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This article was downloaded by: [Cornell University Library]On: 19 November 2014, At: 02:15Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: MortimerHouse, 37-41 Mortimer Street, London W1T 3JH, UK
Acta Agriculturae Scandinavica, Section B — Soil &Plant SciencePublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/sagb20
Crop uptake of 15N labelled fertilizer in spring wheataffected by application timeJ. Petersena Department of Agroecology, Research Centre Foulum , Danish Institute of AgriculturalSciences , P.O. Box 50, DK-8830, Tjele, DenmarkPublished online: 02 Sep 2006.
To cite this article: J. Petersen (2004) Crop uptake of 15N labelled fertilizer in spring wheat affected by application time,Acta Agriculturae Scandinavica, Section B — Soil & Plant Science, 54:2, 83-90, DOI: 10.1080/09064710410024453
To link to this article: http://dx.doi.org/10.1080/09064710410024453
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Crop Uptake of 15N Labelled Fertilizer inSpring Wheat Affected by ApplicationTime
Introduction
Nitrogen (N) fertilizers enriched with the stable 15N
isotope have commonly been used for determination of
crop utilization of fertilizer N as affected by N-form
(Recous et al., 1988, 1992; Malhi et al., 1996; Petersen
et al., 2004), application method (Malhi et al., 1989,
1996; Malhi & Nyborg, 1991), placement geometry
(Petersen, 2001) and precipitation after surface appli-
cation (Hartman & Nyborg, 1989; Powlson et al.,
1992). Similarly, the 15N labelling method can be used
to investigate the crop recovery affected by application
time. In experiments with two or three application
times, the crop 15N recovery at maturity was increased
by postponing the time of N application from the
tillering phase to the beginning of stem elongation
(Leitch & Vaidyanathan, 1983; Riga et al., 1988;
Limaux et al., 1999). However, in these experiments
there was about six weeks between the first and second
application, and the course of change in crop 15N
recovery may not be estimated satisfactorily with such
a wide span in application times.
Detailed knowledge about the variation in crop
utilization of fertilizer N applied during the vegetative
phase is useful for farmers’ decisions regarding time of
application of fertilizer.
In addition, crop 15N recovery at maturity may be
affected by losses during the grain-filling period of
previously recovered 15N (Recous et al., 1988; Peter-
sen, 2001). Several pathways for N losses have been
suggested (Wetselaar & Farquhar, 1980), but the
magnitude of each pathway depends on the growth
conditions. Nevertheless, the sum of losses may be of
significance and may interact with the time of fertiliza-
tion (Yoneyama, 1983). Therefore recordings at har-
Petersen, J. (Department of Agroecology, Danish Institute of Agricultural
Sciences, P.O. Box 50, DK-8830 Tjele, Denmark). Crop uptake of 15N
labelled fertilizer in spring wheat affected by application time. Accepted
February 20, 2004. Acta Agric. Scand., Sect. B, Soil and Plant Sci. 54:
83�/90, 2004. # 2004 Taylor & Francis.
Time of nitrogen fertilizer application on crop recovery was studied in a
field experiment at Foulumgaard, Denmark, in 2001. A solution of 15N-
ammonium-15N-nitrate was applied in bands parallel to a single row of
spring wheat grown in frames of 30 cm�40 cm. The labelled fertilizer
was applied on 16 dates with intervals of 4�/5 days from tillering to the
start of grain-filling. Crop 15N recovery increased by 0.47%-point day�1
when the time of fertilizer application was postponed from tillering untilthe second node stage (GS32). On the other hand, a decrease in crop 15N
recovery of �/0.19%-point day�1 was recorded from GS47 to maturity
(GS85�/87). The effect of the 16 application times on 15N recovery was
described by two straight lines having intersection at the time of full
expanded flag leaf and ear emergence halfway (GS55). It was concluded
that leaf area expansion is important for crop N demand and 15N
recovery.
J. Petersen
Department of Agroecology, ResearchCentre Foulum, Danish Institute ofAgricultural Sciences, P.O. Box 50, DK-8830 Tjele, Denmark
Key words: 15N-ammonium-15N-nitrate, direct injection, fertilizer time,pulse-labelling, recovery.
DOI: 10.1080/09064710410024453 83
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vest may be affected by both the time of fertilization
and losses during the grain filling period.
