fertiliser application, livestock density, crop distribution and crop yields were used as model inputs
together with soil characteristics and climate data. N losses from the root zone of arable land were then es-
timated using the SOILSOILN model Jansson and Halldin, 1979; Johnsson et al., 1987 of the vertical
transport of water and N through profiles of arable soils.
2. Materials and methods
2.1. Calculation of leaching estimates Leaching estimates were calculated for three dif-
ferent soil types, nine different leaching regions, three crops and 14 decades Table 1. The different
Table 1 Calculations were made for each combination of these factors used
as a framework for the calculations Soils
Crop classes Regions
Times Loamy sand
Cereals 9 Regions
14 Decades Loam
Bare fallow Clay
Grass
M. Hoffmann et al. Agriculture, Ecosystems and Environment 80 2000 277–290 279
Fig. 1. Leaching regions for which calculations were done. The regions were separated by differences in discharge of water and annual temperature.
regions Fig. 1 were characterised by differences in annual precipitation 600–1000 mm and discharge
200–400 mm, annual mean temperature 6–8
◦
C and crop management. More detailed information
concerning the leaching regions is given in Johnsson and Hoffmann 1998. A 10-year weather period was
used for each regional calculation; this was consid- ered to represent a ‘normal’ climate reasonably well.
The same weather period was used for calculations for all decades. Three crops dominated the use of
arable land during the period investigated. For each combination in the matrix, the leaching estimate was
calculated by averaging a simulation over the 10-year period using the same management practices crop
and fertilisation every year Hoffmann and Johnsson, 1999. No consideration was taken of crop rotation.
Since the acreage of sugar beets Beta vulgaris, pota- toes Solanum tuberosum and oil plants was small,
the most common preceding crop in the rotation was the one which did not have a strong carry-over effect
of N to the following crop in rotation. The two most common patterns of cultivation are cereals with ce-
reals as the preceding crop, and grass with grass as the preceding crop. Therefore, it was assumed that
there was no significant effect from the preceding crop on the actual leaching rate. Since the acreage
of ploughed grass and that of under-sown grass in cereals were equal, this under- and overestimation of
leaching were considered to have compensated for each other in this study.
Dates for sowing and harvesting, i.e. times for the start and end of plant uptake of N, in the different
regions were the same as those used in the study by Johnsson and Hoffmann 1998. Fertiliser N was ap-
plied according to normal practice at the time of sow- ing, and it was assumed that all manure was applied
during spring operations. In addition, it was assumed that the amount of manure produced and fertiliser N
used was evenly distributed to all arable land in each region.
The fallow was treated as a bare fallow in the calcu- lations, i.e. it was unfertilised and no plant uptake of N
by weeds was assumed. Leaching rates for grass were assumed to be the same for all the decades. Leach-
ing rates from grass are normally low under Swedish conditions Gustafson, 1983; Hoffmann and Ellström,
1993, often in the range 3–10 kg ha
− 1
per year and relatively constant if manure or fertiliser N are not ap-
plied at a super-optimal rate, which was not the case for any decade in this study. Consequently, leaching
from grass can be assumed to have changed less than that from cereals. Calculations were only done for
1951; N fixation was estimated for this year in another study Statistics Sweden, 1995. The calculations for
grass were conducted for continuously growing grass, harvested once a year. Application of both fertiliser N
280 M. Hoffmann et al. Agriculture, Ecosystems and Environment 80 2000 277–290
and manure was calculated as for cereals. The N fixa- tion of clover varied from 29 to 48 kg N ha
− 1
per year in the different regions.
2.2. Input data A complete description of the method and parameter
settings is given in Johnsson and Hoffmann 1998. Input data and parameter settings that are unique for
this study are described below and in accompanying tables and figures.
2.2.1. Harvest export of N Total plant uptake was calculated based on yield
data obtained from official statistics. Average yield of grain for each decade was calculated using data on ac-
tual yields BiSOS-N, 1865–1905; SOS, 1915–1955; Statistics Sweden, 1965–1995. Since yields for spe-
cific years deviate a lot, average yields for 3 years in the middle of each decade were used.
The amount of N exported with harvested products depends not only on yield but also on the N content
in the product. It was assumed that the N concentra- tion was different from today. Hansson 1912 com-
piled data on N content in most agricultural products. These data were used for the period 1865–1925. Data
for current N content was collected from Haak 1993, manuscript and the Swedish Board of Agriculture
1997. The change in N concentration between 1925 and 1995 was assumed to roughly correspond to the
use of fertiliser N.
Fig. 2. Change in animal density in animal units per ha for the whole of Sweden and in some counties. One animal unit is one cow. 10 fattening pigs. three sows and 100 laying hens. Sweden is third line from the bottom.
