204 A. Korsaeth, R. Eltun Agriculture, Ecosystems and Environment 79 2000 199–214
In Step A the annual N balances Bal
x
were used as only predictor z
1
= Bal
x
, z
2
= z
3
= 0. Since precip-
itation has been found to be an important factor for N runoff Jenkinson, 1990, annual total precipitation
Prec
t
was alternatively tested z
1
= Prec
t
, z
2
= z
3
= 0.
In Step B both calculated N balances and precipita- tion were included as predictors z
1
= Bal
x
, z
2
= Prec
t
, z
3
= 0. In Step C we added the total precipitation
of the previous year Prec
t − 1
as a third predictor z
1
= Bal
x
, z
2
= Prec
t
, z
3
= Prec
t − 1
. The rationale for so doing was the assumption that the amount of
leachable N, which is not lost in 1 year e.g. due to shortage of water for N transport in a dry year,
increases the runoff potential the following year due to nitrate storage below the root zone. In order to
eliminate the effect of the climatic variations on the regressions, we finally regressed N balances averaged
over all years against average N runoff Y=average N
runoff
, z
1
= average Bal
x
, z
2
= z
3
= 0 Step D.
All statistical tests were performed at the 0.05 level of probability.
3. Results
3.1. Nitrogen measurements The N content of cattle slurry averaged 2.47 g kg
− 1
SE=0.22, of which 58 SE=4.48 was ammonium- N. Nitrogen input via precipitation was 6.20 kg N ha
− 1
SE=0.40 on average for the 8 years. The amounts of harvested N differed significantly
between the cropping systems Fig. 1, p0.001, and the LSD
0.05
test showed significant differences be- tween all pairs but one; INT-A and ECO-A.
The dry matter yield for ley Table 3 differed significantly p=0.001 between cropping systems
sum of two cuts. Total dry matter production of ley was significantly lower in ECO-F than in CON-F
and INT-F LSD
0.05
. The content of red clover and alsike clover, averaged over cuts, differed signifi-
cantly between cropping systems p=0.014 and be- tween ley years p=0.009. ECO-F had significantly
higher content of clover LSD
0.05
than CON-F and ECO-F, which did not differ statistically. The con-
tent of white clover in ley was very low and had no significant sources of variation. For green fod-
der neither the dry matter yield nor the content of grey peas differed significantly between cropping
systems. Nitrogen concentrations in surface and drainage
runoff are shown on a monthly basis averaged over years in Fig. 2. The N concentrations in drainage
runoff as a yearly average averaged over months and years differed significantly p=0.002 between
the cropping systems. Highest N concentrations were found in drainage water from CON-A, whereas
ECO-A, INT-F and ECO-F had the lowest concentra- tions. The N concentration in surface runoff did not
differ significantly between cropping systems.
In 1988 the N content in topsoil measured at 5–10 and 20–25 cm and subsoil measured at 35–40, 50–55
and 65–70 cm was 2.7 g kg
− 1
SE=0.35, n=24 and 0.5 g kg
− 1
SE=0.06, n=24, respectively Riley and Eltun, 1994. This corresponds to 14.3 tonnes N ha
− 1
95 confidence limits: ±0.38 tonnes N ha
− 1
. Only the upper 25 cm of the soil was analysed in 1995, and
the N content averaged 2.6 g kg
− 1
SE=0.06, n=24. The measurements showed no significant differences
in N content between the cropping systems in 1995. Moreover, the soil N content had not changed signifi-
cantly between 1988 and 1995 for any of the cropping systems.
3.2. Nitrogen estimates The estimated symbiotic N fixation was much
higher for ECO-F than for the other systems with legumes Fig. 3. In all the forage systems the esti-
mated N fixation was larger in the first and the second year than in the third ley year. Clover grass ECO-A
fixed about the same amounts as the first year of ley in ECO-F.
The estimated NH
3
-N losses for crops receiving cat- tle slurry varied considerably, and averaged 13 of
total-N applied with the slurry for ECO-A and 23–27 for the forage systems. Estimates of ammonia volatil-
isation, N in seeds and denitrification are shown in Table 4 as average over all years for the entire crop-
ping system average over eight rotation plots.
3.3. Calculations The N transport via surface and drainage water
Fig. 2 reflected the differences in N concentra-
A. Korsaeth, R. Eltun Agriculture, Ecosystems and Environment 79 2000 199–214 205
Fig. 1. Total harvested N in the arable upper figure and forage systems lower figure for the years 1990–1997 at Apelsvoll.
tions between the systems. Total N runoff drainage plus surface runoff kg N ha
− 1
per year increased in the order INT-F 18.0ECO-F 19.5ECO-A
21.1INT-A 28.8CON-F29.7CON-A 34.9. The runoff losses contributed 10–25 of the total N
output from the systems. The fraction of inorganic N in runoff was 84.
