All simulations were run by considering the crops to be irrigated at three input levels, as in
Sto¨ckle et al. 1997: a ‘no irrigation’: rainfed maize; b ‘deficit’: maize was supplied with 50
mm when the difference between the cumulated Et and the rain reached this value; and c ‘full’:
maize was irrigated through a rigid scheduling which distributed 50 mm every 15 days in July
and August, as is often the case in the considered region, where water availability is high.
A 100 years were simulated for every cropping system, in each of the ten equiprobable soil hy-
drological conditions. The 1000 simulation results available for each
cropping system were described in terms of aver- age leaching over the whole period, standard devi-
ation and breakthrough probability curve of exceeding given levels of nitrate leaching.
3. Results and discussion
3
.
1
. Description of the systems Before analysing the leaching values and their
probabilities, a brief description of the systems, in terms of average values of crop uptake, nitrate
concentration in leaching and gaseous losses ob- tained from the simulations, is reported in Table
3.
Crop uptake was higher in sandy loam soil, but it was not so influenced by the simulated irriga-
tion and fertilization levels. The cropping systems with Italian ryegrass showed higher uptakes; the
N uptake increase due to the catch crop was higher in the sandy loam soil and in the ERM
systems. Gas losses are proportional to the fertil- ization levels in both soils and were higher when
irrigation was lower. They ranged between 14 and 27 of fertilization inputs in sandy loam soil and
between 12 and 22 in the loam one. The average concentration of NO
3
-N in leaching in most situa- tions exceeded the EU threshold for drinkable
water. The concentration was obviously depen- dent on fertilization level, while the effect of irri-
gation was different in the two soils. In the sandy loam soil the lower irrigation reduced the nitrate
concentration maybe because the effect of re- duced mineralization in a dryer soil; in the loam
soil the expected dilution effect prevailed.
3
.
2
. A6erage leaching 6alues The average leaching values for the different
cropping systems are reported in Table 4. As expected, leaching was higher: 1 with the
highest fertilization and irrigation inputs; 2 when crop uptake was smaller i.e. maize only vs.
maize-ryegrass; and 3 in soil with the greatest sand component.
Fertilization is the main leaching cause when it drastically exceeds crop uptake, as it was the case
for the 450 kg N ha
− 1
input level for the two soils and the three irrigation modes: in the case of the
‘full’ irrigation, average leaching losses reached 146 kg N ha
− 1
in the sandy soil and 108 kg N ha
− 1
in the loamy soil. If one excludes such an excessive fertilization
level, there is not one single dominating leaching cause amongst the different factors studied: fertil-
ization, irrigation, cropping system and soil type. All factors interacted creating different average
leaching levels of nitrogen loss. Maize silage MM caused the highest leaching losses 45.6 and
29.4 kg N ha
− 1
year
− 1
in the sandy and loamy soils, respectively. Maize grain MG resulted to
be slightly less risky: leaching was on average 3 kg N ha
− 1
year
− 1
less than MM in both soils. Nitrogen offtake was smaller in MG than in MM,
but the straw returned to the soil, owing to the high CN ratio, seemed to be effective in avoiding
the spring peak of nitrate concentration. This peak is often observed in northern Italy climate
after ploughing, fertilization and the increase of temperature in spring Patruno et al., 1989.
Straw also reduces a rise in nitrate concentration that starts after harvesting and stops when the
winter – autumn rainfalls originated a nitrate leaching. The maize-ryegrass systems show more
interesting possibilities to reduce leaching. The maize-ryegrass combination with the late ryegrass
harvest LRM showed an average leaching of 22.5 and 14.1 kg N ha
− 1
year
− 1
. Compared to MM, in the sandy and the loamy soil respectively
the early harvesting ryegrass system ERM en- hanced such a reduction, since average leaching
M .
Acutis et
al .
Europ .
J .