The present study had two aims. The first aim was
methodological, concerning determination of the in-
fluence of sampling time on the crop uptake of applied
fertilizer N, especially exposure to any interaction
between sampling time and time of fertilizer Napplication. The second and principal aim was to
obtain detailed information about the changes in crop
recovery of fertilizer N applied many times at regular
intervals of a few days during the vegetative growth
phase of a small-grain cereal crop.
Materials and methods
A field experiment on 15N pulse labelling was carried
out in micro-plots at Research Centre Foulum, Den-
mark (56830? N, 9835? E). The soil was a TypicHapludalf according to Soil Survey Staff (2003), with
a topsoil of loamy sand characterized by 9% clay, 13%
silt (2�/20 m), 45% fine sand (20�/200 m), 31% coarse
sand (200�/2000 m), 1.6% C and a pH(CaCl2) of 6.2.
The entire experimental area had 60 kg N
ha�1 applied as ammonium nitrate using a commer-
cial granular NPK fertilizer (16:4:12) for basic fertili-
zation before seedbed preparation. This basicfertilization represents about half the normal N rate
for spring wheat, and may delay a serve shortage of
nutrients without overshadowing the effects of the 15N
treatments. Flexible plastic frames of 30 cm�/40 cm
bordering each of the 114 micro-plots were inserted
into the soil to a depth of 11�/13 cm to prevent 15N
uptake by plants outside the micro-plot. A single 33
cm row of spring wheat (Triticum aestivum L. cv.
Vinjett) was sown on 17 April 2001 in each frame, andemergence was recorded on 3 May. The inter-seed
distance was 0.8 cm to ensure an uninterrupted row.
Weeds were removed frequently, by hand using a claw-
like weeding tool.
Treatment and sampling
A total of 38 treatments (Table 1) in three replicateswere laid out in a randomized complete block design.
A solution of ammonium nitrate (2.624 M , 3.3824 (s.e.
0.0007) and 3.2922 (s.e. 0.0135) 15N atom% for
ammonium-N and nitrate-N, respectively) was applied
by injection as subsurface bands parallel to the crop
row. A 15N pulse was applied at one of 16 dates with
intervals of 4�/5 days from 10 May to 18 July (Fig. 1).
This period corresponds to growth stages (GS) 11�/ 67at the BBCH scale (Lancashire et al. , 1991) and covers
the entire vegetative growth phase of the spring wheat
crop. For all 16 15N pulses, and a treatment without15N pulse (unfertilized), plants were grown to maturity
Table 1. Times of 15N application and sampling: growth stages (GS) are according to the BBCH scale (Lancashireet al., 1991)
Samplings and growth stage
Time of 15N application(pulse number)
I: Opening of flagleaf sheath, GS47
II: End offlowering, GS67
III: Grain-filling,milky ripe,GS75�/77
IV: Yellow ripeness,dough stage,GS85�/87
1 �/ �/ �/ �/
2 �/ �/ �/ �/
3 �/ �/ �/ �/
4 �/ �/ �/ �/
5 �/ �/ �/ �/
6 �/ �/ �/ �/
7 �/ �/ �/ �/
8 �/
9 �/
10 �/
11 �/
12 �/
13 �/
14 �/
15 �/
16 �/
Unfertilized �/
J. Petersen
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and sampled on 15 August, 104 days after emergence
at sampling IV (Table 1). In parallel plots, plants that
had been 15N pulse fertilized at one of the first seven
dates, GS11-32, were sampled on 27 June, 16 July and
30 July, indicated by Samplings I, II and III, respec-
tively (Table 1, Fig. 1).
The fertilizer band was located 2 cm from the crop
row and 8 cm below the soil surface. The depth and
distance from the crop row were both within an
accuracy of less than 9/0.5 cm. The fertilizer band
was divided into 33 points, 1.0 cm apart. A 0.18 cm ¥needle, used for injection at each point, was connected
to a PiP1CKC pump (Fluid Metering Inc., USA). A
total volume of 5.13 ml was applied to each plot. The
pump ensured that 1/33 of the total volume per plot
was applied at each point. The application rate of
0.377 g N plot�1 corresponded to 31 kg N ha�1.
Number of plants, shoots and ears were counted in
plots grown to maturity on 15 May, 12 June and 4 July,
respectively, and the ear:shoot ratio was calculated.
