A. Korsaeth, R. Eltun Agriculture, Ecosystems and Environment 79 2000 199–214 207
Fig. 2. Measured total N concentrations as monthly averages lines and transport of total N averaged over the agrohydrological years May–April 1990–1997 bars, for surface runoff upper two figures and drainage runoff lower two figures from the six cropping
systems at Apelsvoll. Bars with same letter are not significantly different p=0.05.
The statistics for the best fitting regressions are shown in Table 6. From the regression equations it
may be calculated that a difference in total precipita- tion of 100 mm between a dry year and a subsequent
wet year, would increase the N runoff in the wet year by 6 and 5 kg N ha
− 1
per year from the arable and the forage cropping systems, respectively, assuming
the same N balance both years. For the arable systems an increase in the N balance i.e. Bal
simple
by 10 kg N ha
− 1
per year would over time imply an increase in N runoff by 1.5 kg N ha
− 1
per year.
4. Discussion
4.1. Effects of cropping systems on the soil N content The calculated net change in soil organic N 1N
was negative for all systems except CON-F. This re- flects the high initial level of mineralisable N in the
soil at this particular experimental site. As stated in the introduction, the absolute level of 1N depends on
the cultivation history of a field as well as its present regime, and the value for one particular case is not a
208 A. Korsaeth, R. Eltun Agriculture, Ecosystems and Environment 79 2000 199–214
Fig. 3. Estimated N-fixation in leys, greenfodder and peasoats in the cropping systems at Apelsvoll. Mean for 1990–1997.
valid criterion for its sustainability. Thus, the systems sustainability cannot be evaluated by the absolute val-
ues of 1N found in our study. On the other hand, the ranking of the cropping systems is useful as an indi-
cation on the degree to which each cultivation regime is able to sustain soil organic N levels relative to the
others.
Table 4 Measured and estimated nitrogen flows kg N ha
− 1
per year in the cropping systems at Apelsvoll, mean 1990–1997
a
N-flow Cropping system
CON-A INT-A
ECO-A CON-F
INT-F ECO-F
Fertiliser 118.9
75.4 109.1
54.2 0.0
Cattle slurry 0.0
0.0 20.6
82.2 66.7
54.8 Wet atmospheric depositions
6.2 6.2
6.2 6.2
6.2 6.2
Dry atmospheric depositions 2.0
2.0 2.0
2.0 2.0
2.0 Symbiotic N-fixation
0.0 0.0
36.9 24.6
37.2 55.2
N in seeds 10.2
10.2 7.6
2.5 2.5
3.0 Sum input
137.3 93.8
73.4 226.7
168.9 121.2
Harvest
b
106.7 81.5
91.1 162.2
138.6 124.9
NH
3
-N volatile cattle slurry 0.0
0.0 2.6
18.9 14.4
14.5 NH
3
-N volatile crop 2.0
2.0 2.0
2.0 2.0
2.0 Denitrification
8.3 5.3
1.3 12.1
7.5 2.8
Surface N-runoff 2.2
1.5 1.8
2.1 1.7
1.9 Drainage N-runoff
32.7 27.3
19.3 27.6
16.3 17.6
Sum output 151.9
117.6 118.1
224.9 180.5
163.7 1
N
c
input minus output −
14.6 −
23.8 −
44.7 1.8
− 11.6
− 42.5
a
Timestep used is the agrohydrological year May–April, thus covering the period May 1990–April 1998.
b
Including N removed with straw in 1990–1992 otherwise the straw was left on the field.
c
Changes in soil total-N.
4.1.1. Effects of arable cropping systems on the soil N content
A reduction of the soil N pool was calculated for all the arable cropping systems Table 4. These results
are in accordance with other experiments conducted on soils with relatively high 2.0 g kg
− 1
initial N con- tent Christensen, 1990; Uhlen, 1991; Heenan et al.,
A. Korsaeth, R. Eltun Agriculture, Ecosystems and Environment 79 2000 199–214 209
1995; Thomsen and Christensen, 1998. Least reduc- tion was found by the system with the largest N in-
put CON-A, and the depletion of soil N increased with decreasing fertiliser input. Raun et al. 1998 also
reported a positive relationship between fertiliser N input and soil N content in a long term experiment
23 years with continuous wheat. They found, how- ever, mainly positive values of 1N accumulation,
which may be explained by the low initial N content 1.0 g kg
− 1
in their experimental soil. ECO-A had the highest calculated N depletion of
all the systems Table 4. The N export via harvest was larger than the total N input for all crops but
clover grass in this system data not shown, and this relatively high yield level was a major reason for the
calculated large reduction in soil N. A factor, which further increased the N deficits, is the low input of cat-
tle slurry. The amount of cattle slurry followed a fixed plan in the experiment. In practice the livestock num-
ber would be adjusted according to the fodder pro- duction or vice versa, which again would affect the
amount of cattle slurry available to the crops. Consid- ering the relatively high fodder production in ECO-A,
the application rates of slurry may have been too low.