2.2.2. Input of N from manure Until 1920–1930 manure was almost exclusively
used to fertilise the soil. The estimate of the amount of manure produced between 1927 and 1992 was based
on a previous study Statistics Sweden, 1995. The amount of manure produced per animal unit for 1927
was calculated from data obtained from Statistics Swe- den 1995. The amount of manure for each decade
from 1925 back to 1865 was calculated by extrapo- lating the number of animal units using countywide
statistics Fig. 2 for livestock BiSOS-N, 1865–1905; SOS, 1915–1955. Not only did the number of animal
units change but so also did the N content in manure. A correlation between milk production and N content in
manure from dairy cows was used to estimate change in N content in manure. It was roughly assumed that
for every 1000 kg increase in yearly milk production, N in manure increased by 20 kg Steineck, 1997, per-
sonal communication. Data for yearly milk produc- tion per cow were obtained from Statistics Sweden
1959. Most of the manure came from cows and the N content in manure from other animals was assumed
to have changed similarly.
To calculate the N supply from manure to the soil, ammonia emissions had to be subtracted. Total ammo-
nia emissions from manure were calculated by Karls- son 1998, personal communication to be 45 of the
amount of N produced in manure in 1951, and 35 in 1994. Ammonia losses have decreased due to more
frequent use of liquid manure and better handling pro- cedures. The percentage of ammonia losses between
M. Hoffmann et al. Agriculture, Ecosystems and Environment 80 2000 277–290 281
Table 2 Input of N by fertiliser and manure for the different regions in Sweden
Region
a
1865 1875
1885 1895
1905 1915
1925 1935
1945 1955
1965 1975
1985 Fertiliser N kg ha
− 1
parameter FERN Gss
8 14
25 48
85 107
119 Gmb
5 9
17 33
59 82
77 Gsk
3 5
10 19
34 51
52 Gns
4 8
15 27
49 90
96 Ss
4 7
13 24
44 79
80 Ssk
2 4
7 13
23 52
52 Ammonium N in manure kg ha
− 1
parameter MANNH Gss
2.9 3.1
3.6 4.1
4.7 5.0
5.7 6.0
5.6 5.2
7.4 9.7
10.6 Gmb
4.7 4.8
4.6 4.3
5.0 5.2
5.6 5.7
5.4 4.8
7.7 10.9
13.6 Gsk
6.1 5.6
5.8 5.2
5.5 5.9
6.3 6.6
6.5 5.4
7.1 10.1
12.8 Gns
3.5 3.3
3.4 3.4
4.1 4.6
5.0 5.3
5.0 4.1
5.2 6.3
7.8 Ss
3.8 3.7
4.0 3.9
4.1 4.3
4.7 4.6
4.3 3.0
3.6 4.7
5.6 Ssk
9.6 5.1
5.7 5.3
5.2 5.7
6.2 5.9
5.5 3.6
4.4 6.3
8.5 Organic N in manure kg ha
− 1
parameter MANFN Gss
8.7 9.4
10.3 11.8
12.6 13.6
14.6 15.5
13.6 12.7
16.6 18.8
18.1 Gmb
14.0 14.3
13.1 12.3
13.5 14.1
14.5 14.8
13.2 11.8
17.2 21.2
23.1 Gsk
18.2 16.7
16.4 14.7
14.9 16.0
16.2 17.0
15.9 13.2
15.9 19.6
21.8 Gns
10.6 10.0
9.8 9.8
11.0 12.3
12.9 13.5
12.4 10.1
11.6 12.2
13.3 Ss
11.4 11.1
11.3 11.0
11.2 11.6
12.2 11.9
10.5 7.5
7.9 9.1
9.6 Ssk
28.8 15.3
16.1 15.2
14.2 15.4
16.1 15.1
13.4 8.7
9.8 12.2
14.5
a
The abbreviations in the column are for each of the regions which are illustrated in Fig. 1.
1865 and 1945 was assumed to be the same as in 1951 45. Ammonia emissions between 1951 and 1994
were assumed to decrease linearly from 45 to 35. The relation between organic N and ammonium N
in manure changed during the period of this investiga- tion. The fraction of ammonium N in manure in 1865
was assumed to be 25, which is typical for farmyard manure. The present average content of ammonium N
was set at 40. Input of N from manure is accounted for in Table 2.
2.2.3. Input from fertiliser N Data on the usage of fertiliser N was collected from
sales statistics Statistics Sweden, 1995. County data were available from 1965; before 1965 only aggre-
gated data for the whole country existed. When deriv- ing countywide values on fertiliser use in the decades
before 1965, it was assumed that the inter-county re- lationship was the same as for 1965. The use in the
counties was then adjusted to reflect the change in sales for the whole country. To obtain fertiliser use
for production areas the 20 counties were aggregated to six production areas. This was done by weighting
fertiliser use in each county by that county’s acreage of arable land Table 2.
