Introduction

Nitrogen is an important nutrient for the growth and development of crops. Therefore, the application of nitrogen fertilizer has become one of the main measures to increase agricultural yield1,2. In recent years, in order to further improve crop yields, the amount of nitrogen fertilizer application in China has increased year by year3. However, the linear increase in N input did not lead to a significant increase in crop yields, but instead led to a sharp decrease in N use efficiency (NUE)4,5 and environmental damage. In order to increase crop yields and reduce costs without further increasing the use of chemical fertilizers, a series of optimized nitrogen fertilizer management measures were proposed. Compared with conventional fertilizers, slow-release nitrogen fertilizers can improve crop yield and nitrogen use efficiency by slowly releasing nutrients according to crop needs, improving fertilizer timeliness and reducing the number of fertilization applications6,7,8,9. In recent years, slow-release nitrogen fertilizers have been studied and applied in improving crop yield and nitrogen use efficiency10,11. Zhou et al.12 concluded that compared with conventional nitrogen fertilizer, slow/controlled-release fertilizer can significantly improve maize yield and nitrogen use efficiency. Garcia et al.13 and Gautam et al.14 showed that compared with ordinary chemical fertilizers, slow-release nitrogen fertilizers could effectively control the nitrogen release rate, effectively provide nitrogen sources in the later stage of crop growth, and had significant effects on increasing crop dry weight and nitrogen accumulation. Tian et al.15 showed that slow-release nitrogen fertilizer could reduce nitrogen application rate without reducing crop yield and improve nitrogen use efficiency. He et al.16 showed that the application of slow-release nitrogen fertilizer not only promoted the nitrogen supply in the late growth period of crops, but also effectively increased nitrogen conversion and accumulation, coordinated the nutrient absorption and distribution of various organs of crops, and greatly improved crop yield and nitrogen use efficiency. However, the cost increases due to the high price of slow-release nitrogen fertilizers. Therefore, the current research on optimizing fertilization methods mainly focuses on the combination of slow-release nitrogen fertilizer and urea17,18 to achieve the effect of cost saving and yield increase. Yi et al.19 found that the combined application of slow-release nitrogen fertilizer and urea could increase the yield and nitrogen accumulation of summer maize with different dosages of slow-release nitrogen fertilizer and urea. Ma et al.20 and Li et al.21 showed that the combination of slow-release nitrogen fertilizer and urea could significantly improve crop nitrogen partial productivity, apparent nitrogen use efficiency, agronomic efficiency and nitrogen harvest index, and increase yield.

Rational tillage practices can improve soil physical properties, improve crop growth conditions, and better meet crop demand, thereby increasing crop yields22. In recent years, with the popularization of conservation tillage, tillage techniques such as rotary tillage and no-tillage have been widely applied due to their simple operation and low energy consumption23. Conservation tillage is the process of improving soil health and further protecting soil by reducing the frequency and intensity of field preparation operations, thereby increasing crop yields. Rotary tillage and subsoiling are both commonly used conservation tillage techniques24. Studies have shown that long-term rotary tillage can cause problems such as shallow tillage layer and increased bulk density of deep soil25. The advantage of subsoiling is that it can break the bottom layer of the plough, improve soil permeability, enhance soil infiltration capacity, and thus improve soil water storage capacity and deep soil water content without turning over the soil layer, creating a good soil environment for crop growth, thereby enhancing the absorption capacity of roots to nutrients and water, and improving fertilizer absorption and utilization efficiency and yield26,27,28,29. Luo et al.30 and Zhang et al.31 showed that subsoiling could increase the dry weight and number of maize roots per plant, improve the activity of maize root plant protective enzymes, reduce the degree of cell membrane peroxidation, and slow the aging of maize roots, which was conducive to maintaining root vitality and improving the absorption of water and nutrients by roots in the late growth stage of maize. Wang et al.32 and Zhao et al.33 found that subsoiling can loosen the soil and promote the deep growth of maize roots, which is conducive to the absorption of water and nutrients by the roots and improves the yield and water use efficiency of maize grains. Yang et al.34 showed that compared with rotary tillage and subsoiling, deep rotary loosening tillage can improve soil hydrothermal conditions, increase crop dry matter accumulation, and then increase crop yield. Entry point of this study At present, there have been relevant studies on the effects of tillage methods and nitrogen fertilizer types on maize yield, nitrogen uptake and utilization at home and abroad, but there are few systematic studies on the effects of tillage and fertilization on soil moisture, dry matter accumulation, nitrogen uptake and utilization, operation and yield in maize fields in dryland tillage areas. On this basis, the effects of the combined application of slow-release nitrogen fertilizer and urea on soil water use efficiency, dry matter accumulation, nitrogen uptake and utilization, operation and maize yield in maize farmland were studied under two tillage methods, rotary tillage and rotary tillage + subsoiling in dry farming area, so as to screen out better tillage and fertilization methods in dry farming area, in order to provide a theoretical and scientific basis for selecting suitable tillage mode and optimal fertilization ratio in areas with similar ecological types.

