Nitrogen Rate and Planting Density Effects on Yield and Nitrogen
Utilization Efficiency of Direct Seeded Rape (Brassica napus)
Qian Zhao1,2, Xiaoqin Tian1,3, Zhuo Li1,3*
and Yonghong Liu1,3
1Crop Research Institute, Sichuan
Academy of Agriculture Sciences, Chengdu 610066, P.R. China
2College of Landscape
Architecture and Art, Northwest A&F University,
Xianyang 712000, P R. China
3Key Laboratory of Water-Saving
Agriculture for Hilly Areas of Southern China in Sichuan Province, Chengdu
610066, P.R. China
*For correspondence: lizhuo_2000@sina.com
Received
26 August 2020; Accepted 29 November 2021; Published 30 January 2022
Abstract
The high yield of crops mainly depends on the
interaction between nitrogen (N) fertilization and
planting density. The present study evaluated the influence of different N
application rates and planting density on the yield and N
utilization efficiency of direct seeded rape during the 2016–2017 and 2017–2018
growing seasons. The three N application rates including 108 kg N ha−1,
144 kg N ha−1 and 180 kg N ha−1 were laid out
into main plots while planting density including 15.0 × 104, 22.5 ×
104, 30.0 × 104 and 37.5 × 104 plants ha−1
respectively into sub-plots. The results showed that suitable planting density
was the premise to gain high rapeseed yield and the contribution of planting
density to rapeseed yield was small when it
surpassed a certain value. The highest yield in the two growing seasons was
achieved at 144 kg N ha−1 with planting density of 30.0 × 104
plants ha−1. The rapeseed yield increased linearly as N uptake
of the seed part increased, but at high planting density, there was no increase
in yield with the increase N uptake of non-seed parts (stem + husk) when
rapeseed yield exceeded a certain value. High planting densities raised shoot N
uptake and N transfer to rapeseed seeds with the increase of density and N
utilization efficiency was also improved. The too much N application resulted
in more N accumulation in non-seed parts without increasing production. Under
the same target yield, the increased planting density can save 32.4–65.7% of N
fertilization compared with the conventional planting density. The combination
of different N fertilization rates and planting densities is helpful to
increase rapeseed yield. The best N management strategy is to achieve high
yield and reduce the environmental risk to reduce the N fertilization at
suitable high density. © 2022 Friends Science Publishers
Keywords: Directed sowing rape; Nitrogen
fertilization; Planting density; Yield; Nitrogen utilization efficiency
Introduction
Nitrogen (N) fertilizer has been widely used to improve
the yield of crops, but inadequate N inputs resulted in low yields and
food shortages (Sims et al. 2013). In
the modern intensive agricultural production system, up to 50% of the N fertilizer
applied into farmlands is lost to the environment (Cameron et al. 2013). Managing agricultural
nutrients to provide secure and reliable food supply while protecting the
ecological environment remains one of the major challenges today (Zhang
et al. 2007; Cui et al. 2014).
Optimization of the N
application rate is an important direction to achieve
food and environmental safety (Helmers et
al. 2012). The agronomic indicators are necessary to consider in determining
the optimal N application rate (Jin et al.
2012). For instance, plant density management has a crucial impact on the
N application rate (Gao et al.
2009), while higher plant densities can significantly improve the N
uptake and utilization efficiency in crops (Cong et al. 2020). Studies have
reported that for achieving the target yield, high-density planting can reduce
the N application rate by 22.8–25.4% compared with low-density planting (Li et al. 2014).
With urbanization, the rural
labor engaged in agriculture is seriously insufficient and the labor cost is
rising (Hui 2013). In order to coordinate the relationship
among crop production, labor cost and target yield, increased N fertilizer
application under low-density planting to improve the growth of individual
plant or decreased N fertilizer application to reduce the competition among
plants under high-density planting was advocated (Ren et al. 2017). Previous studies have shown that the change of N
fertilizer rates and planting densities would affect N loss and N cycling
between plants and soil, resulting in environmental costs (Gao et al. 2009). Consequently, further
understanding of the interaction between the N fertilizer rate and the planting
density on crop yield and N utilization is of great significance to optimize N
application and achieve high levels of output with low environmental costs
(Abdul et al. 2018; Fang et al. 2018).
