Front. Plant Sci.Frontiers in Plant ScienceFront. Plant Sci.1664-462XFrontiers Media S.A.10.3389/fpls.2022.862088Plant ScienceOriginal ResearchEffects of Nitrogen Application Rate Under Straw Incorporation on Photosynthesis, Productivity and Nitrogen Use Efficiency in Winter WheatWangJinjin123HussainSadam123SunXu123ZhangPeng123JavedTalha45DessokyEldessoky S.6RenXiaolong123*ChenXiaoli123*1College of Agronomy, Northwest A&F University, Yangling, China2Key Laboratory of Crop Physiology, Ecology and Tillage in Northwest Loess Plateau, Minister of Agriculture, Yangling, China3Institute of Water Saving Agriculture in Arid Areas of China, Northwest A&F University, Yangling, China4Department of Agronomy, University of Agriculture Faisalabad, Faisalabad, Pakistan5College of Agriculture, Fujian Agriculture and Forestry University, Fuzhou, China6Department of Biology, College of Science, Taif University, Taif, Saudi Arabia
Edited by: Adnan Noor Shah, Khwaja Fareed University of Engineering and Information Technology (KFUEIT), Pakistan
Reviewed by: Akram Shehata, Alexandria University, Egypt; Mursleen Yasin, Western Sydney University, Australia
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Developing a nitrogen fertilizer (N) reduction method under straw incorporation is essentially important for increasing wheat productivity in terms of improved fertilizer use efficiency and high yield in semiarid areas. A two-year field experiment, with five different nitrogen application rates: control (without N application, N0), low N (75 kg ha–1, N75), medium N (150 kg ha–1, N150), high N (225 kg ha–1, N225) and excessive N (300 kg ha–1, N300), was conducted in 2018 and 2019 to quantify their impacts on the photosynthetic characteristics, nitrogen utilization (in terms of N accumulation, distribution and transportation, and residual soil NO3–-N) and productivity of winter wheat. There was a significant impact of N rates on photosynthetic traits, and N accumulation in different organs. As compared with the N300, N150, and N225 improved the photosynthetic characteristics, increased N accumulation in grains by 5.55 and 10.97%, the N contribution proportion of that accumulated after anthesis by 67.90 and 115.56%, and reduced residual N by 62.50 and 46.48%, respectively, thereby effectively improved N absorption efficiency and N contribution rates. Grain yield remained slightly or unchanged among N treatments. Although N0 and N75 treatments reduced the nitrate-N leaching but caused a significant reduction of 18.13 and 28.37%, respectively, in grain yield. From these results, we conclude that N application at 150 and 225 kg⋅ha–1 under straw incorporation was the most effective fertilization method in achieving the higher photosynthetic characteristics, improving NUE and grain yield. This study provides theoretical and practical guidance for wheat production techniques.
nitrogen fertilizernitrogen use efficiencyphotosynthesisproductivitystraw incorporationwinter wheatTaif University10.13039/501100006261Introduction
Fertilizers, when applied at appropriate rate, have played an important role in increasing the crop productivity (Morari et al., 2011). Good fertilization practices, including as per recommended methods and rates, are very important not only for enhancing crop productivity but ensuring soil and environmental health (Yousaf et al., 2016a,b; Shah et al., 2017a). However, inadequate application of synthetic fertilizers, particularly nitrogenous fertilizers, have a negative impact on crop growth, the soil and the environment; it is predicted that increasing N fertilizers may cause an increase of 23–60% in N2O emissions by 2030 (FAO, 2009). China is among the largest consumer of chemical fertilizers in the world and overuse of synthetic fertilizers, including nitrogenous fertilizers, is a common practice to achieve the higher crops yield (Du et al., 2020). Increasing N inputs would decrease nitrogen use efficiency (NUE) and increase N losses and environmental pollution (Shah et al., 2017b; Bhattacharya, 2019; Zhang et al., 2019; Bhattarai et al., 2021). In a recent study, Gu et al. (2019) also demonstrated that long-term use of nitrogenous fertilizers has been resulted in enhancing the acidification, degradation, and soil compaction problems. Agricultural management practices, for example, the application of N fertilizers greatly influence soil health in term of soil organic carbon (SOC) content, crop growth, and developmental processes. Various reports have reported that reducing mineral N application along with the application of manures would be one of the best practices for enhancing crop performance through promoting soil health and NUE (Duan et al., 2016).
