Fig. 1. Changes in the nutrient solution pH due to NH
4 +
and NO
3 −
uptake by wheat seedlings. Dotted lines show the pH correction by the addition of H
2
SO
4
or KOH, respectively.
filling and full maturation, respectively. These small quantities, which may be accounted for by
losses of root hairs, amount merely to 0.1 and 0.25 of the total
15
N in the plant + soil at the beginning of grain filling and full maturation,
respectively. In the mixed sample soil + roots second pot
60 mg dry matter was needed for
15
N analysis. For this purpose the total amount of soil + roots
was dried and ground together. The mixed sample was than treated twice with a sample separator
Retsch to obtain a representative subsample of
10 g for
15
N analysis. To prove whether the sample separation did influence the analytical re-
sults, several subsamples taken from one soil + root sample were analyzed for
15
N enrichment. The
15
N concentrations of these subsamples did not differ statistically from one another.
It was not possible to measure
15
N in the plants at the date of transplantation because of the very
high
15
N enrichment values over 80, and be- cause with the equipment used Vario EL and
N01-6 only a limited concentration of
15
N in plant dry matter can be measured. We therefore
used the
15
N in the plants + soil at tillering as a control at which stage the
15
N in the plant had been diluted to 66.
The total N and
15
N excess analysis were car- ried out using a Vario EL elementary analyser,
connected with an N01-6 emission spectrometer Fischer, Germany at the Institute of Plant Nu-
trition and Rhizosphere Research in ZALF, Mu¨ncheberg.
In the 1st year the experiment included eight, and the 2nd year, six biological replications.
Statistical elaboration of the data was done using the t-test according to Ko¨hler et al. 1996 for
measuring statistical differences between pairs and ANOVA, and t-test for the statistical differences
between growth stages.
3. Results
3
.
1
. Preliminary experiment in6estigating the relationship between nutrient solution pH and the
depletion of ammonium and nitrate in the nutrient solution
The relationship between the pH in the nutrient solution and the depletion of the ammonium and
nitrate in the nutrient solution is shown in Fig. 1 for the three nutrient concentration levels applied:
14, 12 and full strength nutrient solution. In all three cases the concentration of ammonium de-
clined and as soon as the ammonium concentration attained zero level a stronger decline of the nitrate
concentration was found. From this pattern it is clear that ammonium was preferentially taken up
by plants. Ammonium uptake was associated with a pH decrease, nitrate uptake in the later phase
with a pH increase of the nutrient solution. Hence the pH increase indicated that the N in the nutrient
solution was going to be depleted and new addition of
15
N in the form of ammonium nitrate was required.
3
.
2
.
15
N release into the soil and unaccounted for losses
Table 2 shows the cumulative
15
N amounts released into the soil and losses measured indirectly
by difference Eq. 2 for the 1st and 2nd years. The difference found at tillering between
15
N in the plant and
15
N in the plant + soil was not statisti- cally significant, while these differences at later
stages ear emergence, beginning of grain filling, maturation were significant indicating that
15
N had been released into the soil.
15
N release rates showed a maximum between ear emergence and
the beginning of grain filling Table 2 and were at this stage about three times higher than in the
phase of maturation as found in both years. The cumulative amounts of
15
N released into the soil were 1.06 and 1.89 at tillering, 3.43 and 4.36 at
ear emergence, 8.94 and 7.71 at grain filling, and 13.00 and 11.03 at maturation for the 1st and 2nd
year, respectively Table 2. These percentage val- ues relate to the
15
N present in plant + soil at tillering = 100. The cumulative amounts of
15
N released at the beginning of grain filling and at
maturation were significantly higher than the amounts released at the earlier stages Table 2.
Both years gave practically the same qualitative results.
15
N losses other than those released from roots into the soil indirectly measured are shown
in Table 3. These were only 13 of the
15
N released into the soil by the roots. Similarly to the
losses via the roots, highest release rates for the unaccounted losses were found between ear emer-
gence and the beginning of grain filling.
Table 2
15
N release from the roots into the soil at different developmental stages of the crop as calculated by the difference between
15
N in the plant+soil and
15
N in the plants at the corresponding stage
a
Release rate
15
N in plant
15
N in plant+soil Development stage
Cumulative
15
N release into the soil mg
mg day
− 1
mg mg
1
st year 0.39
a
1.06 Tillering
36.86 0.35 36.47 0.58
35.34 0.22 36.59 0.36
1.25
a
3.43 0.045
Ear emergence 3.26
b
8.94 0.241
Beginning of grain filling 36.19 0.41
32.93 0.71 35.11 0.38
30.35 0.51 Full maturation
4.76
b
13.00 0.064
2
nd year Tillering
0.52
a
27.64 0.15 1.89
27.12 0.27 27.27 0.46
26.07 0.18 Ear emergence
1.21
a
4.36 0.020
26.87 0.31 2.13
b
Beginning of grain filling 24.74 0.21
0.077 7.71
23.61 0.76 26.66 0.29
Full maturation 3.05
b
11.03 0.027
a
release, proportion of
15
N in plant+soil and at tillering. Standard errors are shown in brackets. Significant differences between
15
N release at various stages are indicated by letters; different letters meaning a significant difference. PB0.05, significantly different between
15
N in plant +soil and
15
N in plant. PB0.001, significantly different between
15
N in plant +soil and
15
N in plant.