As one of the major oil crops in China, rape is widely
planted in most regions because of its high oil content and economic value. Rapeseed
oil accounts for 55% of the total output of oil crops in China, which shows that
rape plays an important role in oil crops (Scarisbrick et al. 1982). However, with the increase of labor cost and the low
mechanization of rape production compared with other crop productions, farmers
grow less rape, resulting in strong demand for oil products. At present, the
self-sufficiency rate of oil is only about 40% and the
safety of edible vegetable oil is still under serious threat (Özcan et al. 2014). Since the short
growth period and large population of direct
seeding rape, the increase of population number makes
the branch site move up which is more conducive to mechanized harvesting. The
individual development of direct seeding rape is relatively poor, and the
formation of yield needs to give full play to the group advantage. Therefore,
the establishment of appropriate group structure is an effective way to ensure
the high yield of direct seeding rape.
With the
purpose of save labor, low-density planting is always adopting in practice,
which produces more branches and a great number of pods per plant (Scarisbrick et al. 1977). Whereas, with the planting
density increasing, the number of branches and pods per
plant decreased, which led to the decrease of yield per plant
(Scarisbrick et al. 1977; Velázquez et al. 2018). As one of major factors
contributing to rapeseed yield, N fertilizer could efficiently control the
number of pods per plant. The optimized management of N fertilizer can
promote the growth of plants and increase the number of pods per plant,
and partly balance the number of pods reduced due to the increase of planting
density (Ahmadi and Bahrani 2009; Ren et
al. 2017; Zheng et al. 2020).
The change on planting density could affect the crop
morphology, alter the utilization of resources and induce different yields
react to different N application rates (Zheng et al. 2020). At low density, increasing N fertilizer rates will
increase the number of pods per plant and improve the yield, but the potential
N losses will also increase at the same time. By contrast,
under high density, the relationship between plant population and yield per
plant would be better coordinated by reducing the application rates of N
fertilizer without affecting the seed yield and increasing the environmental
cost (Zhang et al. 2014).
Therefore, this study mainly analyzed the influences of
the N fertilizer rate and the planting density on yield and N utilization of
direct seeding rape while providing the basis for the reasonable N application
under the condition of increasing density.
Materials and
Methods
Site characteristics
The
experiment was carried out from September 2016 to May 2017 and September 2017
to May 2018 in Jianyang city (30°16′26.32″ N,
104°26′30.38″ E), the west of Sichuan Basin, Sichuan province,
southwest China with subtropical monsoon climate
and annual precipitation of 600–1200 mm. The annual average temperature ranges
between -5 to 37.5°C with about 300 days of frost-free period. The soils of the
experimental site are purple. The chemical properties of the soil were
shown in table (Table 1).
Experimental design
and management
A two-factor split plot experiment was designed using a
random arrangement with three replicates. The individual plot area was 20 m2
(4.0 m × 5.0 m). There were three fertilization treatments in main plots, including 108, 144 and 180 kg N ha−1.
The sub-plot planting densities treatments included were four 15.0 × 104
plants ha−1 (plant and row spacing: 20 cm × 33.3 cm), 22.5 ×
104 plants ha−1 (plant and row spacing: 13.3 cm × 33.3
cm), 30.0 × 104 plants ha−1 (plant and row spacing:
10 cm × 33.3 cm), 37.5 × 104 plants ha−1 (plant and
row spacing: 8 cm × 33.3 cm).
Except for the different dosage of N fertilizer, K, P
and B fertilizer was the same under each treatment at 90 kg K2O ha−1 and 90 kg P2O5
ha−1 using urea (46% N), potassium chloride (60% K2O) and calcium
superphosphate (12% P2O5) as fertilizer sources. The
basal fertilizers, including 70% of the N fertilizer and whole K, P fertilizers
were incorporated one day before transplanting. The remaining 30% N fertilizer
was top dressed at the stem elongation period.
Chuanyou 36, a leading rape variety in Sichuan province,
was used as the material in this experiment. In late September, the seedlings
were raised and thinned at first leaf stage. Final singling was carried out in
the three-leaf stage. During the experiment, plant protection measures such as
pest and disease control and herbicide application were followed as local
practice. Nonetheless, no obvious problems of weeds and pests in these two
growing seasons was observed.
Soil sample collection
The soil-sampling scheme was the same during two
seasons. Soil samples were taken from the 0–20 cm and 20–40 cm depth per plot
by five points sampling method before transplanting. The content of total N and
N, including available P and K, organic matter and pH were measured.
Plant sample collection
During both growing seasons, plants were harvested
separately and all the plants per plot were gleaned to evaluate rapeseed yield;
10 samples per plot were randomly selected to count the number of pods per
plant, the number of seeds per pod and 1000-seed weight at maturity.
Dry matter
determination
The mature plant samples were classified and put into
the sample bag according to the organs; the enzymes were deactivated at 105°C
in the oven for 30 min, then dried the plant organs at 80°C and measured for dry weight (Zhang et al.