Guanzhong Plain, having an area of ∼ 34000 km2, is an important grain production base in Shaanxi Province and in China (Liu et al., 2014). Winter wheat is the main grain crop in this region. This region is rich in straw resources, but at the same time, the phenomenon of straw burning is very common (Zhu et al., 2016; Sun et al., 2017). Intensive planting mode without straw retention/incorporation reduced the soil organic matter and soil nutrients which reduced the reward effect of chemical fertilizer (Scotti et al., 2015). In order to pursue high crop yield and reduce cost, the phenomenon of excessive application of N and the “one blast” fertilization method are prevalent (Lou et al., 2011; Ju and Gu, 2014; Zhao et al., 2016; Hafeez et al., 2019; Shah et al., 2021a,b; Yousaf et al., 2021), causing serious loss of N and reduced NUE, which not only wastes resources but also causes serious environmental pollution (Jin et al., 2012; Wei et al., 2015; Zhao and Yin, 2015). Agricultural production in this region lacks scientific and reasonable fertilization guidance.
Straw incorporation can increase soil organic matter, reduce fertilizer input, and increase the reward effect of chemical fertilizers (Zhang et al., 2016). It is commonly considered that fertilizers play a significant role in the continuous increase in the productivity of agricultural crops (Lou et al., 2011). Currently, considering the environmental benefits and better crop yield that sustainable agriculture management provides, a combination of N fertilizer application with straw incorporation is recommended as a sustainable and environment-friendly approach by researchers (Bakht et al., 2009; Liao et al., 2015; Xu et al., 2019; Zhang et al., 2019) and government agencies (MEP, 1999; MOA-PRC, 2015) to increase the soil EC, improve soil fertility status and minimize the environmental problems. Crop straw incorporation or retention is commonly believed to have a positive influence on the soil quality, soil carbon and N dynamics (Ghimire and Machado, 2015). It will promote soil productivity, soil organic C sequestration and NUE (Liu et al., 2014). Various studies have demonstrated that straw incorporation can enhance the carbon inputs which can result in enhancing the organic matter contents and accumulation of essential nutrients that ultimately enhance nutrient use efficiency of crop plants (Li et al., 2010; Tong et al., 2014; Yuan et al., 2021). It is also well documented that straw incorporation positively affected the activities of various beneficial microbes to facilitate in protecting the soil environment for sustainable agricultural production (Garg and Bahl, 2008). However, results of previously published reports on the efficacy of straw incorporation treatments for crop yield and environment were inconsistent, showing either positive (Huang et al., 2017), negative (Yao et al., 2017; Xu et al., 2021), or neutral effects (Zhang et al., 2017). Fertilization had a significant effect on soil health and productivity, and soil GHGs emissions (Liu et al., 2017). N application, when exceeds crop demand, has been reported to increase the primary production of GHGs, particularly N2O, in most terrestrial ecosystems, mainly due to its primary effect on soil N pool (Elser et al., 2007; Xu et al., 2019) and decreasing the NUE (Zhang et al., 2019). Therefore, it is imperative to find the best application rate to reduce the N losses and associated environmental problems.
At present, only limited studies have investigated the effects of reducing N on the basis of straw incorporation for winter wheat production in semiarid areas (Weller et al., 2015; Liang et al., 2017). And the conclusion is still inconclusive (Yin et al., 2018). Furthermore, there is still a lack of knowledge whether better photosynthetic processes, crop productivity and high NUE can be reconciled by optimizing fertilization regimes under straw incorporation. Thus, in this study, in order to find the best fertilization management, we set decreasing N fertilization treatments under the condition of traditional N to find the best choice for increasing the productivity of the wheat crops. Specific objectives of this study were to: (1) investigate the effect of N fertilizer on wheat growth, photosynthetic characteristics, N utilization (N accumulation, distribution and transportation, and residual soil –3–-N) and grain yield; (2) and provide a new direction for optimizing wheat N fertilizer management in semi-arid areas of China. For this study, we hypothesized that the conventional rate of N replaced by reduced N dose would promote the photosynthetic process, crop productivity, increase NUE in terms of high N accumulation in different plant organs and reduce leaching losses under straw incorporation conditions. Results of this experiment would help to identify the most efficient N application rate for achieving better crop growth, sustainable wheat grain yield, and high NUE under straw incorporation in semiarid areas.
