Wide space sowing achieved high productivity and effective nitrogen use of irrigated wheat in South Shanxi, China

Site description

Field experiments were conducted in a farmer’s field in Shangyuan Village, Hougong Township, Wenxi County, Shanxi Province, China (35°24′N, 111°26′E) in 2017–2018 and repeated in a nearby field in 2018–2019. This site has a typical semi-arid warm temperature and continental monsoon climate (Köppen classification) with an average daily temperature of 8.6 °C, average precipitation of 190.5 mm, and 3015.6 MJ m⁻² of total solar radiation during the wheat growing season (from mid–October to early June) from 2005 to 2015. Soil samples from the upper 20 cm layer were randomly collected with five replicates for soil analysis before wheat was sown in 2017 and 2018. The soil type was classified as calcareous cinnamon soil according to Chinese soil taxonomy with a pH of 8.47–8.61, organic matter of 13.61–14.31 g kg⁻¹, total N of 1.01–1.05 g kg⁻¹, alkaline N of 40.05–44.07 mg kg⁻¹, Olsen P of 10.71–11.25 mg kg⁻¹, and available K of 188.87–200.24 mg kg⁻¹ in 2017–2018 and 2018–2019 (Table 1). The cropping pattern of the experimental site is a winter wheat–summer maize double–cropping system. The climate parameters in 2017–2018 and 2018–2019 were collected from a weather station (Watchdog 2000 Series; Spectrum Technologies Inc, Aurora, IL, USA) approximately 200 m from the experimental field.

Experimental design and crop management

The experiment was arranged in a split-plot design with the sowing method as the main plot and N application rate as a subplot with three replicates. Two sowing methods (Fig. 1), wide space sowing (WS, sowing and row width were 8 and 25 cm, respectively) and drill sowing (DS, sowing and row width were 3 and 20 cm, respectively), and four N application rates, 0, 180, 240, and 300 kg ha⁻¹ (represented as N0, N180, N240, and N300, respectively) were applied in this study. DS was performed using a drill sowing machine (2BXF-12; Nonghaha Mechanical Co. Ltd., Shijiazhuang, China), and WS was performed using a wide space sowing machine (2BMYF-10/5; Yuncheng Gongli Co. Ltd., Heze, China).

Each subplot was 8 m in length and 4 m in width. A widely planted wheat cultivar, Liangxing99, was planted on 25^th October and 11^th October 2017 and 2018. The expected plant density was approximately 300 plants m⁻² for both sowing methods. The plant densities at the three-leaf stage (Zadoks code 13) were 295 and 312 plants m⁻² in 2017 and 2018, respectively, and there was no significant difference between WS and DS.

N fertilizer was applied as urea (46% N), with 60% of the N fertilizer being applied before sowing, and 40% as topdressing fertilizer during jointing (Zadoks code 32); 150 kg P₂O₅ ha⁻¹ in the form of calcium super-phosphate (16% P₂O₅) and 90 kg K₂O ha⁻¹ in the form of potassium chloride (52% K₂O) were applied before sowing. Each plot was irrigated thrice, with 60 mm (1.92 m³/plot) water at wintering (Zadoks code 26), jointing (Zadoks code 32), and anthesis (Zadoks code 65). Irrigation water was supplied by a movable sprinkler system, and the amount of water applied was measured using a flow meter. The field was kept free of diseases, pests, insects, and using pesticides as needed. Weeds were well-controlled with herbicide administration two or three times in each experimental year.

Sampling and measurements

The stem number (sum of main stems and tillers) of wheat plants was counted in a typical and central row of 1 m length at jointing, at which the wheat plants had the maximum stems number (Lu et al., 2021). The productive stem percentage was calculated as the ratio of ear number to the maximum stem number. During each growing season, the wheat plants were sampled in a row of 0.5 m length at anthesis and maturity (Zadoks code 91). At anthesis, all green leaves were separated and measured using a leaf area meter (LI–3100C, LI–COR, Lincoln, NE, USA) to calculate the leaf area index. All samples were then divided into ear and vegetative parts (stems, sheaths plus leaves). At maturity, after counting the ear number, the samples were divided into grain and straw (stems, sheaths, leaves, chaff plus rachis). All separated samples were oven dried at 105 °C for 30 min and weighed after further drying at 70 °C to a constant weight. The grain number per ear and 1,000-grain weight were calculated using the grain samples described above. The yield was determined from a 10 m² area at maturity in the center of each plot and adjusted to a standard moisture content of 0.125 g H₂O g⁻¹ fresh weight. The grain moisture content was measured using a digital moisture tester (PM8188A; Kett Electric Laboratory, Tokyo, Japan).