2019).
N content
determination and nitrogen use efficiency
The dried plant samples were ground and the N content
was determined by elemental analyzer. Dry weight and N content of different
plant parts were used to calculate the amount of N shoot uptake of each plot
(Zhang et al. 2019).
The N harvest index was calculated as follows:
N harvest index (NHI) = Rapeseed N accumulation/Total
plant N accumulation (Tirol-Padre et al. 1996; Dong et al.
2007).
N internal utilization efficiency (NUE kg kg-1)
= Yield (kg ha-1) / Total plant N accumulation (kg N ha-1)
(Tollenaar et al. 2006; Dong et al. 2007; Caviglia et al. 2014).
N partial factor productivity (PFP kg kg-1) =
Yield (kg ha-1) / N fertilizer rates (kg N ha-1) (Shapiro
and Wortmann 2006; Zheng et al. 2020).
Statistical analysis
The data of both seasons were analyzed using the
Microsoft Excel 2007 and SPSS 17.0 (SPSS Inc. Chicago, IL, USA) software. The means were compared using Duncan’s test at a 0.05 probability level.
Results
Rapeseed yield
and its components
The contribution of N application to rapeseed yield was
higher than planting density (Table 2). With N
fertilization increased, rapeseed yield improved under low-density planting
treatments. Under low planting density (15.0
× 104 plants ha−1, 22.5 × 104 plants ha−1) and 180 kg N ha−1 rapeseed yields was
highest; and at high planting density (30.0
× 104 plants ha−1, 37.5 × 104 plants ha−1) with 144 kg N ha−1, rapeseed
yields were highest.
Between years, rapeseed yield were higher during 2016–2017
growing season than 2017–2018, similar response of rapeseed yield to N
application and planting density were observed (Table 2). The high-density
planting greatly increased the rapeseed yield. Compared to 108 kg N ha−1 and 15.0 × 104
plants ha−1, the rapeseed yield increased by 51.3 and 45.9% for 108 kg N ha−1 and 37.5 × 104
plants ha−1 in 2016–2017 and 2017–2018 growing seasons,
respectively.
The alterations of pods per plant with N application and
planting density were similar during these two growing seasons (Fig. 1). High-density
planting suppressed the number of pods per plant. The number of pods per plant
under 15.0 × 104
plants ha−1 was higher than other planting density treatments. The
N fertilization remarkably increased the number of pods per plant, and the
number of pods per plant at the treatment of 180 kg N ha−1 was always
the largest no matter what density treatment was.
There was no significant difference in seeds per pod and
1000-seed weight under different treatments (Fig. 1). As the number of pods per
plant was significantly higher in 2016–2017, the number of seeds per pod in
2016–2017 was significantly greater than 2017–2018 (Fig. 1). The average number
of seeds per pod was 21.2 and 15.3 in the 2016–2017 and 2017–2018 seasons,
respectively.
The relationship between rapeseed yield and yield
components showed a significant positive correlation between N fertilization
rate, planting density and rapeseed yield, the correlation
coefficient between N fertilizer rates and rapeseed yields was higher than of
densities and rapeseed yields (Fig. 2). The number of pods per plant and seeds
per pod were positively correlated with rapeseed yield, while the 1000-seed
weight negatively correlated with rapeseed yield. Compared with the correlation
between rapeseed yield and group yield components, the correlation between rapeseed yield and individual yield components was
relatively weak.
Shoot N Uptake
Similar to rapeseed yield, the N application and density
had a significant impact on shoot N uptake of rapeseed at maturity (Table 3).
Under the same N treatment, high-density planting
obviously increased the shoot N uptake of rapes. Compared with 15.0 × 104 plants ha−1 density,
shoot N uptake increased by an average of 12.5, 32.6, 33.0% and 13.5, 36.9, 37.6%
at 22.5 × 104
plants ha−1, 30.0 × 104 plants ha−1
and 37.5 × 104 plants ha−1 in 2016–2017
and 2017–2018, respectively.
Compared with planting density, N fertilizer application
has more effect on shoot N uptake and it was generally the highest under 180 kg N ha−1.
Nevertheless, there was no obvious difference of shoot N uptake at 180 kg N ha−1 under
high-density planting.
The relationships between shoot N uptake and rapeseed
yield at different planting densities was improved linearly as seed N uptake,
and slope at different densities was similar, which indicated that the yield
value of N uptake per seed under different densities was approximately equal (Fig.
3).