In spite of the relatively large calculated reduction of the soil N pool in ECO-A, no changes could be mea-
sured. The high initial content of total-N 14.3 tonnes N ha
− 1
, more than two times the average in agricul- tural soils in Norway, may explain the lack of signif-
Table 5 Coefficients of determination R
2
for linear regressions using the N balance calculations Bal, precipitation prec
t
and precipitation from the previous year prec
t
–
1
as predictors, and N runoff as the dependent variable Data
Predictor 1 Predictor 2
Predictor 3 R
2
p-value Arable
Forage y
a
Bal
simple b
0.29 0.001 0.21 0.001
y Bal
complex c
0.33 0.001 0.14 0.010
y Prec
t
0.41 0.001 0.35 0.001
y Bal
simple
Prec
t
0.58 0.001 0.44 0.001
y Bal
complex
Prec
t
0.50 0.001 0.40 0.001
y Bal
simple
Prec
t
Prec
t − 1
0.87 0.001 0.65 0.001
y Bal
complex
Prec
t
Prec
t − 1
0.82 0.001 0.58 0.001
y
d
Bal
simple
0.86 0.007 0.49 0.122
y Bal
complex
0.86 0.008 0.44 0.154
a
y=Data from each agrohydrological year May–April.
b
Bal
simple
= N
fertiliser
+ N
slurry
− N
harvest
.
c
Bal
complex
= N
fertiliser
+ N
slurry
+ N
wet dep.
+ N
dry dep.
+ N
seed
+ N
fixation
− N
harvest
− N
NH
3
volat. slurry
− N
NH
3
volat. crop
− N
denitrification
.
d
y= Data averaged over all agrohydrological years 1990–1997.
icant changes, since the calculated reductions of soil N during the 8-year experiment were within the statis-
tical error of the measured soil N content Riley and Eltun, 1994. Penfold et al. 1995, who investigated
soil properties of organic, biodynamic, integrated and conventional cropping systems with rotations of grain
and forage crops, found no significant differences in organic C between the systems after 6 years, and con-
cluded that considerable time are needed before iden- tifiable changes in soil fertility emerge.
The results for N runoff corresponded well with findings of other authors, both for measured concen-
trations Bergström, 1987; Uhlen, 1994; Thomsen and Christensen, 1998, and for calculated N transport
Uhlen, 1991; Vinten et al., 1991; Høyås et al., 1997; Vagstad et al., 1997. ECO-A had significantly lower
N runoff than the other arable systems. Decreasing N runoff with decreasing fertiliser level is commonly re-
ported for arable crops e. g. Bergström, 1987; Uhlen, 1994, at least above a certain threshold level.
Summing up, crop production in the low input arable system was clearly at the expense of soil or-
ganic N soil mining, but there was also a negative trend for the other arable systems. What does this
mean at the long term? The long term study of con- tinuous wheat at the Margruder Plots Mitchell et al.,
1991, established in 1892, showed a rapid decline in soil N during the first 35 years 1.6 g kg
− 1
initial N. This was followed by a slower decline during the
210 A. Korsaeth, R. Eltun Agriculture, Ecosystems and Environment 79 2000 199–214
Fig. 4. Total N runoff surface and drainage runoff plotted against the estimated N balance Bal
complex
= N
fertiliser
+ N
slurry
+ N
wet dep.
+ N
dry dep.
+ N
seed
+ N
fixation
− N
harvest
− N
volat. slurry
− N
volat. crop
− N
denitrification
. Data averaged over the agrohydrological years May–April 1990–1997.
next 52 years approaching an equilibrium level. We assume that none of the systems in our experiment are
N limited. A further reduction in the soil N content in the arable systems is thus expected, with a devel-
opment over time similar to that reported by Mitchell et al. 1991.
Table 6 Statistics for the regressions which best described the variation in N runoff from the cropping systems at Apelsvoll, using the linear model:
Y=β +
β
1
z
1
+ β
2
z
2
+ β
3
z
3
+ ε
, where Y is N runoff kg N ha
− 1
per year
a
, β –
3
are parameters, z
1
–
3
are predictors and ε is the random error System
Data Predictors z and parameters β
R
2
z
1
= Bal
simple b
z
2
= Prec
t c
z
3
= Prec
t
–
1 d
β β
1
β
2
β
3
Arable y
e
31.05 4.49
f
0.16 0.02 0.06 0.01
− 0.06 0.01
0.87 Forage
y 24.93 7.82
0.10 0.03 0.05 0.01
− 0.05 0.01
0.65 Arable
y
g
31.52 1.27 0.15 0.03
0.86
a
Agrohydrological year May–April.
b
Bal
simple
= N
fertiliser
+ N
slurry
− N
harvest
, unit: kg N ha
− 1
per year.
c
Precipitation, unit: mm per year.
d
Precipitation from the previous year, unit: mm per year.
e
y=Data from each agrohydrological year.
f
Standard deviation in parentheses.
g
y= Data averaged over all agrohydrological years 1990–1997.