Mean air temperature and precipitation during the
experimental period are shown in Fig. 1.
The plants were cut by shears 0.5�/1 cm above the
soil surface, avoiding contamination by soil particles.
Total plant dry matter (DM) was determined after 18
h of drying at 808C. The grain DM at maturity was
determined after threshing, and the straw was coarse-
ground using a Retsch SM2000 cutting mill. All
samples were finely-ground in a Retsch MM2000 ball
mill (Retsch GmbH & Co, Germany).
Percentage of total N and 15N atom abundance was
measured on a continuous-flow isotope ratio mass
spectrometer (ANA-MS method) (Jensen, 1991). The
natural 15N abundance in plants was determined in
unfertilized plants (0.3682 15N atom%, s.e. 0.00015).Calculation of crop 15N recovery was performed
according to IAEA (1976).
Statistics
The number of days from spring wheat emergence to
the time of 15N application, Fday, and the number ofdays from spring wheat emergence to the time of
sampling, Sday, were both included for the analysis of
crop 15N recovery, Y, at the first seven dates of
fertilization (Eqn 1). In addition, the interaction of
fertilization and sampling time was included using the
procedure GLM (SAS Institute, 1996).
Y �a�b Fday�c Sday�d Fday Sday (1)
Crop 15N recovery at maturity, Y, was analysed using a
segmented model consisting of two linear regressions
(Eqn 2a and 2b), having miximum at (Fday(max ), Ymax ),
where the straight lines intersect.
Y �a1�b1 Fday if Fday 5Fday(max) (2a)
Y �a1�b1 Fday if Fday]Fday(max) (2b)
The segmented model was also used for the ear:shoot
ratio, and the parameters were estimated by the
procedure NLIN (SAS Institute, 1996). For the grain
nitrogen concentration and nitrogen derived fromfertilizer (Ndff) in grain as well as straw, a simple
single linear regression model was applied, using the
procedure GLM (SAS Institute, 1996).
Results
Time of fertilization and sampling
The interaction between time of fertilization and
sampling in Eqn (1) was insignificant with respect to
crop 15N recovery, and therefore was excluded in
estimation of the parameters. This means that the
effects of fertilization and sampling time on crop 15N
recovery are additive. Postponement of the fertilizertime increased crop 15N recovery by 0.47%-point
Fig. 1. Mean air temperature (line) and precipitation (bars) during
the experimental period. The 16 dates for fertilization (triangles) and
the four sampling times (dots) are indicated. Filled triangles
correspond to the seven fertilizer pulses also sampled at times I-
III. All 16 application dates were represented at sampling IV.
Table 2. Recovery of applied 15N (%). Estimates 9/standard errors for the parameters in Eqn (1)
Intercept, a Estimate for b (Fday) Estimate for c (Sday) DF R2
67.39/3.26 0.4679/0.0704 �/0.1879/0.0342 72 0.52
Uptake of 15N affected by application time
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day�1 until the second node stage (GS32) (Table 2).
This positive effect was counteracted by a decrease of
0.19%-point day�1 during the grain-filling period.
Sampling at maturity
Application of the 15N fertilizer solution increased dry
matter yield and N uptake compared to the basic
fertilization only (Table 3). The average grain andstraw dry matter yield of 15N fertilized plots corre-
sponds to 3.9 and 6.2 t ha�1, respectively, using the
framed plot area as the basis. N uptake was not
affected by application time (data not shown), and
only a weak decrease in DM by time was observed
(Fig. 2a). The decrease in DM was estimated by linear
regression (R2�/0.10) to �/0.394 (P�/0.04) and
�/0.166 (P�/0.03) g DM plot�1 day�1 for totalDM and grain DM, respectively.
In contrast, crop 15N recovery (Fig. 2b) and the
ear:shoot ratio (Fig. 2c) depends on the application
time. The description of grain 15N recovery using the
segmented model (Eqn 2) was better than for total
crop 15N recovery (Table 4). The numbers of plants
and shoots were not affected by the time of fertilizer
application (data not shown), whereas the segmentedmodel was able to describe the effect on the ear:shoot
ratio, despite a low R2 value (Table 4).