At the two growing seasons, N uptake of non-seed parts
(stem + husk) was changed at different densities. Under low planting densities,
rapeseed yield improved linearly as the N uptake of non-seed parts increased.
However, rapeseed yield no longer improved with the N uptake of
non-seed parts increasing when the rapeseed yield reached a certain value under
the high-density treatment. These indicate that the non-seed part of rape could
not form higher rapeseed yield when it absorbed more N under high density
treatment.
Fig. 2: Correlation matrix between
different parameters of rapeseed yield. N represents N fertilizer rate, D
represents planting density, P represents pods per plant, S represents seeds
per pod, W represent 1000-seed weight, SP represents seeds per plant, PH
represent pods per ha, SH represents seeds per ha, Y represents rapeseed yield.
The number in the circle represents the determination coefficient
Fig. 1: Yield
components under different N fertilizer rates and planting densities during
2016-2017 and 2017-2018 growing seasons in Jianyang
city, southwest China
N Utilization
Rate
Reducing the application rates of N fertilizer
significantly improved the N harvest index (NHI), internal utilization
efficiency (NUE) and its partial
factor productivity (PFP). All indicators were maximum at 108 kg N ha−1, which were
significantly higher than 144
kg N ha−1 and 180 kg N ha−1 in the two
growing seasons.
The NHI and the PFP increased with the increasing of
planting density, with the highest for 30.0
× 104 plants ha−1 and 37.5 × 104 plants
ha−1, significantly higher than 15.0 × 104 plants ha−1. However,
the NUE decreased with the increase of density.
During both growing seasons, the NHI at the level of 108 kg N ha−1 was 18.7 and
14.9% higher on average than 180
kg N ha−1. NUE was 34.4 and 39.0% higher, respectively While PFP
was 21.3 and 39.5% higher, respectively (Table 4–6).
In the 2016–2017 growing season, the NHI, NUE and PFP
under low N and high planting density (108
kg N ha−1 and 37.5 × 104 plants ha−1) were 43.9,
16.2 and 79.0% higher than under high N and conventional planting density (180 kg N ha−1 and 15.0 × 104
plants ha−1) and 37.5, 4.0 and 74.8% higher than 2017–2018 growing
season, respectively. In the 2016–2017 growing season, the NHI, NUE and PFP
under medium N and high planting density (144
kg N ha−1 and 37.5 × 104 plants ha−1) were 29.3,
6.9 and 50.0% higher than under high N and conventional Table 1: Soil chemical properties at
the experimental sites during both growing seasons
Organic
matter (g kg-1) |
pH |
Total N (g kg-1) |
available N (mg kg-1) |
available P (mg kg-1) |
available K (mg kg-1) |
|
2016–2017(0–20 cm) |
6.65 |
7.22 |
1.14 |
68.45 |
13.48 |
84.75 |
2016–2017(20–40 cm) |
6.86 |
7.35 |
1.29 |
55.46 |
14.38 |
85.64 |
2017–2018(0–20 cm) |
7.78 |
7.12 |
0.07 |
93.47 |
27.30 |
86.50 |
2017–2018(20–40 cm) |
8.82 |
7.23 |
0.07 |
77.08 |
29.27 |
88.50 |
Table 2: Rapeseed yields (kg ha−1) under different N fertilizer
rates and planting densities during 2016–2017 and 2017–2018 growing seasons
|
2016–2017 |
2017–2018 |
||||||
Treatments |
Plant density (× 104 plants ha−1) |
Plant density (× 104 plants ha−1) |
||||||
N application (kg N ha−1) |
15.0
|
22.5
|
30.0
|
37.5 |
15.0
|
22.5
|
30.0
|
37.5 |
108 |
1835f |
2339e |
2499de |
2776c |
1514g |
1826f |
2262d |
2209d |
144 |
2365e |
2806c |
3221a |
3103ab |
1882f |
2221d |
2699a |
2502b |
180 |
2585d |
3013b |
3145ab |
3097ab |
2107e |
2401c |
2453bc |
2371c |
ANOVA |
F value |
P value |
|
|
F value |
P value |
|
|
N |
125.52 |
< 0.01** |
|
|
168.58 |
< 0.01** |
|
|
Density (D) |
101.81 |
< 0.01** |
|
|
139.44 |
< 0.01** |
|
|
N × D |
4.28 |
< 0.01** |
|
|
14.58 |
< 0.01** |
|
|
Note: Different lowercase letters in
each group (2016–2017 and 2017–2018) indicate significant (P < 0.05) differences basing on Duncan’s multiple range test. ** P
≤ 0.01; * P ≤ 0.05.