2.2.4. Soil organic N and mineralisation rate Assumptions concerning the amount of organic N in
the soil profile and the mineralisation rate are decisive to the result of the calculated leaching estimates.
From 1865 to 1935, the acreage of arable land increased by 1.3 million ha Fig. 3. This extra land
originated from three main sources; old grassland, lakes and bogs and forested land. Ploughing of old
grassland is generally considered to have been the main source Sjöström, 1922. When old grassland is
ploughed up, the content of soil organic matter SOM and consequently also organic N, declines from a
level characteristic for grassland soils to a lower level characteristic for soils in arable use. This phenomenon
has been described by a number of authors, e.g. Pers- son 1978, Jenny 1980 and Whitmore et al. 1992.
During the period when SOM declines, mineralisation is larger than the input of new organic material. This
decline results in a likelihood of increased N losses from the cultivation system through leaching, denitri-
282 M. Hoffmann et al. Agriculture, Ecosystems and Environment 80 2000 277–290
Fig. 3. Change over time in total area of arable land and in area of bare fallow. grass and arable crops in Sweden.
fication and harvest export. A decline of up to 35, which is common, represents a loss of 3–4 Mg N ha
− 1
. In this study, SOM was assumed to decline by 35
in the first 30 years after ploughing. This is in accor- dance with the findings from a Swedish field experi-
ment where decline in organic carbon was measured Persson, 1978. The decline in soil organic N was as-
sumed to be proportional to the decline in soil organic carbon by assuming a constant CN ratio of 10. The
bulk density of the soil was assumed to be 10 less in
Table 3 Parameter values of soil organic matter mineralisation rate constant. potential denitrification and content of organic N 0–100 cm soil depth
in the parameter settings
a
Organic matter mineralisation HUMK 10
− 5
Denitrification DENPOT Organic N kg ha
− 1
A B
C A
B C
A B
C 1865
65 59
53 0.16
0.32 0.64
9454 9380
9305 1875
72 63
54 0.16
0.32 0.64
9513 9419
9324 1885
68 61
54 0.16
0.32 0.64
9491 9405
9318 1895
66 60
53 0.16
0.31 0.63
9464 9385
9307 1905
58 55
52 0.15
0.30 0.60
9381 9332
9281 1915
56 54
51 0.15
0.28 0.53
9331 9298
9264 1925
54 52
51 0.13
0.23 0.42
9297 9275
9252 1935
52 51
50 0.11
0.14 0.21
9442 9371
9300 1945
50 50
50 0.08
0.08 0.08
9235 9235
9235 1955
50 50
50 0.08
0.08 0.08
9235 9235
9235 1965
50 50
50 0.08
0.08 0.08
9235 9235
9235 1975
50 50
50 0.08
0.08 0.08
9235 9235
9235 1985
50 50
50 0.08
0.08 0.08
9235 9235
9235
a
A. B and C is a sensitivity analysis where A is ‘High’: the entire increase in arable land originated from ploughing up of old grassland and a low denitrification rate was assumed; B is ‘Normal’: 60 of the increase in arable land originated from ploughing up
of old grassland and a ‘normal’ denitrification rate was assumed; and C is ‘Low’: 20 of the increase in arable land originated from ploughing up of old grassland and a high denitrification rate was assumed.
unploughed grassland compared to the situation after the decline. The parameters for SOM mineralisation
in the model were calibrated to describe this decline in soil organic N. The parameter values of the miner-
alisation rate obtained from this calibration were later used when determining the average mineralisation rate
for all arable land. The average organic N content Table 3 and mineralisation rate for all arable land
were calculated from figures for the increase in arable land for each decade. By assuming that a share of the
M. Hoffmann et al. Agriculture, Ecosystems and Environment 80 2000 277–290 283
increase in arable land originated from grassland it was possible to keep a running tally of how large a
part of the newly cultivated arable land was at a certain phase of the SOM decline. Land that had been arable
for more than 30 years was treated as if the decline had terminated and a steady state had been reached.
The 1930s were the last decade when a larger content of organic N and higher mineralisation rate, due to re-
cent cultivation, were applied. From a previous study Mattsson, 1995, it was estimated that the average
content of SOM today is roughly 45 g kg
− 1
in the top- soil of arable land in Sweden; an estimation that was
used in a previous study by Johnsson and Hoffmann 1998. This amount of SOM was used from 1940 on-
wards. Similarly, the parameter values describing the mineralisation rate from 1940 onwards were also the
same as in Johnsson and Hoffmann 1998.