After the dry matters of all separated sample plants at anthesis and maturity were weighed, all the samples were shredded using a plant ball mill pulverize (Jxfstprp-11; Jingxin Co Ltd, Shanghai, China) for N concentration measurement. The N concentration in the samples was determined using the standard indophenol-blue colorimetric method (Novamsky et al., 1974).

Soil samples were collected from 0–20, 20–40, 40–60, 60–80, and 80–100 cm soil depths in each plot, and were analyzed for total mineral N content (NO₃⁻-N and NH₄⁺-N) using the method described by Wagner (1974) and Benesch & Mangelsdorf (1972).

Statistical analysis and calculations

The experimental data were statistically analyzed using Microsoft Excel 2016 and Statistix 8.0 (Analytical Software, Tallahassee, FL, USA), and figures were generated using Origin Lab Pro 2021b (OriginLab Corporation, Northampton, MA, USA). All data are presented as the means of three replicates (n = 3). Comparisons among multiple groups were performed using Tukey’s honestly significant difference (HSD) test. Differences were considered statistically significant at P < 0.05. Statistix 8.0 software was used for the variance analysis.

The accumulation, partitioning, and translocation of dry matter and N were calculated using the following equations (Laza et al., 2003; Cox, Qualset & Rains, 1986):

(1) $$\begin{array}{l}\mathrm{P}\mathrm{o}\mathrm{s}\mathrm{t}\text{-}\mathrm{a}\mathrm{n}\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{s}\mathrm{i}\mathrm{s}\mathrm{d}\mathrm{r}\mathrm{y}\mathrm{m}\mathrm{a}\mathrm{t}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}(\mathrm{D}{\mathrm{M}}_{\mathrm{p}\mathrm{o}\mathrm{s}\mathrm{t}},\mathrm{t}\mathrm{h}{\mathrm{a}}^{-1})\\ =\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{d}\mathrm{r}\mathrm{y}\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{m}\mathrm{a}\mathrm{t}\mathrm{u}\mathrm{r}\mathrm{i}\mathrm{t}\mathrm{y}-\mathrm{T}\mathrm{D}{\mathrm{W}}_{\mathrm{a}\mathrm{s}}\end{array}$$

(2) $$\mathrm{H}\mathrm{a}\mathrm{r}\mathrm{v}\mathrm{e}\mathrm{s}\mathrm{t}\mathrm{i}\mathrm{n}\mathrm{d}\mathrm{e}\mathrm{x}(\mathrm{\%})=\frac{\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{i}\mathrm{n}\mathrm{d}\mathrm{r}\mathrm{y}\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}}{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{d}\mathrm{r}\mathrm{y}\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{m}\mathrm{a}\mathrm{t}\mathrm{u}\mathrm{r}\mathrm{i}\mathrm{t}\mathrm{y}}$$

(3) $$\mathrm{P}\mathrm{o}\mathrm{s}\mathrm{t}\text{-}\mathrm{a}\mathrm{n}\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{s}\mathrm{i}\mathrm{s}\mathrm{a}\mathrm{c}\mathrm{c}\mathrm{u}\mathrm{m}\mathrm{u}\mathrm{l}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{d}\mathrm{N}({\mathrm{N}}_{\mathrm{p}\mathrm{o}\mathrm{s}\mathrm{t}},\mathrm{k}\mathrm{g}\mathrm{h}{\mathrm{a}}^{-1})=\mathrm{T}\mathrm{N}-\mathrm{T}{\mathrm{N}}_{\mathrm{a}\mathrm{s}}$$

(4) $$\mathrm{N}\mathrm{h}\mathrm{a}\mathrm{r}\mathrm{v}\mathrm{e}\mathrm{s}\mathrm{t}\mathrm{i}\mathrm{n}\mathrm{d}\mathrm{e}\mathrm{x}(\mathrm{N}\mathrm{H}\mathrm{I},\mathrm{\%})=\mathrm{G}\mathrm{N}/\mathrm{T}\mathrm{N}$$where TDW_as (t ha⁻¹) is the total dry weight at anthesis. TN (kg ha⁻¹) and TN_as (kg ha⁻¹) are the total N quantity at maturity and anthesis, respectively. GN (kg ha⁻¹) is grain N content.