Table 3: Shoot N uptake (kg N ha−1) under different N fertilizer rates and planting densities during both
growing seasons
|
2016–2017 |
2017–2018 |
||||||
Treatments |
Plant density (× 104 plants ha−1) |
Plant density (× 104 plants ha−1) |
||||||
N application (kg N ha−1) |
15.0
|
22.5
|
30.0
|
37.5 |
15.0
|
22.5
|
30.0
|
37.5 |
108 |
87.1g |
101.8fg |
118.2def |
124.5cd |
73.4f |
81.3f |
100.8d |
106.6c |
144 |
105.6ef |
122.8cde |
151.7ab |
148.1ab |
82.9e |
95.7de |
115.4b |
112.3b |
180 |
130.7cd |
139.2bc |
158.9a |
157.6a |
101.1cd |
115.1bc |
136.1a |
135.4a |
ANOVA |
F value |
P value |
|
|
F value |
P value |
|
|
N |
43.90 |
< 0.01** |
|
|
42.23 |
< 0.01** |
|
|
Density (D) |
26.16 |
< 0.01** |
|
|
31.32 |
< 0.01** |
|
|
N × D |
0.61 |
0.72 |
|
|
0.25 |
0.95 |
|
|
Note: Different lowercase letters in
each group (2016–2017 and 2017–2018) indicate significant (P < 0.05) differences basing on Duncan’s multiple range test. ** P ≤
0.01; * P ≤ 0.05.
Table 4: N harvest index
(NHI) under different N fertilizer rates and planting
densities during both growing seasons
|
2016–2017 |
2017–2018 |
||||||
Treatments |
Plant density (× 104 plants ha−1) |
Plant density (× 104 plants ha−1) |
||||||
N application (kg N ha−1) |
15.0
|
22.5
|
30.0
|
37.5 |
15.0
|
22.5
|
30.0
|
37.5 |
108 |
0.50c |
0.53bc |
0.54b |
0.59a |
0.46d |
0.50bc |
0.51b |
0.55a |
144 |
0.45d |
0.48cd |
0.52bc |
0.53bc |
0.42e |
0.45de |
0.47cd |
0.50bc |
180 |
0.41e |
0.44de |
0.49cd |
0.48cd |
0.40f |
0.44de |
0.46d |
0.46d |
ANOVA |
F value |
P value |
|
|
F value |
P value |
|
|
N |
32.15 |
< 0.01** |
|
|
27.52 |
< 0.01** |
|
|
Density (D) |
16.89 |
< 0.01** |
|
|
19.38 |
< 0.01** |
|
|
N × D |
0.74 |
0.62 |
|
|
0.55 |
0.76 |
|
|
Note: Different lowercase letters
in each group (2016-2017 and 2017-2018) indicate significant (P < 0.05) differences basing on
Duncan’s multiple range test. ** P ≤ 0.01; * P ≤ 0.05
Table 5: N internal utilization
efficiency (NUE kg kg–1)
under different N fertilizer rates and planting
densities during both growing seasons
|
2016–2017 |
2017–2018 |
||||||
Treatments |
Plant density (× 104 plants ha−1) |
Plant density (× 104 plants ha−1) |
||||||
N application (kg N ha−1) |
15.0
|
22.5
|
30.0
|
37.5 |
15.0
|
22.5
|
30.0
|
37.5 |
108 |
23.67a |
24.47a |
21.68b |
20.90b |
21.29a |
15.23b |
14.30c |
15.20b |
144 |
20.82b |
20.17c |
20.62bc |
19.24cd |
17.32ab |
14.32c |
14.50bc |
15.11b |
180 |
17.99d |
17.82de |
16.01de |
15.69e |
14.62bc |
10.38d |
11.42d |
11.07d |
ANOVA |
F value |
P value |
|
|
F value |
P value |
|
|
N |
41.25 |
< 0.01** |
|
|
39.97 |
< 0.01** |
|
|
Density (D) |
4.36 |
< 0.05* |
|
|
21.84 |
< 0.01** |
|
|
N × D |
0.69 |
0.66 |
|
|
0.67 |
0.68 |
|
|
Note: Different lowercase letters
in each group (2016–2017 and 2017–2018) indicate significant (P < 0.05) differences basing on
Duncan’s multiple range test. ** P ≤ 0.01; * P ≤ 0.05
Fig. 3: The relationship
between rapeseed yield and N uptake of seed and non-seed parts under different
planting density treatments during
growing seasons
Fig. 4: Effect of N
fertilizer rate and planting density on direct seeding rape yield per unit.