Materials and methods

Overview of the test site

The test site will be conducted in 2022–2023 at the Dongyang Experimental Base of Shanxi Agricultural University (Shanxi Academy of Agricultural Sciences) (37° 33′21″ N, 112° 40′2″ E, 802 m above sea level). The region belongs to the warm temperate semi-humid continental monsoon climate, with an average annual temperature of 9.7 °C, an annual average frost-free period of 158 days, rain and heat in the same season, and the rainfall is mainly concentrated in July and September, with 385 mm of precipitation in the growth period in 2022 and 222.9 mm in the growth period of 2023. The soil of the experimental plot was yellow clay, and the content of organic matter in the 0–20 cm tillage layer was 14.08 g kg−1, the total nitrogen content was 0.96 mg kg−1, the alkaline nitrogen content was 67.1 mg kg−1, the total phosphorus content was 0.775 mg kg−1, the available phosphorus content was 11.0 mg kg−1, the total potassium content was 17.06 g kg−1, the available potassium content was 110 mg kg−1, and the pH value was 8.55. The rainfall during the growing season in 2022 and 2023 is shown in Fig. 1.

Fig. 1
figure 1

Precipitation the growth period of Dong yang Demonstration Base in 2022 and 2023.

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Test materials

(1) Seeds: Drought-resistant maize variety Qiangsheng 388 is selected, with compact plants, 317 cm high, 116 cm ears, 22 leaves, and a growth period of 129 days. The seedling leaf sheath is dark purple, the leaf margin is red, the filament is green, medium resistance to large spot disease, and high resistance to stem rot. Provided by Shanxi Qiangsheng Seed Industry Co., Ltd. (2) Nitrogen fertilizer: urea, net content of 40 kg, total nitrogen content ≥ 46.0%, particle size range of 0.85 mm ~ 2.80 mm ≥ 90%, provided by PetroChina Co., Ltd. slow-release nitrogen fertilizer: Jugu Neng brand was selected, with a net content of 40 kg, a total nutrient content of ≥ 50% (the mass ratio of N:P2O5:K2O was 26:12:12), and a slow-release nitrogen ≥ 8%. Provided by Jiashili (Yingcheng) Fertilizer Co., Ltd.

Experimental design

The two-factor randomized block design was adopted, and the tillage methods were rotary tillage (R) and rotary tillage + subsoiling (R + S), in which the depth of rotary tillage reached 30 cm and the depth of subsoiling reached 50 cm. The fertilization methods were CK (no fertilization treatment), 100% urea (U), 100% slow-release nitrogen fertilizer (S), and combined application of urea and slow-release nitrogen fertilizer (UNS), with the ratios of 2:8 (UNS1), 3:7 (UNS2) and 4:6 (UNS3), respectively. The planting density was 66,000 plants hm−2, the plot area was 60 m2, and each treatment was set up for 3 replicates.The specific tillage design and fertilization amount are shown in Table 1.

