Experimental site
This study was conducted from 2020 to 2022 at the Xinji Experimental Station(37°99′N, 115°20′E), Hebei Agricultural University, situated at an altitude of 37 meters above sea level (Fig. 1). Xinji experiences a warm continental monsoon climate characterized by a mean annual temperature of 12.5°C, an average annual rainfall of 472.5 mm. The frost-free period spans 188 days, with a mean annual sunshine duration of 2571 hours. The soil type at the experimental station corresponds to Eum-Orthosol according to the Chinese Soil Taxonomy. The topsoil layer (0–40 cm) in the experimental field exhibits an average bulk density of 1.56 g cm−³. Notably, the soil contains 22.4 g kg− 1 of organic matter, 1.26 g kg− 1 of total nitrogen, 125.0 mg kg− 1 of quick-acting potassium, and 25.8 mg kg− 1 of quick-acting phosphorus. The initial pH value of the soil at the commencement of the field experiment was 7.6. Each year of the trial featured summer maize cultivation preceding the wheat crop. Figure 2 illustrates the average daily temperature and precipitation patterns during the three years of wheat growth. The cumulative rainfall for the two wheat-growing seasons amounted to 103.4 mm in 2020–2021 and 99.2 mm in 2021–2022.
Experimental design and field management
The experiment was structured as a two-factor split plot design. The primary factor encompassed five different nitrogen (N) application levels: 0, 180, 240, 300, and 360 kg N ha− 1, denoted as N0, N1, N2, N3, and N4, respectively. Only the harvest yield of the N0 plots was used to calculate the nitrogen agronomic efficiency (NAE). The secondary factor comprised three irrigation levels: traditional conventional irrigation (W2) involving 75 mm of water at the 3-leaf stage and an additional 75 mm at the anthesis stage; water-saving irrigation (W1) comprising a single 75 mm irrigation event at the 4-leaf stage in spring; and no irrigation in spring (W0). Only the harvest yield of the W0 plot was used to calculate irrigation water utilization efficiency (IWUE). Figure 3 shows the plant growth and spikelet growth process of wheat during irrigation at the spring 3-leaf stage, spring 4-leaf stage and anthesis stage. The ratio of base N fertilizer to topdressing was 1:1, and the topdressing fertilizer was applied at the 3- (W2) and 4-leaf stages (W1) in the spring. Each treatment was replicated three times; the plot size was 60 m2. The chosen planting material was the high-yielding semi-winter wheat variety, ‘Gao You 2018’, known for its robust tillering capacity and wide cultivation in Hebei Province. Planting density was set at 3.75 × 106 plants per hectare, and the row spacing was 15 cm. The irrigation method employed was micro-sprinkling irrigation, utilizing micro- sprinkling hoses with a length of 10 m. The flow rate was maintained at 6.0 m3/h under a working pressure of 0.02 MPa with a micro- sprinkling hose spray angle of 80°. To prepare the field, prior to sowing, summer maize straw was finely crushed and returned to the soil. A one-time base fertilizer was applied, which included 112.5 kg ha− 1 of P2O5, and 112.5 kg ha− 1 of K2O) for each plot. This was followed by two rotary cultivator tilling operations at a depth of 15 cm. Wheat sowing took place on October 8, 2020, and October 16, 2021, with harvests conducted on June 10, 2021, and June 9, 2022.
Sampling and measurements
Crop phenology
Crop phenology was defined using the Zadoks scale (1974), following the mean phenology of plots (when 50% of shoots reached the main developmental stage).
Soil water content and crop water use characteristics
Assessed across a depth range of 0–200 cm at 20 cm intervals during key growth stages, including sowing, turning green, jointing, anthesis, and maturity stage. This measurement was conducted using a Trimer Pico 64 portable soil moisture meter (TDR, IMIKO, Bochum, Germany).
Water consumption amount (WCA) is the water consumption amount for a given stage, water consumption percentage (WCP) is the percentage of water consumption of that stage, and water consumption intensity (WCI) characterizes the average intensity of water consumption at different wheat growth stages. These calculations are as follows (Ye et al. 2022):
$$\text{W}\text{C}\text{A}={\text{P}}_{\text{i}}+{\text{I}}_{\text{i}}+{\varDelta \text{S}}_{\text{i}}$$
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where \({\text{P}}_{\text{i}}\),\({\text{I}}_{\text{i}}\) and \({\text{I}\varDelta \text{S}}_{\text{i}}\) are the precipitation, irrigation and the amount change of soil water stored in 200 cm soil mass for a given period.
where \(\text{E}\text{T}\text{i}\) is the water consumption for a given period (mm), and ET is the total water consumption (evapotranspiration, mm) for the entire growth period.
$$\text{W}\text{C}\text{I}=\frac{\text{E}\text{T}\text{i}}{\text{G}\text{D}}$$
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where \(\text{W}\text{C}\text{I}\) is the water consumption intensity (mm d− 1), \(\text{G}\text{D}\) (d) is the effective growing degree days for a given period.
Crop water and nitrogen productivity of wheat
Water use efficiency (WUE kg ha− 1 mm− 1) and irrigation water use efficiency (IWUE ha− 1 mm− 1) were calculated by the following formula:
$$\text{W}\text{U}\text{E}=\frac{\text{G}\text{Y}}{\text{E}\text{T}}$$
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$$\text{I}\text{W}\text{U}\text{E}=\frac{\text{G}{\text{Y}}_{\text{I}}-{\text{G}\text{Y}}_{0}}{{\text{E}}_{\text{i}}}$$
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where \(\text{G}\text{Y}\) is the grain yield (kg ha− 1), \(\text{G}{\text{Y}}_{\text{I}}\) and \({\text{G}\text{Y}}_{0}\) are the GYs (kg ha− 1) of winter wheat in irrigation application and irrigation-free areas, respectively, \(\text{E}\text{T}\) (mm) is the total water consumption (evapotranspiration, mm) for the entire growth period, and \({\text{E}}_{\text{i}}\)(mm) is the irrigation amount.