Table 6: Partial factor productivity (PFP kg kg–1) under different N fertilizer rates and planting densities during both growing
seasons
|
2016–2017 |
2017–2018 |
||||||
Treatments |
Plant density (× 104 plants ha−1) |
Plant density (× 104 plants ha−1) |
||||||
N application (kg N ha−1) |
15.0
|
22.5
|
30.0
|
37.5 |
15.0
|
22.5
|
30.0
|
37.5 |
108 |
16.99e |
21.65c |
23.14b |
25.71a |
14.02e |
16.91c |
20.94a |
20.46a |
144 |
16.42e |
19.49d |
22.37bc |
21.55c |
13.07f |
15.43d |
18.75b |
17.38c |
180 |
14.36f |
16.74e |
17.47e |
17.21e |
11.71g |
13.34f |
13.63ef |
13.17f |
ANOVA |
F value |
P value |
|
|
F value |
P value |
|
|
N |
175.33 |
< 0.01** |
|
|
159.54 |
< 0.01** |
|
|
Density (D) |
109.80 |
< 0.01** |
|
|
90.12 |
< 0.01** |
|
|
N × D |
9.99 |
< 0.01** |
|
|
6.75 |
< 0.01** |
|
|
Note: Different lowercase letters
in each group (2016–2017 and 2017–2018) indicate significant (P < 0.05) differences basing on
Duncan’s multiple range test. ** P ≤ 0.01; * P ≤ 0.05
Table 7: N fertilizer rates of rapeseed based on regression equations
under different planting densities
|
N fertilizer rates in 2016–2017 |
N fertilizer rates in 2017–2018 |
||||||
Target yield (kg ha-1) |
Plant density (× 104 plants ha−1) |
Plant density (× 104 plants ha−1) |
||||||
|
15.0
|
22.5
|
30.0
|
37.5 |
15.0
|
22.5
|
30.0
|
37.5 |
Average yield of Sichuan Province (2389) |
146.69 |
110.78 |
88.52 |
89.47 |
- |
168.42 |
115.18 |
123.63 |
Decrement |
- |
35.91 |
58.17 |
57.22 |
- |
- |
53.24 |
44.79 |
Decrease |
- |
32.4% |
65.7% |
64.0% |
|
|
46.2% |
36.2% |
planting density (180
kg N ha−1 and 15.0 × 104 plants ha−1),
respectively. And the NHI, NUE and PFP were 25, 3.4 and 48.5% higher in the
2017–2018 growing season.
Nonetheless, the NHI and NUE decreased when the
application of N fertilizer was high, which was
very easy for plants to absorb a large amount of N, and most of N absorbed was
mostly accumulated in non-economic parts, thus reducing the N application and
increasing planting density can not only realize rapeseed yield and promote N
to rapeseed distribution, but also effectively improve the utilization
efficiency of N fertilizer at the same time.
Comparison of
N Fertilizer Dosage under Four Densities
Regression analysis was carried out between the yield of
rapeseed under four densities and the corresponding N fertilizer dosage (Fig. 4). The fitting degree of the four models has
reached an extremely significant level.
According
to the model and regression equation, the rates of N fertilizer were calculated
when the rapeseed yield reaches 2389 kg ha-1 (Table 7).
During 2016–2017 growing season, four kinds of density of rapeseed N fertilizer
were 146.69 kg ha-1, 110.78 kg ha-1, 88.52 kg ha-1 and
89.47 kg ha-1. During 2017–2018 growing season, 15.0 × 104 plants ha−1 level of rapeseed yield has yet to
reach an average yield of Sichuan province, so it was unable to calculate the N
fertilizer usage, the N fertilizer dosage of the other three-density rapeseed
was 168.42 kg ha-1,
115.18 kg ha-1, 123.63 kg ha-1, respectively.
Compared with the conventional density (15.0 × 104
plants ha−1),
increasing the planting density could save 32.4–65.7% N fertilizer. Under the
same target yield condition, the rape of high density requires less exogenous N
and the increase of planting density could effectively reduce the rates of N
fertilizer.
Discussion
The N fertilization and planting density had crucial
impacts on crop yield (Liang et al.
2013; Fang et al. 2018). Previous
studies have shown that crop yield frequently exhibited a curve response to N
fertilizer rates or planting densities, and reaches the maximum level under the
optimum N application or planting density (Roques and Berry 2016). The present
study showed that rapeseed yield improved linearly with the increase of
planting density at low N levels. However, planting density had a quadratic
response at high N treatments. In this study, the yield of rapeseed at
high-density treatments increased by 20.2–32.3% and 17.2–28.1% under the
condition of constant N application compared with low-density treatment (15.0 × 104 plants ha−1) in two
growing seasons, respectively. The rapeseed yield at 180 kg N ha−1 were the
highest under low density treatments, but rapeseed yield no longer improve
while N application exceed 144 kg N ha-1 at high density treatments.