Table 1 Presentation of tillage practices and fertilizer application design.
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Determination items and methods

Soil moisture

Before sowing and after harvesting, each test plot was sampled in the 0–100 cm soil layer with a ring knife with a volume of 100 cm3, and every 20 cm was a layer, repeated three times. The soil moisture content and soil bulk density were determined by drying method, so as to calculate the water consumption and water use efficiency during the growth period.

$$\begin{aligned} \theta &= \, \left( {{\text{fresh soil sample }} - {\text{ dry soil sample}}} \right)/{\text{dry soil sample}}*{1}00\% \\ {\text{R }} &= {\text{ dry soil sample}}/{1}00 \\ {\text{W}} &= \theta *{\text{h}}*{\text{R}}*{1}0 \\ {\text{ETa}} &= \left( {{\text{W}}_{{1}} - {\text{W}}_{{2}} } \right) + {\text{P}} \\ {\text{WUE}}\left( {{\text{kg}}\;{\text{hm}}^{{ - {2}}} \;{\text{mm}}^{{ - {1}}} } \right) &= {\text{Y}}/{\text{ETa}} \\ \end{aligned}$$

wherein: θ is the soil moisture content (%), R is the soil bulk density (g cm-3), h is the soil depth (cm), and 10 is the unit conversion factor. W is the soil water storage (mm), W1 is the soil water storage before sowing (mm), and W2 is the soil water storage after harvest (mm). ETa is the water consumption of crops (mm); P is the rainfall during the growth period (mm). WUE is water use efficiency (kg hm−2 mm−1); Y is the crop yield (kg hm−2)

Aboveground dry matter quality and total nitrogen in aboveground parts of plants

Sampling was carried out at the jointing stage, the large flare stage, the tasseling stage, the grain filling stage and the maturity stage, and 3 maize plants were selected for each plot at one time and repeated 3 times. Among them, the jointing stage, the large flare stage and the tasseling stage were divided into two parts according to the stem and leaves, and the grain filling stage and the mature stage were divided into four parts according to the stem, leaf, leaf wrapping + cob and grain. Samples were placed in an oven at 105 °C for 0.5 h and then at 75 °C to constant weight, recording the dry matter mass. The dried samples were sieved at 0.5 mm, boiled with H2SO4-H2O2, and analyzed for total nitrogen content by Kjeldahl method.

Formula:

  • Accumulation of dry matter weight of plants = dry matter weight per plant × number of plants per unit area

  • Plant nitrogen accumulation (kg hm−2) = plant nitrogen content (%) × dry matter mass × hectare density

  • Nitrogen uptake efficiency (kg kg−1) = nitrogen accumulation/nitrogen application

  • Nitrogen use efficiency (kg kg−1) = grain yield/plant nitrogen accumulation

  • Nitrogen fertilizer use efficiency (kg kg−1) = grain yield/nitrogen application rate

  • Nitrogen harvest index (%) = total nitrogen accumulation in grains/total nitrogen accumulation in plants × 100

  • Nitrogen transfer of vegetative organs = nitrogen uptake of vegetative organs at the tasseling stage − nitrogen uptake by vegetative organs at maturity stage

  • Nitrogen uptake after tasseling stage = total nitrogen uptake at mature stage − nitrogen uptake by vegetative organs at tasseling stage

  • Nitrogen transfer rate of vegetative organs = nitrogen transfer of vegetative organs/nitrogen uptake of vegetative organs at the tasseling stage × 100%

Yield

The middle 2 rows of each plot were taken to measure the yield, and the total number of plants and the actual number of harvested ears were recorded. At the same time, 20 fruit ears were randomly selected to be naturally air-dried, and when the moisture was less than 20%, the panicle length (cm), bald tip length (cm) and grain number per spike were measured, and the 100-grain weight (g) was determined after threshing and air drying, and the grain yield was calculated according to the grain moisture content of 14% by combining the seed test data with the field yield measurement data.