The estimates in Table 4 were used for preparing the
straight lines in Fig. 2b and c. Fday(max ) was solved
setting the right sides of Eqn 2a and 2b equal to each
other and inserting the estimated parameters, and then
Ymax was calculated inserting Fday(max ) in Eqn 2 (Table
4).The NLIN procedure was not able to fit Eqn. 2 to
straw 15N recovery, and a simple linear regression
(R2�/0.24) shows a significant decrease of only
�/0.055%-point day�1. Therefore the recovery in
grain has a major influence on the recovery in the
total crop, and the difference between the estimates of
the slopes (b1 and b2 in Eqn. 2) for grain and total crop
is moderate (Table 4), resulting in a near parallelcourse of both parts of the segmented model in Fig. 2.
Both 15N recovery in grain and total crop are at
maximum at fertilization on day 58, whereas
the ear:shoot ratio is at maximum 3 weeks earlier
(Table 4).
In contrast to both the 15N recovery and the
ear:shoot ratio, the grain N-concentration was in-
creased linearly (Fig. 3), corresponding to an increase
by 0.04%-point protein for every day the N application
was postponed (calculated from Table 5). However, the
Table 3. Average dry matter yields and N uptake in plots with and without application of 15N fertilizer solution.9/ standard errors
Basic fertilized (n�/3) Basic fertilized�/15N fertilizer solution (n�/46)
Dry matter (g plot�1) N uptake (g plot�1) Dry matter (g plot�1) N uptake (g plot�1)
Grain 41.89/2.43 0.829/0.057 46.89/1.66 0.999/0.030Straw 75.69/5.66 0.279/0.013 73.99/2.44 0.309/0.011Total 117.39/7.61 1.099/0.069 129.69/4.03 1.299/0.040
Fig. 2. Dry matter (a), crop recovery of applied 15N (b) and ear:
shoot ratio (c). The recorded values for the 16 application dates are
mean 9/ standard error. The regression lines in (b) and (c) are based
on the estimates in Table 4 using Eqn. (1). The regressions for the
lines in (a) are mentioned in the text.
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segmented model was also able to describe changes in
N concentration (R2�/0.61), but the higher degree ofexplanation depends very much on the low values
recorded on days 34 and 39 (Fig. 3), that may be
caused by the low temperatures during a rainy period
(Fig. 1). Grain Ndff also increased linearly with
postponement of fertilization time, accompanied by a
decrease in straw Ndff. In total, Ndff for the total crop
increased.
Discussion
Sampling time and loss of 15N
There was a minimum time interval of three weeks
between fertilizer application and sampling, to ensure
that maximum crop 15N recovery was obtained
(Petersen, 2001). However, the crop is not exclusively
a sink for fertilizer N (Recous et al. , 1988), as therealso are losses of 15N previously taken up. The mean
decline of crop 15N recovery was 0.19%-point day�1,
corresponding to 9.3%-point from sampling I to
sampling IV for the first seven dates of N application.
In this experiment, the decline in crop 15N recovery
during the grain-filling period was not significantly
affected by the fertilizer application time, but it is likely
that losses during grain filling may reduce earlier and
more pronounced differences. Thus, during the growth
of winter wheat to maturity, Yoneyama (1983) re-ported reductions of 15N recoveries for fertilizer
applied at the very beginning of the growth cycle.
The average decline in crop 15N recovery during the
grain-filling period was below the 12%-point observed
by Petersen (2001), but above the values reported by
Nielsen & Jensen (1986) and Recous et al. (1988). The
decline depends on application rate and year (Schjør-
ring et al., 1989) who suggested ammonia volatilizationfrom the aerial parts of the plants as the major source.
Thus, sampling time had an important influence on
the evaluation of treatments earlier applied. Losses
from the crop need to be explored further, and
attention must be drawn to the crop as more than
simply a sink for N. Despite a significant loss of
previously recovered 15N and the potential for inter-
action with time of sampling, the recordings atsampling IV for the 16 treatments were used for
estimation of the effect of fertilization time.
Time of fertilization
Averaged over of the four sampling dates, the crop 15N
recovery increased by postponement of the time of
fertilization from tillering to the second node stages
(GS32). However, the recovery at the sixth fertilization
pulse was low compared with the fifth and seventhfertilization pulse, irrespective of sampling time (see
Fig. 2b for sampling IV). Powlson et al. (1992)
reported that unrecorded 15N (expressing the loss
from the plant:soil system) was linearly related to the
accumulated precipitation during the three weeks
succeeding fertilizer N application. Thus, the 21 mm
precipitation received on the first two days succeeding
the sixth application pulse (Fig. 1) may have caused Nlosses, explaining the low recovery for this pulse.