N application could promote crop growth, improve dry
matter accumulation and leaf area per plant and reduce variation among plants
at the same time (Rossini et al.
2011). The crop growth was poor with a relative low N supply, and the dry
matter of individual plant at the stem elongation period was significantly
higher at high N input treatments (Dong et
al. 2012). The competition among plants was healthy even
under high-density treatments, and individual plants could still
give full play to its yield potential (Tollenaar et al. 2006; Ren et al.
2017). By contrast, high N supply increased the dry matter
accumulation of individual plants (Jiang et al. 2002). The crowding stress among crops under suitable
conditions often associated with high N application and high-density planting,
which accelerated the senescence of low canopy leaves, enhancing plant
competition, increasing the incidence rate of crops and resulting in low yield
eventually (Tollenaar et al.
2006; Dong et al. 2012; Antonietta et al. 2014). Therefore, the key to high
yield and high efficiency was to pay attention to the coordination of N
application and planting density.
It has been pointed out in previous studies that
rapeseed yield in each unit area depends on the planting density, the number of
pods per plant, the seeds per pod and the individual seed weight (Diepenbrock
2000). In present study, the impacts of planting density and N application rate
on the number of seeds per pod and the 1000-seed weight were not significant,
which might be related to the characteristics of varieties. Consequently, the
relationships between rapeseed yield and individual yield components was not
very obvious.
The number of pods per plant was remarkably affected by
N fertilizer rate and planting density. The planting density was negatively
correlated with the number of pods per plant. The number of pods per plant
decreased greatly under high-density treatments. Improving the N fertilizer
rate could promote the growth of individual plants and increase the number of
pods per plant (Riffkin et al. 2012; Ren
et al. 2017). Studies have shown that
the optimal N fertilization is closely related to the improvement of rapeseed
yield components while the number of pods per plant is the most sensitive to N
fertilization. The positive effect of N application on pods per plant could
partially balance the decrease of pod numbers per plant at high density (Ma et al. 2014).
The purpose of rational close planting of rape is to
cultivate a population structure with high light efficiency to improve the
yield per unit area to obtain high yield (Wholey and
Booth 1979; Ren et al. 2017).
Although to some extent, the increase of planting density of rape inhibited
some characters (such as pods per plant) of rape, however, with the increase of
density, the leaf area index and light energy utilization rate of the
population were greatly improved, which gave full play to the group effect and
finally reflected in the increase of rape groups´ yield (Ozer 2003). This study
showed that there was a strong correlation between rapeseed yield and yield
components, and the number of pods per ha was significant positively correlated
with rapeseed yield and the correlation coefficient is relatively high, which
was consistent with previous study results (Ozer 2003; Ren et al. 2017; Zheng et al.
2020).
The application of N fertilizer and planting density
significantly affected rapeseed yield and yield components. High-density
planting improved rapeseed yield. The number of pods per plant decreased
obviously as the planting density increased. The N fertilizer rate greatly
increased the number of pods per plant and the yield of rapeseed, and
significantly affected the response of planting density to rapeseed yield. At
low density, the yield of rapeseed improved with the increase of the N fertilizer
rate. Under high density, excessive N application reduced rapeseed yield. The
income of rapeseed was the best under the treatment of 144 kg N ha-1 and
30.0 × 104 plants ha−1.
One of the main measures to reduce the loss of N is to
reduce the N fertilization and improve the N utilization rate. The N uptake
reflects the N utilization rate (Nakamura et al. 2008; Caviglia et al. 2014).
The rapeseed yield improved linearly as N uptake
increased, and the N uptake of each seed was similar under different densities.
Nevertheless, for the non-seed part (stem + husk), the response of N uptake to
rapeseed yield varied with changes of planting densities. The N uptake of
non-seed part increased without obtaining high rapeseed yield at high density
treatments, showing the obvious phenomenon of luxury N uptake. This change was
consistent with previous research results, the competition among plants was
invariably fierce under high density and high N supply, so much more N was
distributed to the non-seed part to promote the competitiveness of plants (Bennett
et al. 2011; Ren et al. 2017).