Table 3 Cumulative unaccounted for losses of
15
N at different developmental stages Daily release
Development stage Further losses
15
N in plant+soil mg day
− 1
mg mg
1
st year –
– 36.86
– Tillering
36.59 Ear emergence
0.27 0.7
0.011 0.66
1.8 Beginning of grain filling
0.039 36.19
1.74 4.7
35.11 0.027
Full maturation
2
nd year Tillering
– 27.64
– –
0.37 1.34
27.27 0.010
Ear emergence 26.87
Beginning of grain filling 0.77
2.79 0.033
Full maturation 0.99
26.66 3.58
0.007 Indicates significances at P = 0.05 between the
15
N amount in plant+soil at tillering and the
15
N amounts in the plant+soil at the respective development stages.
These unaccounted losses may result from gaseous losses NH
3
, NO
2
or losses of plant material. Under our experimental conditions
losses of plant material could only have occurred by dropping of anthers. At flowering, anthers
were therefore collected in both years and analysed for total N and
15
N enrichment. It was found that the anthers contained 1 and 0.8 of
the total
15
N in the soil-plant system at the begin- ning of the experiment for the 1st and the 2nd
year, respectively.
15
N enrichment shown in Table 4 considerably decreased from tillering to ear emergence in
shoots and roots, and then remained at almost the same level until maturation. This decrease in
15
N was due to the uptake of
14
N from the soil which occurred practically from tillering until ear emer-
gence. Thereafter practically no net uptake of nitrogen was found as shown in Table 5.
This finding is in line with results of other authors who also found that most plant species
need a lot of nitrogen during the vegetative phase while in the reproductive phase nitrogen uptake
by roots is low and retranslocation of organic nitrogen in the plant plays an important role
Imsande and Touraine 1994. At tillering roots had a lower level of
15
N enrichment than shoots which shows that at this stage the
15
N of roots was more diluted than shoots by newly taken up
14
N from the soil. Table 5 shows the dry weight, the N concentrations, and the N content of shoots
and roots at the four growth stages as well as comparative values for the ears at maturation. At
ear emergence no further dry matter production of shoots occurred. The same is also true for the
total N uptake
14
N +
15
N of the plants. Table 6 shows the amounts of
15
N present in the plant organs at the various growth stages.
Table 4
15
N enrichment in atom excess in different plant organs during the different developmental stages
Development stages Roots
Shoots Ear
1
st year 66.86
49.87 Tillering
18.55 Ear emergence
17.52 Beginning of grain filling
18.70 16.64
14.67 14.91
19.74 Full maturation
2
nd year 54.17
Tillering 35.99
12.56 Ear emergence
14.41 11.85
Beginning of grain filling 12.52
11.91 Full maturation
10.76 12.18
12.82 PB0.001, significant difference between shoots and
roots, and shoots and ear.
Table 5 Weight, N concentration and N content in shoots, roots and ears at different developmental stages
Tillering Ear emergence
Beg. grain filling Full maturation
1
st year Shoots
Weight, g 1.15
8.11 9.17
7.53 Total N, g kg
− 1
40.3 17.5
15.3 7.2
46.18 142.23
Total N, mg 140.61
53.78 Weight, g
Roots 0.62
3.63 3.85
3.97 Total N, g kg
− 1
17.2 12.9
11.9 7.6
10.57 46.88
Total N, mg 45.79
30.19 Weight, g
Ears 4.38
Total N, g kg
− 1
24.4 Total N, mg
106.72 Weight, g
Total 1.76
11.74 13.02
15.88 56.76
189.11 186.40
Total N, mg 190.69
2
nd year 1.03
7.70 Weight, g
6.94 Shoots
7.88 Total N, g kg
− 1
40.5 18.4
16.3 7,4
Total N, mg 41.7
141.5 112.8
58.1 0.87
3.38 Weight, g
4.23 Roots
3.98 Total N, g kg
− 1
16.0 17.0
14.7 12.7
13.9 57.4
Total N, mg 62.0
50.5 Weight, g
Ears 1.97
3.97 16.5
Total N, g kg
− 1
26.6 Total N, mg
32.6 105.5
Weight, g Total
1.89 11.09
13.14 15.83
55.6 198.9
207.4 214.1
Total N, mg
This shows clearly that in the stage of ear emer- gence and at the beginning of grain filling the
15
N amounts in roots were increased, and that from
the beginning of grain filling to full maturation the highest proportion of labelled N was present
in the ears.
4. Discussion