N use-related traits were calculated using the following equations (Moll, Kamprath & Jackson, 1982; Foulkes et al., 2009):

(5) $$\mathrm{N}\mathrm{u}\mathrm{p}\mathrm{t}\mathrm{a}\mathrm{k}\mathrm{e}\mathrm{e}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y}(\mathrm{N}\mathrm{U}\mathrm{p}\mathrm{E},\mathrm{\%})=\mathrm{T}\mathrm{N}/\mathrm{s}\mathrm{o}\mathrm{i}\mathrm{l}\mathrm{N}(\mathrm{p}\mathrm{r}\mathrm{e}\text{-}\mathrm{s}\mathrm{o}\mathrm{w}\mathrm{i}\mathrm{n}\mathrm{g}\mathrm{s}\mathrm{o}\mathrm{i}\mathrm{l}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{l}\mathrm{N}+{\mathrm{N}}_{\mathrm{f}})$$

(6) $$\mathrm{N}\mathrm{u}\mathrm{t}\mathrm{i}\mathrm{l}\mathrm{i}\mathrm{s}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{e}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y}(\mathrm{N}\mathrm{U}\mathrm{t}\mathrm{E},\mathrm{k}\mathrm{g}\mathrm{k}{\mathrm{g}}^{-1})=\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{i}\mathrm{n}\mathrm{d}\mathrm{r}\mathrm{y}\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}/\mathrm{T}\mathrm{N}\times 1000$$

(7) $$\mathrm{N}\mathrm{u}\mathrm{s}\mathrm{e}\mathrm{e}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y}(\mathrm{N}\mathrm{U}\mathrm{E},\mathrm{k}\mathrm{g}\mathrm{k}{\mathrm{g}}^{-1})=\mathrm{N}\mathrm{U}\mathrm{p}\mathrm{E}\times \mathrm{N}\mathrm{U}\mathrm{t}\mathrm{E}$$

(8) $$\mathrm{A}\mathrm{g}\mathrm{r}\mathrm{o}\mathrm{n}\mathrm{o}\mathrm{m}\mathrm{i}\mathrm{c}\mathrm{N}\mathrm{u}\mathrm{s}\mathrm{e}\mathrm{e}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y}(\mathrm{A}{\mathrm{E}}_{\mathrm{N}},\mathrm{k}\mathrm{g}\mathrm{k}{\mathrm{g}}^{-1})=({\mathrm{Y}}_{\mathrm{N}}-{\mathrm{Y}}_0)/{\mathrm{N}}_{\mathrm{f}}\times 1000$$

(9) $$\mathrm{N}\mathrm{r}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{y}\mathrm{e}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y}(\mathrm{R}{\mathrm{E}}_{\mathrm{N}},\mathrm{\%})=(\mathrm{T}{\mathrm{N}}_{\mathrm{N}}-\mathrm{T}{\mathrm{N}}_0)/{\mathrm{N}}_{\mathrm{f}}$$

(10) $$\mathrm{P}\mathrm{a}\mathrm{r}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}\mathrm{f}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{o}\mathrm{r}\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{y}\mathrm{o}\mathrm{f}\mathrm{a}\mathrm{p}\mathrm{p}\mathrm{l}\mathrm{i}\mathrm{e}\mathrm{d}\mathrm{N}(\mathrm{P}\mathrm{F}\mathrm{P}\mathrm{N},\mathrm{k}\mathrm{g}\mathrm{k}{\mathrm{g}}^{-1})=\mathrm{Y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}/{\mathrm{N}}_{\mathrm{f}}\times 1000(10)$$where Y_N and Y₀ are the yield (t ha⁻¹) in the N fertilization and N0 treatments, respectively. TN_N and TN₀ are the total N quantity at maturity (kg ha⁻¹) in the N fertilization and N0 treatments, respectively. N_f is the total input of N fertilizer (kg ha⁻¹).