Materials and MethodsExperimental Site Description
Field experiments were conducted over two consecutive years (2018–2019) at the Institute of Water-saving Agriculture in Arid Areas of China (IWSA), at Northwest A&F University, Yangling (E108°04′, N35°20′) Shaanxi Province, China (Figure 1). This area is typically a semi-arid area which is located at the northern foot of the Qinling Mountains and on the Toudaoyuan in the west of the Weihe Plain. The experimental site is characterized as a hilly, dry flat land in the abdomen of the Loess Plateau with Lou soil type. Table 1 shows the nutrient content of wheat straw for returning to the field during the next wheat season and the soil nutrient content of the 0–60 cm soil layer before planting. Soil physical and chemical properties of above 0–20 cm were as follows: (i) soil bulk density 1.25 ± 0.3 g⋅cm–3, SOM 11.97 ± 0.95 g⋅kg–1, available N 53.12 ± 2.45 mg⋅kg–1, available P 22.34 ± 1.24 mg⋅kg–1, available K 97.37 ± 4.56 mg⋅kg–1 and pH 7.59 ± 0.58.
Location of the experimental site and layout of the field plots randomly distributed with N application rates. N0, unfertilized control; N75, 75 kg N⋅ha– 1; N150, 150 kg N⋅ha– 1; N225, 225 kg N⋅ha– 1; N300, a traditional N application rate 300 kg N⋅ha– 1.
Nutrient contents of incorporated wheat straw and soil nutrient contents in different soil layers before planting (g⋅kg–1).
Nutrient
Soil layer (cm)
Crop straw (g⋅kg–1)
0–20
20–40
40–60
Organic C (g⋅kg–1)
11.97
12.03
8.25
/
Total N (g⋅kg–1)
1.31
0.78
0.56
6.64
Total P (g⋅kg–1)
0.729
0.54
0.39
0.47
Total K (g⋅kg–1)
14.9
20.14
25.48
20.62
In the past 40 years, the average annual temperature is 13.5°C, annual average rainfall was 540–625 mm, where most of rainfall occurred during July and September, and annual evaporation was 993.2 mm. The average temperature and rainfall during the growth period of wheat are shown in Figure 2. The average temperature during the whole growth period of winter wheat (from October to June) in the first year (2017–2018) and second year (2018–2019) of the experiment was 9.0 and 8.9°C, respectively. The rainfall during the whole growth period in the first year (2017–2018) and second year (2018–2019) of the experiment was 179.0 and 186.0 mm, respectively.
Rainfall and average temperature during the winter wheat growing seasons in 2018 and 2019 in Yangling, China.
Experimental Design and Treatments
The area planting system is winter wheat - leisure. The seedbed preparation included mechanical crushing of the whole wheat straw and return to the soil, this was followed by 15 cm deep plowing. The wheat straw (4500 kg⋅ha–1) was returned to the field before sowing. The nutrient contents of used straw were: (i) 6.64 ± 0.56 g kg–1 total N, (ii) 0.47 ± 0.01 g kg–1 total phosphorus (P), (iii) 20.62 ± 1.21 g kg–1 total potassium (K). The site was planted with winter wheat, where its cultivar Xinong-979 was sown, keeping an inter-row spacing of 20 cm. The seeds of winter wheat were sown in the mid-October and harvested in the first week of June during both study years. The plot area was 18 m2 (6 m × 3 m), with 3 replicates and a completely random arrangement. No irrigation was applied during the growing period of crop and other management practices were the same as local practices. The recommended dose of phosphorus and potassium was applied at 150 kg⋅ha–1 P2O5 and 60 kg⋅ha–1 K2O, using calcium phosphate and potassium chloride. N fertilizer using urea, five N treatments (0, 75, 150, 225, and 300 kg⋅ha–1, abbreviated as N0, N75, N150, N225, N300), was applied as the basal application to be consistent with the local farmer community.
Sampling Methods and MeasurementsPlant SamplingLeaf Area Index and Photosynthetic Indices
In the crucial growth period of winter wheat, at the flowering stage, values were recorded during a sunny day between 9:00 – 11:00 a.m. The net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr) and intercellular CO2 concentration (Ci) of flag leaves were measured by li-6400 photosynthetic apparatus manufactured in the United States, and the SPAD values using flag leaves were measured by SPAD apparatus, which was repeated for 10 times. The leaf area per plant was measured at the jointing stage, flowering stage, middle and late filling stage. Ten plants having similar growth trends were selected for each experimental plot, and the length and width of leaves were measured from all green fully expanded leaves. Then, the length and width coefficient method was applied to calculate the leaf area of each leaf by using the following equation: leaf area = length × width × 0.75. Leaf area index (LAI) was calculated as follows:
LAI=Totalleafarea(m2)Totalareaofcultivatedland(m2)Indicators Related to Nitrogen Accumulation and Transport
At the flowering and maturity stage, samples were taken from the center of the experimental plot dedicated to sampling, and ten (10) plants were sampled from each plot, and further separated into different parts as: leaf, stem + leaf sheath, glume + cob, grain and other organs. For analyzing the N concentration through the Kjeldahl method, firstly, the samples were oven-dried at 110 °C for 30 min, and later at 80°C to get a constant weight. Nitrogen accumulation and translocation in plant organs were calculated according to the method of Stevens et al. (2005) as follows:
Where, NAVOF and NAVOM are the accumulation of nitrogen in vegetative plant organs at flowering and maturity stage, respectively (kg⋅ha–1).