Agronomy
13 2000
191 –
206
Table 3 Simulated drainage, crop uptake above ground and root biomass, nitrate leaching and gaseous losses for the different cropping systems in the two soils
Cropping system Sandy-loam soil
Irrigation Loam soil
Crop uptake Drainage
Leaching Gaseous losses
Drainage Crop uptake
Gaseous losses Leaching
kg N ha
− 1
kg N ha
− 1
kg N ha
− 1
mg N l
− 1
mm y
− 1
mm y
− 1
kg N ha
− 1
mg N l
− 1
Maize silage 23.5
Full 35
114 316
13.1 31
134 398
25.2 37
83 315
12.9 399
31 92
MM; 200 Deficit
42 68
293 17.2
40 kg N ha
− 1
No irrigation 80
397 10.1
70.1 57
114 375
134 21.4
417 39
Full Maize silage
82.3 MM; 300
61 83
375 23.8
38 Deficit
92 417
78 68
371 24.4
54 55.4
kg N ha
− 1
417 80
No irrigation 108.6
86 114
428 Maize silage
95.9 Full
59 134
417 123.5
93 83
424 125.2
417 56
92 Deficit
MM; 450 110
68 399
119.9 72
kg N ha
− 1
No irrigation 80
417 95.1
27.5 45
107 309
126 17.3
Full 36
411 Maize grain
29.6 42
74 318
16.6 32
Deficit 87
MG; 200 410
52 60
298 21.5
45 13.6
No irrigation 410
kg N ha
− 1
71 416
Full 69.2
64 107
373 37.7
50 126
Maize grain 78.9
59 74
373 55.4
416 46
87 MG; 300
Deficit 81
60 355
59.4 64
kg N ha
− 1
No irrigation 71
416 51.7
457 12.3
28 112
Full 322
133 11.6
25 Late ryegrass-
452 10.3
28 78
318 11,5
maize LRM; 25
Deficit 92
5.3 31
64 297
14.4 37
451 No irrigation
200 kg N 75
ha
− 1
484 40.4
43 Late ryegrass-
112 Full
428 21.4
39 133
490 39.9
50 78
424 92
23.8 maize LRM;
38 Deficit
19.1 63
300 kg N 64
399 24.4
54 No irrigation
75 490
ha
− 1
474 7.3
22 Early ryegrass-
112 Full
336 10.0
30 132
474 6.3
28 84
334 10.1
maize ERM; 27
Deficit 99
4.5 29
66 318
11.4 34
459 200 kg N
No irrigation 78
ha
− 1
Early ryegrass- 132
548 18.7
41 112
444 13.1
41 Full
551 16.7
42 84
441 99
14.5 Deficit
40 maize ERM;
6.3 45
No irrigation 66
423 15.1
48 300 kg N
78 547
ha
− 1
Table 4 Simulated nitrate leaching for different cropping systems in the two soils
a
Sandy-loam soil Loam soil
Cropping system Irrigation
Average St. dev
Cv Average
St. dev Cv
31.6 Maize silage MM; 200 kg N ha
− 1
28.3 Full
89.6 14.5
11.3 77.9
23.1 24.5
106.1 Deficit
10.7 9.7
90.7 8.1
12.9 159.3
11.3 11.0
No irrigation 97.3
93.3 80.2
86.0 Maize silage MM; 300 kg N ha
− 1
53.7 Full
42.6 79.3
76.0 77.7
102.2 48.7
45.1 92.6
Deficit 41.7
59.3 142.2
37.2 No irrigation
36.6 98.4
Maize silage MM; 450 kg N ha
− 1
Full 146.2
127.4 87.1
108.2 86.5
79.9 Deficit
121.1 125.0
103.2 101.6
93.6 92.1
76.1 105.2
138.2 79.4
No irrigation 79.6
100.3 Maize grain MG; 200 kg N ha
− 1
Full 34.8
36.8 105.7
18.3 15.8
86.3 25.8
31.9 123.6
Deficit 12.1
13.3 109.9
9.7 17.2
177.3 12.9
14.8 No irrigation
114.7 87.5
81.8 93.5
Maize grain MG; 300 kg N ha
− 1
40.8 Full
36.6 89.7
68.8 77.8
113.1 41.2
Deficit 43.8
106.3 36.9
57.7 156.4
36.0 No irrigation
40.4 112.2
Late ryegrass-maize LRM; 200 kg N ha
− 1
Full 16.3
16.9 103.7
12.3 8.9
72.4 Deficit
9.5 13.4
141.1 8.2
7.5 91.5
4.0 8.8
220.0 8.6
No irrigation 8.2
95.3 Late ryegrass-maize LRM; 300 kg N ha
− 1
Full 53.8
50.9 94.6
23.1 16.9
73.2 36.9
43.5 117.9
Deficit 17.6
16.1 91.5
14.4 24.5
170.1 15.0
14.6 No irrigation
97.3 9.7
9.8 101.0
Early ryegrass-maize ERM; 200 kg N ha
− 1
10.7 Full
8.2 76.6
6.3 8.5
Deficit 134.9
7.9 7.4
93.7 3.5
7.7 220.0
7.0 No irrigation
7.0 100.0
24.8 22.8
91.9 Early ryegrass-maize ERM; 300 kg N ha
− 1
14.4 Full
11.1 77.1
16.6 18.8
113.3 Deficit
11.6 10.7