Non-leaf organs played an important role in the
photosynthesis and yield formation of rape since the pod development period
beginning (Beccafichi et al. 2003; Amanullah
2010; Zhang et al. 2014). High
density would lead to shading between plants, so more N was distributed to the
non-seed part to compensate for the decrease of light intensity and the
maintenance of photosynthesis (Delagrange 2011; Ren et al. 2017). Although crops at high planting densities absorbed
much more N, the absorbed N was used to improve the competition among plants
rather than to obtain high yield. Therefore, it was necessary to optimize the
population structure and reduce competition among plants by coordinating N
application and planting density.
High density increased the shoot N uptake of rapeseed.
Under high density and high N application treatment, the non-seed parts (stem +
husk) did not make more contribution to rapeseed yield with the condition of
absorbing higher N. Increasing the planting density could promote the transfer
of N to rapeseeds and improve the N utilization rate. However, excessive
application of N fertilizer would lead to more accumulation of N in
non-economic parts of rapeseed, which could not achieve the purpose of
increasing rapeseed yield.
At present, in order to give full play to the potential
of increasing yield of rapeseed, the application rates of N fertilizer are too
large, resulting in low N fertilizer utilization rate and surplus N
fertilizer (Du et al.
2019). Studies have shown that crop yield was closely related to N uptake and
the effects of N application on its uptake and utilization of crops are different.
Instead of increasing crop yield, high N will lead to decreased N utilization
efficiency, resource waste and environmental security (Anbessa and
Juskiw 2012). Some studies also showed that by reducing N fertilizer
and increasing planting density, high crop yield and N utilization rate
can be achieved (O’Beirne and Cassidy 1990).
The results showed that dense planting could improve the
N utilization efficiency, increase the NHI and the NUE of rapeseed. The NHI
reflects the N transfer to the seeds. In this study, the increase of N
application reduced the NHI and the NUE, which indicates that the increase of N
application is not conducive to the N transfer to the
rapeseeds, and more N will be used for the physiological activities of the
plants themselves. The results were consistent with previous research (Tirol-Padre et al. 1996; Dong
et al. 2007). Under low N fertilizer and high density, PFP is higher,
it can be seen that reducing N application and density planting is the key
measure to improve the utilization efficiency of N fertilizer and effectively
save the N fertilizer rates (Shapiro and Wortmann 2006; Zheng et al.
2020).
From the relationship between rapeseed yield and N
fertilizer dosage, in this study, when the same target yield was achieved, the
N fertilizer dosage of high-density rapeseed was 35.9–58.2 kg per ha less than
regular density (15.0
× 104 plants ha−1) rapeseed, which means that
increasing the planting density could save 35.1–61.2% of N application. The
dense planting is a key method to improve the utilization efficiency of N
fertilizer and effectively save the application of N fertilizer (Zhang et al.
2019). Thus, reducing the application of N fertilizer and increasing the
planting density at the same time can not only achieve the high yield of rape,
promote the distribution of N to seeds and slow down the N loss, but also
effectively improve the efficiency of N utilization while protecting the
ecological environment.
Conclusion
The application of N fertilizer and planting density significantly
affected rapeseed yield and yield components. High-density planting improved
rapeseed yield. At low density, the yield of rapeseed improved with the
increase of the N fertilizer rate. Under high density, excessive N application
reduced rapeseed yield. The income of rapeseed was the best under the treatment
of 144 kg N ha-1 and 30.0 × 104 plants ha−1. High
density increased the shoot N uptake of rape. However, excessive application of
N fertilizer would lead to more accumulation of N in non-economic parts of
rape, which could not achieve the purpose of increasing rapeseed yield.
Reducing the nitrogen rate and increasing the planting density can realize the
high yield of rape and effectively improve the nitrogen use efficiency.
Acknowledgments
This research was supported by the National Key R & D Program of China
(2018YFD0200703), the Sichuan Provincial Financial Innovation Ability
Improvement Project (2016GYSH-007), the Study on the Disaster Mechanism and
Control Techniques of Rape Autumn Floods and Spring Droughts in Chengdu-Chongqing
Economic Zone (2020ZYD029), the Study on Key Technologies and Integrated Models
of Green, High-quality, High-yield and High-efficiency of New Double Cropping
Production System in Rape-Maize Dryland (2021XKJS007).
Author Contributions
QZ and
ZL conceived and designed the experiments. QZ, ZL and XQT performed the experiments.
QZ, ZL, XQT, and YHL analyzed and interpreted the sequence data. QZ wrote the
paper. All authors read and approved the manuscript.
Conflicts of Interest
The authors declare that the research
was conducted in the absence of any commercial or financial relationships that
could be construed as a potential conflict of interest.
Data Availability
All data included in this study are
available upon request by contact with the corresponding author.
Ethics Approvals
Not applicable.
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