Nitrogentransportrateofvegetativeorgans%=NitrogentransportvolumeofvegetativeorgansNAVOFSoil Sampling and Measurements
Soil samples were collected from three random soles from a 2 m soil layer from the center of each plot after harvesting winter wheat in early June of 2018 and 2019. Three sets of soil samples from each experimental unit were randomly taken from three locations within the 0–200 cm soil profile by using a core sampler (15 cm high and 5 cm diameter). Well sieved soil samples (2 mm), from deep 2 m soil layer, were refrigerated to measure the soil NO3–-N concentration for consideration of NH3–-N leaching. A mixture containing 5.0 g of fresh sieved soil (<2 mm) extracted with 50 ml of 1 mol⋅L–1 KCL solution was used for determining the soil total NH4+-N and NO3–-N. After filtration with Whatman filter paper (No. 42), AA3 continuous flow analyzer was used to analyze the extract solution, according to the method of Norman and Stucki (1981). The soil bulk density was determined by the ring knife method, for that, five replicates were taken from the top-soil layer and three replicates were from below the soil layer. The samples were put in an oven at 105 ± 2 °C for 4 h, and then cooled the samples in a desiccator and weigh to a constant weight.
The nitrogen harvest index (NHI), agronomic efficiency nitrogen (AEN), partial factor productivity (PFP) and recovery efficiency nitrogen (REN) were used for the determination of nitrogen use efficiency (NUE), according to the procedure of (Dobermann, 2005; Jin et al., 2012; Hartmann et al., 2015) as follows:
NHI=GN/TPNAEN=Y–Y0FPFP=Y/FREN=TPN-TP0F
Where GN is nitrogen content in grains; TPN denotes the total uptake of N by the wheat plant; TP0 denotes the total uptake of nitrogen by the wheat plant under control condition; F denotes the amount of applied N; Y is the grain yield with applied N; Y0 is the crop yield under N0 treatment.
NO3–-N content was calculated as follows:
c=c0×vT×50×1000v×w×1000
Where c denotes the quantity of soil NO3– -N (mg⋅kg–1); C0 is the mass concentration of test solution obtained from the curve (μg⋅mL–1); 50 is the total volume of the colorimetric test solution used (mL); vT denotes the total extract volume used (mL); v is the volume of the filtrate drawn; w is the mass of the dry soil sample (g).
The accumulation of NO3–-N in the soil is the sum of the individual accumulated amount of NO3–-N in each soil layer at a depth of 0–2 m, the formula is as follows:
A=c×h×BD×10100
Where A and c are the accumulation of NO3–-N (kg⋅ha–1) and soil layer NO3–-N concentration (mg⋅kg–1), respectively; BD and h are the soil bulk density (g⋅cm–3) and thickness of soil layer (cm), respectively.
Residual nitrogen was calculated as follows:
Fertilizernitrogenresidue=An-A
Where An and A are the accumulation of NO3–-N in fertilized area after planting the crop (kg⋅ha–1) and the accumulation of NO3–-N in the non-fertilized area (kg⋅ha–1), respectively.
Fertilizernitrogenapparentresiduerate(%)=An-Ac
Where c is the total N application (kg⋅ha–1).
Grain Yield
At maturity, three central rows were harvested from each plot. The harvested plants were threshed using a mini thresher to separate the grains. The grains were weighed to record the grain yield.
Statistical Analysis
Primary data from the experimental unit were computed using Microsoft Excel 2010. Differences among the nitrogen treatments were separated by analysis of variance technique, and mean values were compared to calculate the least significant difference (LSD) at the 0.05 level using SPSS-19.0 software. Origin 2021 software was used for regression analysis and graphing.
ResultsPhotosynthetic Indicators
Nitrogen treatments significantly affected the photosynthetic traits during both study years. As sown in Figures 3, 4, maximum LAI at different growth stages and SPAD values were recorded for N300 which were statistically the same as N225 during both study years. However, minimum values of these traits were recorded for N0 followed by N75, N150, and N225 (Figures 3, 4). Similarly, higher net photosynthetic rate (Pn), stomatal conductance (Gs), and transpiration rates were recorded for N225 during both study years as compared with N0. When compared with control, N225 increased Pn by 113.12 and 25.43%, Gs by 188.89 and 25% and transpiration rate by 155.92 and 17.39% in 2018 and 2019, respectively (Table 2). In contrast, N225 treatment recorded a maximum reduction in intercellular CO2 concentration by 27.08 and 9.03% in 2018 and 2019, respectively, compared to the control treatment.