Evapotranspiration (\(\text{E}\text{T}\)) during the wheat growth period was calculated using the water balance method by Li et al. (2010) as follows:
$$\text{E}\text{T}=\text{P}+\text{I}-\text{D}-{\text{W}}_{\text{g}}-\text{R}+\varDelta \text{S}$$
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where \(\text{P}\) is precipitation during the growth period (mm), \(\text{I}\) is the irrigation amount (mm), \(\text{D}\) is soil displacement (mm), \({\text{W}}_{\text{g}}\) is the amount of water produced by crops rising through groundwater capillaries (mm), \(\text{R}\) is surface runoff (mm), and \(\varDelta \text{S}\) is the amount change of soil water stored in 200 cm soil mass between sowing and maturity. In this study, micro-sprinkling irrigation was used, and there was no surface water accumulation; thus, \(\text{R}\) was ignored. In addition, the depth of groundwater in the study region during winter wheat growth was greater than 5 m, making \({\text{W}}_{\text{g}}\) negligible. In this study, the irrigation system was controlled by a water meter, and the irrigation volume was low; thus, there was no drainage, and \(\text{D}\) was negligible.
Crop nitrogen fertilizer agronomic efficiency (NAE, kg kg− 1) Is an index to estimate the grain yield increase from fertilizer application. \(\text{N}\text{A}\text{E}\) was calculated using the following formula (Mon et al. 2016):
$$\text{N}\text{A}\text{E}=\frac{\text{G}{\text{Y}}_{\text{N}}-{\text{G}\text{Y}}_{0}}{{\text{E}}_{\text{N}}}$$
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where \(\text{G}{\text{Y}}_{\text{N}}\) and \({\text{G}\text{Y}}_{0}\) are the GYs (kg ha− 1) of winter wheat in nitrogen application and nitrogen-free areas, respectively, and \({\text{E}}_{\text{N}}\) is the amount of nitrogen applied (kg ha− 1) in nitrogen application areas.
Photosynthesis parameters
Net photosynthesis rate (Pn) was recorded using the LI-6800 Portable Photosynthesis System (Li-cor, Lincoln, NE, USA). Three selected flag leaves in each plot were measured between 9:00 and 11:00 am at anthesis and grain filling stage (20 days after anthesis). The leaf chamber was maintained under control, with a reference CO2 concentration of 400 µ mol mol− 1, a leaf temperature of 25°C, a saturated photosynthetic photon flux density of 1000 µ mol m–2 s− 1, and a relative humidity of 60%-70%.
At anthesis and the grain filling stage (20 days after anthesis), 20 flag leaf samples were randomly collected. The chlorophyll content of flag leaves (Chl) was extracted with 95% alcohol and measured with a spectrophotometer at A649 and A665.
Plant dry matter, grain yield and composition
In the same plots, spike number was measured in 1-m double rows. The grain number per spike was determined by counting the kernels in each spike from 60 randomly selected plants in each plot before harvest. At the wheat maturity stage, three 3m2 plots of wheat were randomly selected from each plot and threshed and weighed after natural drying to determine the grain yield. The actual grain yield was reported on a 13% moisture basis. The 1000-grain weight was calculated by weighing 1000 seeds from each sample and averaged from three replicates.
To determine plant dry matter, plants with a floor area of 0.15 m2 were sampled at maturity and all plant samples were dried at 105°C for 30 minutes and then at 75°C to balance weight. The harvest index (HI) was considered as the ratio of grain yield to total above-ground dry matter at maturity.
Calculation of economic benefits
The net income (CNY ha− 1) for each treatment is determined by subtracting the total input from the total output, while the benefit-cost ratio is determined by dividing the net income by the total input (Kamran et al. 2023).
\(\text{N}\text{e}\text{t} \text{i}\text{n}\text{c}\text{o}\text{m}\text{e}=\text{t}\text{o}\text{t}\text{a}\text{l} \text{o}\text{u}\text{t}\text{p}\text{u}\text{t}-\text{t}\text{o}\text{t}\text{a}\text{l} \text{i}\text{n}\text{p}\text{u}\text{t}\) (8) \(\text{B}\text{e}\text{n}\text{e}\text{f}\text{i}\text{t}-\text{c}\text{o}\text{s}\text{t} \text{r}\text{a}\text{t}\text{i}\text{o}=\text{n}\text{e}\text{t} \text{i}\text{n}\text{c}\text{o}\text{m}\text{e}/\text{t}\text{o}\text{t}\text{a}\text{l} \text{i}\text{m}\text{p}\text{u}\text{t}\) (9)
The cost of the total input includes the cost of materials (seeds, irrigation equipment, irrigation water, fertilizers and pesticides), and the cost of labor (preparation, sowing, fertilization, irrigation, weeding, harvesting). Total revenue is calculated based on total wheat grain production and average annual market price per unit (CNY ha− 1).
Data analysis
In this study, Microsoft Excel 2021 was used for data arrangement and SPSS21.0 (IBM Inc., Armonk, NY, USA) was used for data analysis. The images were drawn using ArcGIS 10.2, Origin 2021 (OriginLab Corporation, Northampton, MA, USA), and GraphPad Prism 9.0 (GraphPad Software, Inc., San Diego, CA, USA).