Plant availability of the remaining 15N may be
further reduced by immobilization, which potential
depends on the distribution of the applied mineral N
in the soil volume. Petersen et al. (2004) used the same
soil type as reported here and found that 13% of the
applied N was immobilized by broadcasting, but
Table 4. Estimates9/approximately standard errors of the parameters in the segmented model (Eqn. 2) for crop15N recovery and the ear:shoot ratio
Increasing line (Eqn. 2a) Decreasing line (Eqn. 2b) Maximum
Parameter a1 b1 a2 b2 DF R2 Fday(max) Ymax
Recovery; grain (%) 33.49/1.65 0.4529/0.0465 101.89/20.74 �/0.7229/0.300 42 0.72 58.3 59.7Recovery; total (%) 47.79/1.95 0.3979/0.0547 119.69/24.40 �/0.8289/0.353 41 0.59 58.7 71.0Ear:shoot ratio (%) 64.69/3.47 0.5099/0.1575 93.89/6.38 �/0.3309/0.111 42 0.31 35.0 82.3
Fig. 3. Grain nitrogen concentration in grain for 16 application
dates (mean 9/ standard error). The regression line is based on the
estimates in Table 5.
Uptake of 15N affected by application time
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immobilization did not occur by banding ammonium
sulphate. A similar difference has been obtained for
urea, but the quantity depends on the soil immobiliza-
tion potential (Tomar & Soper, 1981). Thus, fertilizer
banding increases the amount of plant available
fertilizer N compared with incorporation into the
topsoil. These results are in accordance with the
findings of increased crop recovery of banded 15N-
urea (Tomar & Soper, 1981, 1987; Carter & Rennie,
1984; Malhi et al., 1989, 1996; Malhi & Nyborg, 1991).
In a pulse-labelling experiment applying eight pulses at
two-week intervals, Recous & Machet (1999) recorded
immobilization of 13�/16% of applied N, irrespective
of the date of application, and in addition, immobili-
zation was affected by neither the applied N-form
(Recous et al., 1992) nor soil type (Powlson et al.,
1992). These observations, together with the use of
fertilizer banding, make it implausible that immobili-
zation is the cause of the variation in the obtained data
for crop 15N recovery.
Hence, minimizing immobilization by banding, the
crop 15N recovery was described by a segmented linear
function using Fday, the day of fertilization counted
from emergence, as an independent variable. The
maximum recovery in the total crop was estimated to
58 days after emergence corresponding to fully ex-
panded flag leaf and ear emergence halfway (GS55).
The ability of cereals to translocate N from vegetative
to reproductive organs is presumably the reason why
the straw 15N recovery was only modestly affected by
the application time. Therefore, the total crop 15N
recovery was nearly parallel to the grain 15N recovery
(Fig. 2) with an estimated maximum also on day 58.
Applying the segmented linear model (Eqn 1) to the
results of Recous & Machet (1999), the maximum
recovery of total crop sampled one or two weeks after
pulse-labelling was obtained about two weeks before
ear emergence (heading). Applying the segmented
linear model to the recorded total crop 15N recovery
in a pot experiment using seven pulse-labellings with
7�/14 day intervals (Esala, 1991) maximum was
estimated to day 42, corresponding to the flag leaf
stage. Thus, there is a general indication that max-
imum crop 15N recovery takes place in the final part of
the elongation phase, around the flag leaf stage.
Leitch & Vaidyanathan (1983) suggested that the
crop N demand expressed by crop growth rate and
development stage may explain the increased 15N
recovery with applications late in the vegetative growth
phase. However, due to crop growth, the crop N
demand changes with time and therefore the effects
of time and crop N demand are linked. Limaux et al.
(1999) isolated the effect of crop N demand per area
unit by changing the plant density, and they found that
crop 15N recovery at maturity was linearly related to
the crop growth rate at fertilization. For pre-anthesis
growth of winter wheat, Olesen et al. (2002) found that
N uptake was a linear function of the green leaf area
(indexed and designated as GAI), and a declining
function of crop dry matter. Thus the green leaf area is
a very important parameter for crop N demand, and
combining three well-known functions, the daily leaf
expansion may be described as the minimum of either,
1) the exponential increase of GAI in thermal time, 2)
a minimum leaf area:soil area ratio, or 3) a minimum
GAI:N-uptake ratio (Olesen et al., 2002). In wheat, the
flag leaf represents a significant part of the green area,
and the flag leaf emerges and unfolds in the final part
of the elongation phase. It is therefore suggested that
maximum 15N recovery is related to the flag leaf
growth.