Impact of different nitrogen (N) application rates on leaf area index at different growth stages in winter wheat during 2018 and 2019. Above bars, different small letters show significant differences among N treatments at each growth stage. N0: unfertilized control; N75: N application at 75 kg N⋅ha– 1; N150: N application at 150 kg N⋅ha– 1; N225: N application at 225 kg N⋅ha– 1; N300: traditional N application rate of 300 kg N⋅ha– 1.
Influence of different nitrogen (N) application rates on SPAD values at flowering stage in winter wheat during 2018 and 2019. Above bars, different small letters show significant differences among N treatments. N0: unfertilized control; N75: N application at 75 kg N⋅ha– 1; N150: N application at 150 kg N⋅ha– 1; N225: N application at 225 kg N⋅ha– 1; N300: traditional N application rate of 300 kg N⋅ha– 1.
Effects of different N application rates on net photosynthetic rate, stomatal conductance, intercellular CO2 concentration and transpiration rate at flowering stage of winter wheat in 2018 and 2019.
Year
Treatments
Index
Net photosynthetic rate
Stomatal conductance
Intercellular CO2 concentration
Transpiration rate
(μmol m–2 s–1)
(mmol m–2⋅s–1)
(μmol mol–1)
(mmol m–2 s–1)
2018
N0
12.35e
0.09d
180.98a
2.70e
N75
13.91d
0.09d
163.38b
2.84d
N150
18.98c
0.16c
140.62e
4.36c
N225
26.32a
0.26a
131.96d
6.91a
N300
22.62b
0.22b
152.31c
6.18b
2019
N0
8.81c
0.32b
362.10a
1.61b
N75
10.16b
0.33b
348.82b
1.70b
N150
10.89a
0.37ab
338.60c
1.78ab
N225
11.05a
0.40a
332.85c
1.89a
N300
10.62ab
0.34b
350.94b
1.69b
N0: unfertilized control; N75: N application at 75 kg N⋅ha–1; N150: N application at 150 kg N⋅ha–1; N225: N application at 225 kg N⋅ha–1; N300: traditional N application rate of 300 kg N⋅ha–1. Different small letters show significant differences at p < 0.05.
Distribution of Nitrogen
Nitrogen distribution in different plant organs was significantly affected by N treatments during both experimental years at the flowering and maturity stage (Tables 3, 4). At the flowering stage, maximum N accumulation in spike axis + husk and leaf were recorded for N300 which was statistically the same as N225, however, maximum N accumulation in stem + sheath was recorded for N225 during both years. For total N accumulation, maximum values were recorded for N330 during both years, which was statistically similar to N225 in 2018. At maturity stage, maximum N accumulation in grains, spike axis + husk, and leaf + spike + sheath was recorded for N300 followed by N225 during both experimental years (Table 4). Similarly, for total N accumulation, maximum values were recorded for N300 which were statistically similar to N225. For total distribution proportion, maximum values (18.40 and 16.88%, in 2018 and 2019, respectively) were recorded for N300 flowered by other N treatments.
Nitrogen accumulation and distribution proportion in different organs at flowering stage of winter wheat in 2018 and 2019 under different nitrogen application rates.
Year
N treatments
Nitrogen accumulation amount (kg⋅hm–2)
Distribution proportion (%)
Spike axis+Husk
Leaf
Stem+Sheath
Total
Spike axis+Husk
Leaf
Stem+Sheath
2018
N0
11.23c
20.02c
20.69b
51.94d
21.57a
38.46b
39.97a
N75
15.64c
30.73c
26.41b
72.78c
21.67a
42.17ab
36.16ab
N150
22.63b
45.26b
33.62ab
101.51b
22.42a
44.55a
33.03ab
N225
29.65a
56.34ab
38.56a
124.55a
24.15a
44.99a
30.86b
N300
32.85a
60.35a
42.16c
135.36a
24.15a
44.44a
31.41ab
2019
N0
15.62c
22.16e
21.56b
59.34e
26.27a
37.52b
36.21a
N75
17.92bc
32.85d
26.39b
77.16d
23.27ab
42.58ab
34.15a
N150
22.03b
43.33c
31.48ab
96.84c
22.78ab
44.75a
32.47a
N225
26.54ab
56.32b
36.85a
119.71b
22.13ab
47.16a
30.71a
N300
29.53a
65.12a
41.59c
136.24a
21.61b
47.81a
30.58a
N0: unfertilized control; N75: N application at 75 kg N⋅ha–1; N150: N application at 150 kg N⋅ha–1; N225: N application at 225 kg N⋅ha–1; N300: traditional N application rate of 300 kg N⋅ha–1. Different small letters show significant differences at p < 0.05.