The changes in soil N 1N are shown as average values in Table 4. All the arable systems had a reduc-
tion in the soil N content. INT-A and ECO-A had re- ductions of about 10 and 30 kg N ha
− 1
per year more than CON-A, respectively. The soil N changes were
small for CON-F and INT-F. The greatest reductions were calculated for ECO-A and ECO-F, which accu-
mulated to 357 and 341 kg N ha
− 1
over 8 years, re- spectively. This corresponds to approximately 2.5 of
the initial soil N pool. Linear regression analyses showed that both the cal-
culated mass N balances and precipitation Step A were positively related to N runoff selected results are
shown in Table 5. The annual precipitation explained more of the variation in N runoff between systems
and years than did the N balances. When combining these variables in a two-predictor model Step B, the
variation in N runoff was described better Table 5. Further improvement was made by adding the annual
precipitation from the previous year as a third predic- tor Step C, when 87 and 65 of the variation in
206 A. Korsaeth, R. Eltun Agriculture, Ecosystems and Environment 79 2000 199–214
Table 3 Dry matter yields of the crops including legumes and visually estimated legume content in the cropping systems at Apelsvoll mean
1990–1997
a
Crop Yield Mg DM ha
− 1
Legumes Red and alsike clover
b
White clover
d
Grey peas
c
Common vetch
e
Cut 1 Cut 2
Cut 1 Cut 2
Cut 1 Cut 2
ECO-A
f
Clover grass 4.10 0.36
3.88 0.15 50 4.6
b
70 4.9
b
CON-F
g
1st year of ley 5.31 0.23
5.48 0.23 22 1.5
b
24 4.2
b
2nd year of ley 6.19 0.19
5.03 0.35 18 4.3
b
21 4.1
b
0 0.1
d
3rd year of ley 5.28 0.45
4.54 0.35 12 3.3
b
14 4.5
b
Green fodder 4.33 0.40
2.83 0.14 48 4.9
c
3 1.4
c
INT-F
h
1st year of ley 5.39 0.32
5.23 0.23 40 4.0
b
35 5.2
b
2nd year of ley 5.97 0.23
4.58 0.33 24 5.6
b
27 4.6
b
3rd year of ley 5.76 0.22
3.90 0.31 13 4.5
b
16 4.6
b
0 0.3
d
Green fodder 4.49 0.28
1.96 0.20 47 5.2
c
6 2.3
c
ECO-F
i
1st year of ley 4.41 0.31
4.25 0.11 45 4.5
a
63 4.0
b
1 0.5
d
0 0.1
d
2nd year of ley 4.95 0.25
3.71 0.32 32 6.1
a
58 3.9
b
1 0.3
d
0 0.4
d
3rd year of ley 4.43 0.50
3.08 0.47 16 4.6
a
30 0.7
b
1 0.5
d
1 1.4
d
Green fodder 5.32 0.24
1.62 0.21 36 4.1
b
7 2.9
c
18 2.6
e
13 4.2
e
Peas and oats 4.08 0.27
12 2.0
b a
Standard error of mean in parentheses.
b
Red clover Trifolium pratense L. and Alsike clover Trifolium hybridum L..
c
Grey peas Pisum arvense L..
d
White clover Trifolium repens L..
e
Common vetch Vicia sativa L., sown in ecological greenfodder only ecological forage cropping.
f
Ecological arable cropping.
g
Conventional forage cropping.
h
Integrated forage cropping.
i
Ecological forage cropping.
runoff from the arable and the forage systems, respec- tively, could be explained. The linear regressions gave
generally poorer fits to data from the forage systems than from the arable systems. No improvement in the
regressions was gained when all negative N balances where excluded or when only data from drainage N
runoff was used i.e. excluding surface N runoff or by using ungrouped data data from both arable and
forage systems data not shown. Only three N flows, fertiliser-N, N in slurry and harvested N, were needed
to calculate the N balance which was best suited to pre- dict N runoff Bal
simple
= N
fertiliser
+ N
slurry
− N
harvest
. Including more N flows in the N balance did not im-
prove the model performance. When we used data averaged over all 8 years Step
D, instead of data on a yearly basis, most of the vari- ation in N runoff between the arable systems could
be explained by Bal
simple
. No such significant rela- tion between accumulated balances and N runoff was
found when using the same model on the data from the forage systems. Plots of N runoff against N bal-
ance for the different systems shown for Bal
complex
, Fig. 4 suggest, however, that a positive relationship
exists also in the forage systems. The plots further in- dicate the existence of a threshold for N balance, be-
low which the N runoff is unaffected by the N balance. This threshold appears to be different for arable and
forage systems.
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