4.1.2. Effects of forage cropping systems on the soil N content
The system CON-A had a 1N value close to zero, i.e. the soil organic N level was close to equilibrium
Table 4. This is a very likely result, since the culti- vation regime for the site prior to the experiment was
A. Korsaeth, R. Eltun Agriculture, Ecosystems and Environment 79 2000 199–214 211
very close to CON-A. Although INT-F was similar to CON-F with regard to calculated 1N, the underlying
N flows differed considerably between the two sys- tems. INT-F had about 25 lower N input, but only
15 lower N yields than CON-F. The N runoff via drainage water from INT-F was among the lowest of
all the cropping systems Fig. 2. Conservation or even an increase in soil N has also been reported for other
rotations containing pasture or ley receiving organic N on relatively N-rich 2.0 g kg
− 1
soils Uhlen, 1991; Heenan et al., 1995.
Our calculations suggest that the soil N pool was re- duced by 45 kg N ha
− 1
per year every year in ECO-F. Why were the soil N losses in ECO-F so large com-
pared to the other systems? The considerations regard- ing slurry application are the same for ECO-F as for
ECO-A as discussed earlier. Underestimation of fixed nitrogen Fig. 3 is one possible reason for the low
1
N, although the estimated amounts were within the range of findings made by other authors Høgh-Jensen
and Steen Kristensen, 1995; Whitehead, 1995; Fis- cher, 1996. The legume content in ley decreased with
increasing N input and age, also reported by Petterson et al. 1998. This may be a result of the negative ef-
fect of fertiliser N on clover growth, as has been com- monly reported Høgh-Jensen and Steen Kristensen,
1995; Petterson et al., 1998.
Our results on N runoff showed in general that the forage systems had lower N runoff than the arable
systems. This confirms earlier results from the present experiment Eltun, 1995; Eltun and Fugleberg, 1996,
and is in accordance with other studies Bergström, 1987; Gustafson, 1987; Uhlen, 1991; Solberg, 1995.
In summary, it appeared that the system CON-F had a N input which balanced its N export, but that it also
had higher N losses to the environment than the other forage rotations. The calculated 1N in INT-F was
similar to that in CON-A. When taking into account the low N runoff from INT-F, it appeared to be the
most favourable in terms of both 1N and N runoff. Major adjustments are needed for ECO-F to avoid a
further decrease in the soil N pool.
4.2. Mass balance of total-N as a predictor of N runoff
On an annual basis there was a positive correlation between N balance and N runoff Step A, but an-
nual precipitation explained more of the variation in N runoff from the cropping systems than did the N
balance Table 5. Other authors have also found high correlations between precipitation and N runoff Jenk-
inson, 1990; Eltun and Fugleberg, 1996; Vagstad et al., 1997. N runoff from agricultural fields depends
primarily on the mobility of the soil N present, and surplus water to transport soluble N out of the field
Vagstad et al., 1997. Even if mobile soil nitrogen is present, no runoff occurs if surplus water for N trans-
port is lacking. This means that some of the leachable N may be left in the soil in a dry year, thereby increas-
ing the potential for N runoff the following year. Such ‘delayed N runoff’ would cause reduced efficiency in
the annual regressions. By combining annual N bal- ances, total precipitation from the same year and from
the previous year in a three-predictor model Step C, some of this effect was obviously reduced. In this way
up to 87 and 65 of the variation in N runoff between systems and years could be explained for the arable
and the forage systems, respectively Table 5.
A factor, which may have contributed to the ob- served differences between arable and forage systems,
is the difference in the composition of the N inputs. The fraction of inorganic N was much higher in the
N input to the arable systems than in that of the for- age systems. As a result, the proportion of inorganic
N would generally be higher in the arable than in the forage systems after N application, and thus more sus-
ceptible to leaching. This is confirmed by measure- ments of relatively high N concentration in drainage
runoff from CON-A soon after fertiliser application Month Numbers 5–7, Fig. 2.
Runoff of N from agricultural fields originates, however, primarily from organic N Macdonald et al.,
1989; Jenkinson, 1990; Lyngstad, 1990. Decay rates for different organic compounds Minderman, 1968
show that some of the N applied in 1 year is not min- eralised and thus exposed to N runoff within the same
year. Later release and runoff of this N would thus weaken the regression model when based on yearly
data. We speculate that such a delay also has con- tributed to the poorer description of the N runoff from
the forage systems, as these systems received a larger fraction of organic N than did the arable systems. This
effect was mainly eliminated in Step D, when the N balances were averaged over years. The effect of the
climatic variation was also eliminated by using
212 A. Korsaeth, R. Eltun Agriculture, Ecosystems and Environment 79 2000 199–214
average values, and a one-predictor model could then be used. The differences between the arable systems
were well described by this model Table 5, but those between the forage systems were not. Plotting the
results, however, revealed a different pattern between the two groups of cropping systems. The forage sys-
tems appeared to have a higher N balance threshold, below which the N runoff was insensitive to the N bal-
ance Fig. 4. We speculate that this may be explained by the effect of the perennial ley. Perennial grassland,
with a long growing period, takes up inorganic N at times when it would otherwise be exposed to runoff
e.g. Bergström, 1987; Gustafson, 1987. Our mea- surements of the N concentrations in drainage water
Month Numbers 10–12, Fig. 2 substantiate this hy- pothesis. The largest discrepancies between arable
and forage systems were found in autumn, after crop harvest.
5. Conclusions