Nitrogen accumulation and distribution proportion in different organs at maturity stage of winter wheat in 2018 and 2019 under different nitrogen application rates.
Year
Treatment
Nitrogen accumulation amount (kg⋅hm–2)
Distribution proportion
Grain
Plant organs
Grain
Plant organs
Spike axis+Husk
Leaf+Stem+Sheath
Total
Spike axis+Husk
Leaf+Stem+Sheath
Total
2018
N0
66.14c
3.24b
4.34c
7.58d
89.72a
3.49d
6.10b
10.28b
N75
87.40c
4.47b
6.35c
10.82cd
88.98a
4.78cd
6.78b
11.02b
N150
120.56b
6.35b
10.15bc
16.50c
87.95a
6.73c
7.80b
12.05b
N225
173.56a
10.56a
14.26b
24.82b
97.39a
10.76b
7.63b
12.61b
N300
169.87a
13.59a
25.00a
38.59a
81.60b
14.27a
12.77a
18.40a
2019
N0
86.24d
3.74d
8.85c
12.59e
87.27a
4.11d
9.30a
12.73b
N75
110.26c
5.10cd
12.06c
17.16d
86.40ab
5.58cd
9.98a
13.60b
N150
135.86b
6.37c
15.96b
22.33c
85.87ab
6.90c
10.53a
14.13b
N225
170.65a
8.56b
20.67a
29.23b
85.28ab
9.13b
10.89a
14.72b
N300
168.52a
11.51a
22.63a
34.14a
83.12b
12.15a
11.87a
16.88a
N0: unfertilized control; N75: N application at 75 kg N⋅ha–1; N150: N application at 150 kg N⋅ha–1; N225: N application at 225 kg N⋅ha–1; N300: traditional N application rate of 300 kg N⋅ha–1. Different small letters show significant differences at p < 0.05.
Transport of Nitrogen
Nitrogen accumulation amount in kernels after flowering stage, N translocation amount from vegetative organs to kernel, N contribution proportion of that accumulated after anthesis stage and N contribution proportion from vegetative organs varied significantly for N treatments where maximum N accumulation amount in kernels after flowering stage and N translocation amount from vegetative organs to kernels was recorded for N300 followed by N225 whereas minimum was recorded for control treatment without N application during both study years (Figure 5). For N contribution proportion of that accumulated after anthesis stage, maximum values were recorded for N75, N150, and N225 followed by control treatment without N application, however, minimum was recorded for N300. Similarly, for N contribution proportion from vegetative organs, N300 and N0 significantly increased the values followed by other treatments (Figure 5).
Influence of different nitrogen (N) application rates on N accumulation amount of kernel after flowering stage, N translocation amount from vegetative organs to the kernel, N contribution proportion of that accumulated after anthesis stage and N contribution proportion from vegetative organs in winter wheat during 2018 and 2019. In bars of specific colors, different small letters show significant differences among N treatments. N0: unfertilized control; N75: N application at 75 kg N⋅ha– 1; N150: N application at 150 kg N⋅ha– 1; N225: N application at 225 kg N⋅ha– 1; N300: traditional N application rate of 300 kg N⋅ha– 1.
Nitrogen Efficiency
As shown in Table 4, NHI was decreased with increasing N rates during both study years. However, for N fertilizer recovery efficiency, values were increased with increasing N rate in 2018, and the values were statistically the same for all N treatments in 2019. For N fertilizer contribution rate, the maximum percentage of 22.69 and 40.02% in 2018 and 2019 were recorded for N225 as compared with N0 where minimum values were recorded (Table 5). However, there was an opposite trend for N-fertilizer partial factor productivity and agronomic efficiency of N fertilizer where maximum values were recorded for control treatment without N application.
Effect of nitrogen (N) application rates on grain yield, N fertilizer recovery, N fertilizer contribution efficiency, N fertilizer partial factor productivity and agronomic efficiency of N fertilizer in winter wheat during 2018 and 2019.
N0: unfertilized control; N75: N application at 75 kg N⋅ha–1; N150: N application at 150 kg N⋅ha–1; N225: N application at 225 kg N⋅ha–1; N300: traditional N application rate of 300 kg N⋅ha–1. Different small letters show significant differences at p < 0.05.
