M. Hoffmann et al. Agriculture, Ecosystems and Environment 80 2000 277–290 287
Fig. 6. Calculated N leaching expressed in kg of leached N per harvested kg of N in foodstuff and fodder per ha of arable land in Sweden.
mates from cereals were at their lowest because the utilisation of N had increased.
From the 1930s until 1970, leaching rates from ce- reals increased by two-thirds; the load from bare fal-
low ceased whereas leaching from grassland remained unchanged.
The net effect of this was an increase in average N leaching rates from arable land of 60 Fig. 5a, while
the increase in gross load was only 30 Fig. 5b. One explanation for the smaller increase in gross load
compared to the increase in average leaching rates was the decrease in area of arable land. Fig. 3 shows that
practically the entire decrease in arable land was due to reduced acreage of grassland. Since leaching rates
from grassland were very low the effect on load was small. Furthermore, the decrease in arable land was
largest in the forest regions Gsk and Ssk, which had low leaching rates. Leaching has increased since the
1930s in all regions except the forest regions Gsk and Ssk. The largest increase in leaching rates occurred in
the two most southerly regions Gss and Gmb. These regions also witnessed the biggest increase in animal
density and input of N from manure Fig. 2. Gross load has decreased somewhat in the two forested areas
Gsk and Ssk.
To evaluate how much the change in acreage of arable land and change in crop distribution affected
the change in gross load to watercourses, the leaching rate estimates for 1865 were applied to all decades.
The result was that the change in load to watercourses followed the development in acreage of arable land,
i.e. gross load to watercourses increased from 1865 to a maximum in about 1920–1930 when the acreage
of arable land was largest, and decreased thereafter. This shows that, since change in leaching for bare
fallow was small, the decrease in leaching rates from cereals had a large influence on calculated gross load to
watercourses. Thus, the leaching estimates for cereals were critical for the final calculation of gross load to
watercourses via leaching. Leaching rates can, besides being expressed in ab-
solute values, also be expressed in terms of efficiency. Relating leaching rates to yields showed that ‘leach-
ing efficiency’ has decreased during the period inves- tigated since yields have increased more than leaching
Fig. 6. Production of a certain amount of food causes less leaching today compared to in the 19th century.
Efficiency of N application by manure and fertiliser was expressed as leached amount of N per applied kg
of N. It has decreased in about the same magnitude as leached amount of N per kg of N harvested in the
field. This illustrates the difficulty of making a sim- ple connection between input of N and leaching for
moderate inputs of N since the significance of miner- alisation for leaching is not taken into account in such
a comparison.
4. Discussion
The most remarkable finding in this study is that leaching of N from agricultural land in Sweden was
approximately the same in the 1860s as it is today. A post-war increase could also be discerned. This in-
crease was about 60, in Sweden as a whole. Only in southernmost Sweden, where agriculture is rela-
tively intensive, did the post-war increase approach 100. The latter estimate agrees well with the results
of a Danish study, in which it was suggested that the leaching of N from arable land in Denmark had in-
creased by slightly more than 100 from 1950 to 1980 Schröder, 1985. Yet other studies demonstrate that
temporal changes can vary strongly depending on the area. Boers et al. 1997, for example, estimated that
288 M. Hoffmann et al. Agriculture, Ecosystems and Environment 80 2000 277–290
Fig. 7. Number of drained lakes for each year in Sweden and an estimation of lost retention of N leached from arable land resulting.
the leaching of N from agricultural land in the Nether- lands increased by 300–400 from 1950 to 1990. It
is generally recognised that super-optimal fertilisation rates can dramatically increase losses of N from the
root zone of arable land. This is most likely one of the main causes of the large post-war increase in the
Netherlands, and may explain the large difference in the increased loss of N compared to this study.
There is a strong correlation between specific N leaching rates, gross load, retention of N and net load
to the sea. Gross load is all the specific leaching rates for different regions summarised for the whole coun-
try. Part of this gross load of N is retained in lakes and rivers and net load is the amount of N that reaches the
sea via river mouths. Even though N leaching rates and gross load are estimated in this study to be in the
same order of magnitude today as in the middle of the 19th century, it does not mean that net load to the sea
is in the same order of magnitude. Current average re- tention of N lost from arable land has been estimated
at approximately 40 Arheimer et al., 1997. In other words, 40 of the N leached from arable land is lost
in rivers and lakes mainly by denitrification, and 60 reaches surrounding seas via river mouths. During the
second half of 19th century and first half of 20th cen- tury, lakes and wetlands were extensively drained to
gain arable land Fig. 7. About 2500 lakes situated close to arable land were drained Swedish Meteoro-
logical and Hydrological Institute, 1995. Since these lakes were recipients of discharge water from nearby
arable land, it is reasonable to assume that they had a significant role as N-sinks. An estimate of the decline
in retention indicates that approximately 30 000 Mg of N may previously have been lost in these lakes Fig. 7.
The calculation indicates that a probable increase in net load of N, i.e. the difference between gross load
from all arable land and total retention in lakes and wa- terways, for the period as a whole was more affected
by change in retention than by a change in gross load of N from arable land.
It is also important to note that these different phe- nomena, all leading to increase in net load occurred
at different times. The draining of lakes and wetlands, which has led to less retention capacity for N in the
landscape was mainly done at the end of the 19th century and in the first decades of the 20th century.
However, the increase in gross load, which is the other factor of importance for the net load occurred mainly
in the post war period. This indicates that most of the increase in gross load occurred at a time when most
of the drainage was completed, and consequently, retention capacity lost.
The rise in N concentration in rivers has often been connected to the increased input of fertiliser N. This
study shows that there are other mechanisms, notably draining of lakes and wetlands, which are as important
as the input of fertiliser N in affecting net losses to the surrounding seas from arable land. However, it should
M. Hoffmann et al. Agriculture, Ecosystems and Environment 80 2000 277–290 289
be noted that this does not influence the process of nitrate leaching to groundwater.
Model calculations in general and large-scale model calculations in particular are impaired by uncertain-
ties. The result depends on the accuracy of assump- tions made. However, regardless of the exact outcome
of the model calculations, none of the three explana- tory factors for high historical N leaching rates are
particularly controversial and are more likely to be significant than other factors indicating low leaching
rates.
Long-term studies on N leaching raise the question of what type of model tool should be used. Since
essential assumptions were made concerning impor- tant soil processes such as N turnover from newly
cultivated land, the model chosen must describe the significance of such processes for leaching. The
SOILSOILN model and many other deterministic models have that capability. A more simple empir-
ical model generating N leaching estimates model would probably not have been sufficient since such
models’ capability to calculate leaching estimates is built on modern field experiments on leaching, and
consequently, may not be valid for totally different inputoutput conditions of N.
Perhaps the largest constraint in this type of appli- cation is not in the different models capability to de-
scribe soil processes, but in the large uncertainties of input data, especially assumptions of gas losses den-
itrification from the soil and mineralisation rate. This is why these two key factors for leaching were chosen
to illustrate uncertainty in the sensitivity analysis.
5. Conclusions