92.2 4.9
10.1 206.1
9.5 9.2
96.8 No irrigation
a
Average and standard deviation values are expressed in kg N-NO
3
ha
− 1
year
− 1
; the coefficients of variation cv are expressed in .
was only 11.0 and 10.2 kg N ha
− 1
year
− 1
. The LRM average leaching reduction was 51 and 52,
while the ERM reduction was 76 and 65, in each soil. This reduction in leaching for both
cropping systems can be explained by the increase in nitrogen uptake and by the known mitigating
effect of the winter crop cover that reduces the bare soil period. Early harvested ryegrass is more
effective in reducing leaching than late harvested ryegrass: this shows that, under these conditions,
it is more important to enhance maize uptake during summer than Italian ryegrass uptake in
early spring. When decreasing fertilization to a sub-optimal
level from 300 to 200 kg N ha
− 1
year
− 1
leach- ing was substantially reduced: from 46.6 to 15.2
kg N ha
− 1
year
− 1
in the sandy soil − 67 and from 29.0 to 11.3 kg N ha
− 1
year
− 1
in the loamy soil − 61,while the reduction in nitrogen up-
take and DM production was much lower. Water input is another way of controlling the
leaching risk. By irrigating maize with a fixed amount of water every 15 days leaching was on
average 44.0 and 23.5 kg N ha
− 1
year
− 1
in the two soils. When irrigation was limited to the soil
water deficit, the average leaching was reduced to 32.9 and 19.6 kg N ha
− 1
year
− 1
in the two soils, respectively − 25 and − 17. Without any irri-
gation, the leaching values were 15.4 and 17.2 kg N ha
− 1
year
− 1
− 65 and − 27. A variation of the irrigation management had a
more pronounced impact on leaching in soils with a higher conductivity at saturation and a lower
water holding capacity, as with sandy soil.
3
.
3
. Variability of leaching The standard deviations reported in Table 2 are
closely correlated to the means and show values that often exceed the means, particularly in the
sandy soil. These results confirm the importance of considering leaching variability when evaluat-
ing the environmental impact of cultivation sys- tems. This is particularly true when general
assumptions are based on experimental data, which often refer to a limited number of locations
and years.
The standard deviation to mean ratio coeffi- cient of variation increased when irrigation was
not applied. Irrigation therefore reduced leaching variability, which is otherwise dominated by
rainfall.
The breakthrough curves with the probability of exceeding a given value of leaching are re-
ported in Figs. 2 and 3, for the maize alone and the combination of maize and Italian ryegrass,
respectively.
The general shape of these curves is hyperbolic. The probability of exceeding a given value of
leaching changes rapidly in the first part of the curve leaching values lower than average but
much less in the second part leaching values higher than average. In other words, given a
particular cropping system, the change in proba- bility of exceeding two different leaching values is
higher when leaching is low.
This observation seems more relevant in crop- ping systems that are more exposed to the risk of
leaching i.e. MM highly fertilized and fully irri- gated in the sandy soil, while the shape of the
curve seems more linear in situations where aver- age leaching is lower i.e. ERM in the loam soil.