Residual Nitrogen
The NH3–-N, NH4+-N, residual N and N apparent residual rates in 2018 and 2019 are shown in Figure 1. The NH3–-N contents increased with the increase in soil depth and N application rates (Figure 6). The straw effect gradually appeared over time, which reduced the NH3–-N leaching during the second year. The positive influence of N treatments on NH4+-N was as N150 > N225 > N75 > N0 > N300. Residual N amount and rates were increased gradually with the increase of N application rate in 2018 and 2019, indicating that appropriate N can reduce N losses (Figure 6).
Effect of different nitrogen (N) fertilization rates on NH3–-N, NH4+-N, residual N and N apparent residual rate in winter wheat during 2018 and 2019. The line segment is the LSD-t value obtained by multiple comparisons using the LSD-t method. N0: unfertilized control; N75: N application at 75 kg N⋅ha– 1; N150: N application at 150 kg N⋅ha– 1; N225: N application at 225 kg N⋅ha– 1; N300: traditional N application rate of 300 kg N⋅ha– 1. The left-side figures show data for 2018, and the right-side figures show data for 2019.
Yield and Yield-Related Traits
Nitrogen treatments significantly affected the yield traits including grains per spikes, 1000-grain weight, and grain yield of winter wheat during both years (Table 6). Among N treatments, maximum 1000-grain weight and seed yield were recorded for N225 which increased 1000-grain weight by 4.46 and 31.02% and grain yield by 29.36 and 66.78% in 2018 and 2019, respectively, compared to the control treatment (N0) (Table 6).
Effect of nitrogen (N) application rates on number of ears per hectare, grains per spike, 1000-grain weights, and grain yield of winter wheat during 2018 and 2019.
Year
Treatment
Number of ears (10000 ha–1)
Grains per spikes
1000-grain weight (g)
Grain yield (kg⋅ha–1)
2018
N0
480.01d
30.96c
34.90d
5102.61d
N75
503.89c
33.48b
35.61c
6006.38c
N150
509.44bc
35.40a
35.90c
6480.00b
N225
521.44b
34.68ab
36.86a
6600.00a
N300
535.15a
33.84b
36.37b
6510.00b
2019
N0
473.89e
28.70d
37.97d
4104.16d
N75
487.78d
31.97c
43.62c
5254.69c
N150
496.11c
33.80ab
48.20ab
6500.86b
N225
505.00b
34.57a
49.75a
6845.47a
N300
516.67a
33.00b
47.70b
6405.68b
N0: unfertilized control; N75: N application at 75 kg N⋅ha–1; N150: N application at 150 kg N⋅ha–1; N225: N application at 225 kg N⋅ha–1; N300: traditional N application rate of 300 kg N⋅ha–1. Different small letters show significant differences at p < 0.05.
DiscussionPhotosynthetic Process
Leaf area index (LAI) can be used to reflect the growth of plant population and determine the supply status of N. Photosynthetic rates directly reflect the strength of photosynthesis. The SPAD value at flowering stage indicates the relative chlorophyll content of flag leaf In our study, the SPAD value and LAI at flowering stage increased linearly with an increase of N application. Latiri-Souki et al. (1998) also demonstrated that increasing the applied amount of N improved LAI, relative chlorophyll content of flag leaf, and photosynthetic rate for better crop growth and seed yield. Net photosynthetic rate, Ci and Tr are among the important indicators of gasses exchange and photosynthetic activity of plant leaves. In this study, Pn, Ci and Tr of flag leaf were increased with increasing N and maximum values were recorded for N225. On the contrary, Gs was decreased with increasing N rate and minimum values were recorded for N225 in comparison to other treatments. Our results showed that excessive N application at 300 kg⋅ha–1 reduced the photosynthetic performance of winter wheat. However, N application at 225 kg⋅ha–1 was the most beneficial for stomatal gases exchange and photosynthesis. Some published reports also demonstrated that leaves photosynthetic rates increase with increasing N till a threshold level is reached. The number of panicles in wheat increased correspondingly under high N rate, but large population of applied N was not conducive to increase the leaf photosynthesis (Latiri-Souki et al., 1998).
Plant N Distribution and Transportation
Nitrogen application below the recommended rate retarded the growth and developmental processes in wheat. On the other hand, higher N rates caused significant losses by adding the excess N to soil residual N pools; thereby pollutanting the environment as well (Anas et al., 2020). Nitrogen application rates significantly affected the uptake and transportation of N in winter wheat. In our work, N accumulation in different organs at flowering stage follow the trend as leaf > stem sheath > glume + cob, and N accumulation in grains was significantly increased, ranging from 66.14–173.56 kg⋅ha–1, accounting for 81.60–89.72% of the total N in above-ground plant parts. Also, about 53.26–71.23% of total N was transferred from vegetative organs after flowering stage, indicating that vegetative organs contribute a relatively high proportion to N accumulation in grains. Excessive N application is beneficial for accumulating N in vegetative organs at maturity stage.