However, the farmer’s decision regarding applica-
tion time may not exclusively be based on maximum
crop 15N recovery. An essential criterion is also that
the applied fertilizer has to be available to the crop. An
immediate availability of surface applied fertilizer N is
obtained by precipitation or irrigation events immedi-
ately after application, whereas the fertilizer may
remain at the surface in dry conditions (Hartman &
Nyborg, 1989). However, excessive precipitation in-
creases the losses of applied N (Powlson et al., 1992).
Thus weather forecast information is important for
decisions regarding application time. The direct injec-
tion application method used in this experiment
prevents fertilizer stranding at the soil surface, and
the applied N is assumed to be immediately available
to the roots. Another criterion for the timing of
application may be a high concentration of protein
in the grain. Contrary to crop 15N recovery, a linear
model was able to describe the increase in grain N
Table 5. Estimates9/standard errors of linear regression parameters for grain N concentration and Ndff for grain,straw and total crop
Parameter Intercept, a Slope, b DF R2
Grain N concentration (%) 1.919/0.039 (6.129/0.86)�/10�3 42 0.56Grain Ndff (%) 13.99/1.34 0.1329/0.0251 42 0.44Straw Ndff (%) 18.29/1.01 �/0.0639/0.0190 41 0.29Total crop Ndff (%) 14.99/1.25 0.0879/0.0234 41 0.33
J. Petersen
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concentration from application time 1 to 16 (Fig. 3).
The increased grain N concentration may be related to
the increase in grain-N derived from fertilizer (grain-
Ndff) with postponement of the application time (Table
5). The increase in grain-Ndff was in favour of straw-
Ndff, and Riga et al. (1988) observed a similar change
in N partitioning between plant parts for threeapplication times during the elongation phase of
winter wheat. Along with N utilization expressed as
crop 15N recovery, and grain quality expressed as
protein concentration, the grain yield is also a
significant parameter for optimizing decisions for
fertilization. The basic N application plus the miner-
alization of organic N created a high concentration of
mineral N in the soil, corresponding to 90 kg N ha�1.Thus, the crop in this experiment had been well
supplied with N for growth, and this may be the
reason for the limited effect of the 16 fertilization
pulses on dry matter grain yield and N uptake in grain.
This illustrates that effects of N application time may
be difficult to reveal without the use of a 15N labelled
source. Also the ear:shoot ratio may be a part of the
basis for decision, but in contrast to 15N recovery themaximum ear:shoot ratio was obtained very early in
the growth period on day 35 (Table 4) corresponding
to GS32�/33 where the second and third nodes were
visible.
Even though N taken up in the vegetative phase was
lost during grain-filling, and the crop for that reason
may not be considered exclusively as a sink for applied
N, the crop recovery of applied fertilizer N depends onthe time of fertilizer application. It is likely that crop
recovery of fertilizer N is related to the crop N demand
determined by the expansion of green leaves, and this
may explain that maximum crop recovery of fertilizer
N was obtained from application at the time of
expansion and unfolding of the flag leaf. However,
this time did not coincide with the obtained maximum
in the ear:shoot ratio or the continuous increase ingrain N concentration.
The consequence of these findings is that, irrespec-
tive of the time of fertilizer N application, it may not
be possible to maximize all parameters simultaneously.
Thus, farmer’s decision on the optimal time for
application may depend on the actual crop condition
and how he wishes the crop to be managed.
Acknowledgements
Thanks are expressed to technician Kai Eskesen for
invaluable assistance during the experiment, and for
preparing the samples for analysis. Senior clerk Margit
Schacht has contributed with valuable linguistic com-
ments on the manuscript. The experimental work was
funded by the Ministry of Food, Agriculture and
Fisheries, Danish Directorate for Development (pro-
ject TEK97-DJF-2). The 15N analysis was carried out
at Research Centre Risø and funded by the Norsk
Hydro Foundation, Denmark.
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