Weather conditions and crop growth duration

Seasonal precipitation was 51.2 mm greater in 2017–2018 than that in 2018–2019 due to the greater rainfall from jointing to booting and from anthesis to maturity in the former growing season (Table 2). However, there was a higher mean temperature, more accumulated temperature, and greater incident solar radiation during all growing periods in 2018–2019 than those in 2017–2018, especially from jointing to booting and from anthesis to maturity.

The growing durations in each period (sowing to jointing, jointing to booting, and anthesis to maturity) were longer in 2018–2019 than those in 2017–2018; thus, the total growing durations were longer in 2018–2019 (Table 3). It should be noted that the wheat crop was sowed late (by approximately 10 days) in 2017–2018 because of the continuous rainy weather before sowing.

Yield and yield related attributes

The sowing method and N application rate significantly affected grain yield (Table 4). The wheat crop under WS produced higher grain yield by 16.38% and 13.57%, averaged across N application rates, than that under DS in 2017–2018 and 2018–2019, respectively. There were significant increases in yield when the N application rate increased from 0 to 180 kg ha⁻¹ and then to 240 kg ha⁻¹ under both sowing methods and during the two growing seasons. The sowing method and N application rate had a significant combined effect on grain yield. Grain yield improved slightly with an increase in the N application rate (from 240 to 300 kg ha⁻¹) under WS in both growing seasons, whereas significant yield reductions (11.78% in 2017–2018 and 7.21% in 2018–2019) were observed under DS (lodging occurred during grain filling under DS with 300 kg N ha⁻¹ applied). In addition, the wheat crop under WS treated with 180 kg ha⁻¹ N produced a commensurate yield compared with that under DS with 240 kg N ha⁻¹ applied in both growing seasons.

The positive effect of WS on grain yield was mainly because of the increased ear number per hectare (Table 4). Averaged across N application rates, the wheat crop under WS exhibited a 20.36% and 18.95% higher ear number per hectare than that under DS in 2017–2018 and 2018–2019, respectively. There was little or no difference in the grain number per ear and 1,000-grain weight between sowing methods. With an increase in the N application rate, although it was observed that the ear number increased under both sowing methods. The rates of ear number increased under WS with improved N application rate (from 180 to 240 kg ha⁻¹ and from 240 to 300 kg ha⁻¹) were significantly larger than those under DS (8.91–13.81% vs. 4.62–8.81%). However, with an increase in the N application rate, a decreasing trend in grain number per ear and 1,000-grain weight was recorded under both sowing methods and in the two growing seasons.

The maximum stem number and productive stem percentage under WS were both significantly higher than those under DS (Figs. 2 and 3). The improvement in maximum stem number (14.52–16.09%), rather than productive stem percentage (3.24–3.85%), mainly accounted for the wheat crop under WS producing more ears per hectare. With an increase in the N application rate, the maximum stem number of winter wheat significantly and continuously increased. However, a decreasing trend was recorded in the productive stem percentage under both sowing methods and in the two growing seasons.

The leaf area index at anthesis under WS was significantly higher by 6.70% and 7.97%, averaged across N application rates, than that under DS in 2017–2018 and 2018–2019, respectively (Fig. 4). With an increase in the N application rate, the leaf area index at anthesis significantly increased under both sowing methods and in the two growing seasons.

The total dry weight at maturity under WS was significantly higher by 13.62% and 15.26%, averaged across N application rates, than that under DS in 2017–2018 and 2018–2019, respectively, whereas no significant difference was observed in the harvest index between sowing methods (Table 5). The total dry weight at maturity significantly increased when the N application rate increased from 0 to 180 kg ha⁻¹ and then to 240 kg ha⁻¹ under both sowing methods and in the two growing seasons. With a further increase in the N application rate (from 240 to 300 kg ha⁻¹), the total dry weight at maturity under WS slightly improved in both growing seasons, but significant reductions of 6.49% and 7.65% were observed under DS (lodging occurred during grain filling under DS with 300 kg N ha⁻¹ applied) in 2017–2018 and 2018–2019, respectively. The harvest indices with N fertiliser application were significantly lower than those without N application. There was no significant difference in the harvest index among treatments with N application, except that the N application rate of 300 kg ha⁻¹ resulted in significantly lower values than N application rates of 180 and 240 kg ha⁻¹ under DS (lodging occurred during grain filling under DS with 300 kg N ha⁻¹ applied) in 2017–2018.