Nitrogen Efficiency
The effect of various nitrogen application rates on N efficiency of winter wheat was mainly reflected in absorption efficiency and utilization efficiency. Nitrogen absorption and utilization efficiency were different under different N rates. In this study, with exceeding N rates, nitrogen harvest index, nitrogen partial productivity and agronomic nitrogen efficiency were decreased gradually, and nitrogen absorption efficiency and N contribution rates were consistent where maximum values were recorded for N225. Similar to our findings, Wang et al. (2010) reported that increasing N rates significantly increased the wheat yield, N uptake and utilization efficiency, however, excessive rates caused a significant decline in wheat N utilization efficiency.
Residual Nitrogen
In this study, the residual soil N was increased gradually with increase of N rate. In Guanzhong Plain area, remaining soil N after crop harvest tends to move-down after rainfall, resulting in leaching and denitrification losses. Our results showed that the nitrate-N contents are positively related to N application rates. This is consistent with previous studies where increasing N rate caused a significant increase in nitrate-N content (Wang et al., 2010). In this study, the nitrate nitrogen (NO3-N) was mainly concentrated in deeper soil layers (below 120 cm) and varied in different experimental years. As compared with second year of the experiment (2019), nitrate nitrogen contents (NO3-N) in different soil layers were lower in the first year (2018), which may be related to the effect of straw retention. Straw retention combined with N fertilizer is beneficial to improve the plant N utilization efficiency and reduce the leaching N losses. In a previous study, Oikeh et al. (2003) demonstrated that under high N application, soil residual N content kept on increasing with increasing soil depth and were maximum below 1 m soil layer. In our study, ammonium nitrogen content did not vary significantly for tested soil layer.
Yield
Fertilizer application combined with straw retention plays an important role in sustainable agricultural production. In this study, compared with N0, grain yield was improved to some extent with exceeding N application rates with increase of 17.71, 26.99, 29.35, and 27.58% in 2018 and 28.03, 58.40, 66.79, and 56.08% in 2019 for N0, N150, N225, and N300, respectively. Straw retention combined with synthetic N fertilizer application effectively increased the wheat yield where maximum yield was recorded during second study year. N150 and N225 treatments recorded higher yield than N300. Previous published reports also showed that N application rates and grain yield showed a quadratic curve where N application increased grain yield within a certain threshold range. When N application rate exceeds the critical range, it results in lower nitrogen use efficiency and environmental pollution (Yang et al., 2017). Reduced number of grains per spike, thousand grain weight, grain yield and water and nutrient use efficiency under excessive N rate were also reported by Liu et al. (2021).
Conclusion
N application rates significantly affected the photosynthesis, plant growth, N use efficiency and productivity of winter wheat under straw incorporation. N application at 150 and 225 kg⋅ha–1 was the most economical method under straw incorporation; it significantly increased the N uptake, N translocation in different plant organs including grains. Compared with N300, N150 and N225 reduced the soil nitrogen losses and in this way contributed to the sustainable agricultural development.
Data Availability Statement
The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author/s.
Author Contributions
JW: conceptualization, methodology, investigation, formal analysis, writing – original draft, and writing – review and editing. SH: investigation, formal analysis, and writing – review and editing. XS: investigation, formal analysis, and writing – review and editing. TJ: writing – review and editing. ED: writing – review and editing. PZ: writing – review and editing. XC: conceptualization, methodology, and writing – review and editing. XR: conceptualization, methodology, and writing – review and editing. All authors contributed to the article and approved the submitted version.
Conflict 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.
Publisher’s Note
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.
Funding
This study was supported by the Program of Shaanxi Province Key R&D Program (Nos. 2021NY-073 and 2022NY-196), National Natural Science Foundation of China (Nos. 31871580 and 31871562), and Ningxia Hui Autonomous Region Key R&D Program (No. 2019BBF03011). This study was also financially supported by Taif University Researchers Supporting Project number (TURSP-2020/85), Taif University, Taif, Saudi Arabia.
The authors extend their sincere appreciation to Taif University for funding current work by Taif University Researchers Supporting Project number (TURSP-2020/85), Taif University, Taif, Saudi Arabia. The authors grateful to Junfeng Nie, Baoping Yang, Ruixia Ding, and Hui Li for their help with the experiments.
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