However, in all cases, lower than average leach- ing values are more frequent than higher ones: the
median is lower than the average. A possible application of these results concerns
the criteria to use when judging experimental re- sults: measured differences between treatments in
years or soil conditions that reduce leaching tend to be more consistent and predictive than in the
opposite conditions.
Figs. 2 and 3 show that curves rarely intersect or only do so when the differences are very small.
If other contrasts were studied for example com- paring different crops at the same fertilization and
irrigation level, some interactions would show up. For example, in the sandy soil at 300 kg N
ha
− 1
fertilization and full irrigation, MM shows a 50 probability of exceeding a leaching value of
75.6 kg N ha
− 1
, and a 20 probability of exceed- ing 167.8 kg. Under the same conditions MG
shows a lower risk at 50 breakthrough probabil- ity 65.3 kg N ha
− 1
, but a higher one at 20 probability 171.3 kg. Nevertheless, interactions
are few and always concern small differences from a practical point of view, as in the given example.
The data would therefore suggest that the classifi- cation of cropping systems with respect to NO
3
leaching, does not change according to the chosen breakthrough probability.
Ranking can also be used to judge cropping systems.
In each
environment, a
leaching threshold can be chosen, for example, on the basis
of the EU limit of 50 mg l
− 1
N-NO
3
in the predicted drainage flux.
The cultivation systems could be classified into three groups:
low risk L: average leaching is lower than the leaching threshold and the probability of ex-
ceeding the leaching threshold is lower than 20;
medium risk M: average leaching is lower than the leaching threshold and the probability
of exceeding is between 20 and 50;
high risk H: on average, leaching is higher than the leaching threshold.
The following is an example based on the pro- posed simulations.
For the environment characterized by the sandy soil and higher water drainage, the average water
Fig. 2. Breakthrough probability of exceeding nitrate leaching levels for maize cultivation systems with different fertilization and irrigation levels. Abscissa denotes soil profile depth in cm.
flux allows one to accept a leaching impact of 50 kg N ha
− 1
at the most leaching threshold for the sandy soil. In the loamy soil, character-
ized by a lower water drainage, this limit is 40 kg N ha
− 1
. An application of these criteria is reported in
Table 5. Cropping systems with silage maize or maize grain, fertilized with 300 kg N ha
− 1
or more, are at high risk of leaching when full irriga-
tion is applied to the sandy soil. The high fertiliza- tion of maize silage shows a high risk even when
irrigation is more carefully applied. In the other eight cropping systems, where the crop is more
likely to produce high leaching silage maize, fertilization is high, or irrigation exceeds the crop
requirements, the leaching level could be classified as medium. The remaining cropping systems are
in the low risk class.
Fig. 3. Breakthrough probability of exceeding nitrate leaching levels for maize-ryegrass cultivation systems with different fertilization and irrigation levels. Abscissa denotes soil profile depth in cm.
Table 5 Classification of different cropping systems in term of leaching risk H: high; M: medium; L: low to exceed a given level of nitrate
loss 50 and 40 kg N-NO
3
in the sandy-loam soil and in the loam soil, respectively Cropping system
Sandy-loam soil Fertilization
Loam soil kg N ha
− 1
No irrigation Deficit
Full No irrigation
Deficit Full
Silage maize MM 450
M H
H H
H H
M 300
M H
M M
H L
L M
L 200
L L
Grain maize MG 300
M M
H M
M M
200 L
L M
L L
L 300
L M
Late ryegrass maize M
L L
L LRM
200 L
L L
L L
L 300
L L
Early ryegrass maize L
L L
L ERM
200 L
L L
L L
L
The same criteria applied to the other soil show a sharper defined situation. Silage maize and grain
maize are at high or medium risk when fertilized with 300 kg N ha
− 1
. All other cases show to belong to the low leaching risk class. When com-
pared to the crop or to the crop fertilization, irrigation seems less important in the definition of
the leaching risk.
4. Conclusions