2.2.5. Denitrification Denitrification is the most uncertain sink of N in this
study. Since nitrate in the soil profile can be subject to both denitrification and leaching a large denitrification
will decrease the amount of N available for leaching. It is obvious that the drainage status of the arable soils
has gradually improved during the period investigated LBS, 1950 resulting in dryer soils. Denitrification
was, therefore, assumed to be higher in 1865 compared to the present Table 3. The denitrification rate was
assumed to decline gradually during the period when
Table 4 Used values of wet and dry deposition for each decade and each region in Sweden
Region
a
1865–1905 1915
1925 1935
1945 1955
1965 1975
1985 1995
Wet deposition DEPWC mg l
− 1
Gss 0.1
0.2 0.2
0.3 0.4
0.5 0.7
0.9 1.3
1.2 Gmb
0.1 0.2
0.2 0.3
0.3 0.4
0.6 0.8
1.1 1.0
Gsk 0.1
0.1 0.2
0.2 0.3
0.4 0.5
0.7 1.0
0.9 Gns
0.1 0.1
0.1 0.2
0.2 0.3
0.4 0.5
0.8 0.7
Ss 0.1
0.1 0.1
0.2 0.2
0.3 0.4
0.6 0.8
0.7 Ssk
0.1 0.1
0.1 0.1
0.2 0.2
0.3 0.4
0.6 0.5
Dry deposition DEPDRY g N m
− 2
day
− 1
10
− 4
Gss 0.82
0.12 0.15
2.0 2.4
3.1 4.4
5.7 8.2
7.4 Gmb
0.55 0.8
1.0 1.3
1.6 2.1
3.0 3.9
5.5 4.9
Gsk 0.55
0.8 1.0
1.3 1.6
2.1 3.0
3.9 5.5
4.9 Gns
0.42 0.6
0.8 1.0
1.2 1.6
2.3 2.9
4.2 3.8
Ss 0.14
0.2 0.3
0.3 0.4
0.5 0.8
1.0 1.4
1.3 Ssk
0.14 0.2
0.3 0.3
0.4 0.5
0.8 1.0
1.4 1.3
a
The abbreviations in the column are for each of the regions which are illustrated in Fig. 1.
intensive drainage work was undertaken. The decline was assumed to be proportional to the intensity in
drainage activities on agricultural land and the end of enhanced denitrification was set to 1940.
2.2.6. Sensitivity analysis To illustrate the uncertainty in the assumptions con-
cerning denitrification and mineralisation, calculations were conducted for three different assumed situations
Table 3. 1. ‘High’: the entire increase in arable land originated
from ploughing up of old grassland and a low den- itrification rate was assumed.
2. ‘Normal’: 60 of the increase in arable land orig- inated from ploughing up of old grassland and a
‘normal’ denitrification rate was assumed. 3. ‘Low’: 20 of the increase in arable land origi-
nated from ploughing up of old grassland and a high denitrification rate was assumed.
This approach results in an interval between highest and lowest leaching rates in which it is likely that the
actual leaching rate occurred. 2.2.7. Input of N from atmospheric deposition
Deposition of N was assumed to increase by 100 from 1900 to 1985, and thereafter decrease by 10 un-
til 1995 Table 4 according to a study by the Swedish Environmental Research Institute Swedish Environ-
mental Protection Agency, 1997. Deposition of N
284 M. Hoffmann et al. Agriculture, Ecosystems and Environment 80 2000 277–290
from 1865 until 1900 was assumed to be the same con- sidering that the change in ammonia emissions due to a
change in the way manure was handled was relatively small and NO
x
emissions from internal-combustion engines were still very low.
2.3. Calculation of average leaching and gross load Eighty-one leaching estimates were calculated for
each decade three crops, three soils and nine regions. Average leaching rates for each of the nine regions
were calculated by 1 weighting the leaching esti- mates according to the distribution percentage of
area of each of the three soil types for each crop class. The same soil distribution was used as in previous
study Johnsson and Hoffmann, 1998; 2 multiplying average leaching rates for each of the crops by acreage
of each crop; and 3 summing up the three gross loads from each crop and dividing by total acreage of arable
land in each region. Average leaching for the whole country was calculated as the sum of gross load in
each region divided by the total acreage of arable land.
Fig. 4. Input of N from both manure and fertiliser N and output as yield in tonnes A and expressed as per hectare of utilised arable land B.
2.4. Calculation of reduced N retention in lakes An estimate of the effect of draining lakes on N re-
tention was also done. This was calculated by estimat- ing the area of drained lakes and assigning a retention
to this lake area. The average N retention used in this estimate was 104 kg N ha
− 1
of lake area as reported in a study by Arheimer et al. 1997. Since retention
is dependent on nitrate concentration, it was adjusted to change in simulated average concentrations in dis-
charge from arable land, assuming that retention is proportional to concentration if all other factors are
unchanged.
3. Results