Total dry weight at anthesis and post-anthesis dry matter production were both significantly higher under WS than those under DS in the two growing seasons, which accounts for the advantage in total dry weight of the wheat crop under WS over DS (Table 5). Pre-anthesis dry matter production and post-anthesis dry matter production significantly increased when the N application rate increased from 0 to 180 kg ha⁻¹ and then to 240 kg ha⁻¹ under both sowing methods and in the two growing seasons. When the N application rate further increased (from 240 to 300 kg ha⁻¹), pre-anthesis dry matter production and post-anthesis dry matter production under WS exhibited an increasing or unchanging trend in both growing seasons, but significant reductions of 5.35–6.34% and 8.76–10.10% were recorded under DS (lodging occurred during grain filling under DS with 300 kg N ha⁻¹ applied) in 2017–2018 and 2018–2019, respectively.

N uptake, utilization, and related efficiencies

There was no significant difference in grain N concentration between the sowing methods, averaged across N application rates, in the two growing seasons (Table 6). Among the N treatments, there was no significant difference in grain N concentration under DS. However, significant improvements (5.02–5.98%) were recorded at 300 kg N ha⁻¹ under WS in the two growing seasons. The grain N content under WS was significantly higher than that under DS by 18.67% and 15.24% in 2017–2018 and 2018–2019, respectively. The grain N content significantly increased when the N application rate increased from 0 to 180 kg ha⁻¹ and then to 240 kg ha⁻¹ under both sowing methods and in the two growing seasons. When the wheat crop received more N fertiliser (300 kg ha⁻¹), the grain N content under WS significantly and continuously improved in both growing seasons; however, significant reductions of 9.78% and 6.22% were observed under DS (lodging occurred during grain filling under DS with 300 kg N ha⁻¹ applied) in 2017–2018 and 2018–2019, respectively.

The wheat crop under WS had a higher total N quantity at maturity and N harvest index, averaged across N application rates, than those under DS in both growing seasons (Table 6). With an increase in the N application rate, the total N quantity at maturity significantly and consistently increased under both sowing methods and in the two growing seasons, whereas the N harvest index exhibited a decreasing trend. Pre-anthesis N uptake and post-anthesis N uptake were both significantly higher under WS than those under DS in the two growing seasons, which accounts for the advantage in total N quantity at maturity under WS over DS. With an increase in the N application rate, both pre-anthesis N uptake and post-anthesis N uptake significantly and continuously increased under both sowing methods and in the two growing seasons. In addition, total N (including pre-anthesis N uptake and post-anthesis N uptake, and total N quantity at maturity) under DS with 240 kg N ha⁻¹ applied was commensurate with that under WS with 180 kg N ha⁻¹ applied.

The wheat crop under WS exhibited a higher NUE by 15.00% and 12.44%, averaged across N rates, than that under DS in 2017–2018 and 2018–2019, respectively (Table 7). The recorded NUE decreased with increases in N fertiliser administration. NUpE under WS was significantly higher by 11.98% and 10.15%, averaged across N rates, than that under DS in 2017–2018 and 2018–2019, respectively, whereas there was no significant difference in NUtE during the two growing seasons. The higher NUE under WS largely resulted from the advantage of NUpE over DS.

The AE_N under WS was significantly higher by 16.51% and 21.05%, averaged across N application rates, than that under DS in 2017–2018 and 2018–2019, respectively (Table 7). AE_N significantly decreased when the N application rate increased from 180 to 240 kg ha⁻¹ and then to 300 kg ha⁻¹ under both sowing methods and during the two growing seasons, except that there was no significant difference between 180 and 240 kg ha⁻¹ under WS in 2017–2018.

A higher RE_N was observed under WS than that under DS by 11.55% and 9.75%, averaged across N application rates, in 2017–2018 and 2018–2019, respectively (Table 7). When the N application rate increased from 180 to 240 kg ha⁻¹ and then to 300 kg ha⁻¹, RE_N under DS significantly and continuously decreased, but did not decrease under WS in 2017–2018 and 2018–2019, respectively.

The wheat crop exhibited a significantly higher PFP_N under WS by 15.11% and 13.51%, averaged across N application rates, than that under DS in 2017–2018 and 2018–2019, respectively (Table 7). With an increase in the N application rate, PFP_N significantly and continuously decreased under both sowing methods and in the two growing seasons.