Data processing and analysis

Use Microsoft Excel 2023 software for data collation; SPSS 22.0 statistical analysis software was used to analyze the significance of the test data, and the LSD method was used to compare the test data under the condition of 5% significance, and the difference was significant when the P < 0.05 was used. The data results are represented as “mean ± standard error”, and the data is graphed using Origin2021.

Results and analysis

Dry matter accumulation

As can be seen from Fig. 2, the dry matter accumulation of all treatments gradually increased with the advancement of maize growth period. At the jointing stage, the higher the proportion of urea in the treatment, the greater the dry matter accumulation, in the order of U > UNS3 > UNS2 > UNS1 > S > CK. With the advancement of the growth period, the effect of slow-release nitrogen fertilizer on dry matter accumulation was significant, and the order was UNS > S > U > CK.

Fig. 2
figure 2

Effects of different tillage and fertilizer types on dry accumulation in maize plants in 2022 and 2023. Note R: rotary tillage; R + S: rotary tillage + subsoiling; CK: no fertilization; U: 300 kg hm−2 urea, 0 kg hm−2 slow-release nitrogen fertilizer; S: 0 kg hm−2 urea, 530.7 kg hm−2 slow-release nitrogen fertilizer; UNS1: 60 kg hm−2 urea, 424.6 kg hm−2 slow-release nitrogen fertilizer; UNS2: 90 kg hm−2 urea, 371.5 kg hm−2 slow-release nitrogen fertilizer; UNS3: 120 kg hm−2 urea, 318.4 kg hm−2 slow-release nitrogen fertilizer; JT: jointing stage; BS: Big trumpet stage; TS: Tasseling stage; FS: Filling stage; MT: maturity stage;

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Under the same tillage method, the dry matter accumulation in the nitrogen application treatment was significantly higher than that in the non-nitrogen application treatment. At the maturity stage, UNS2 was the best under R and R + S tillage methods in 2022, which were 24,898.5 kg hm−2 and 24,409.44 kg hm−2, respectively. CK was the lowest, with R CK and R + S CK being 19,074.66 kg hm−2 and 19,534.02 kg hm−2, respectively. The treatment of R UNS2 was 2.00% higher than that of R + S UNS2.

In 2023, UNS2 was the best treatment under the two tillage methods, with R UNS2 and R + S UNS2 being 23,464.98 kg hm−2 and 24,056.34 kg hm−2, respectively, which were 25.24% and 31.17% higher than those of R CK and R + S CK, respectively. R + S UNS2 treatment was 2.52% higher than that of R UNS2.

Nitrogen accumulation

It can be seen from Fig. 3 that with the advancement of maize growth period, the nitrogen accumulation of all treatments were consistent with the previous dry matter accumulation, and showed a gradual increasing trend. At the jointing stage, the higher the proportion of urea in the treatment, the greater the nitrogen accumulation, in the order of U > UNS3 > UNS2 > UNS1 > S > CK. With the advancement of the growth period, the effect of slow-release nitrogen fertilizer on nitrogen accumulation was significant, and the order was UNS > S > U > CK.

Fig. 3
figure 3

Effects of different tillage and fertilizer types on nitrogen accumulation in maize plants in 2022 and 2023. Note: R: rotary tillage; R + S: rotary tillage + subsoiling; CK: no fertilization; U: 300 kg hm−2 urea, 0 kg hm−2 slow-release nitrogen fertilizer; S: 0 kg hm−2 urea, 530.7 kg hm−2 slow-release nitrogen fertilizer; UNS1: 60 kg hm−2 urea, 424.6 kg hm−2 slow-release nitrogen fertilizer; UNS2: 90 kg hm−2 urea, 371.5 kg hm−2 slow-release nitrogen fertilizer; UNS3: 120 kg hm−2 urea, 318.4 kg hm−2 slow-release nitrogen fertilizer; JT: jointing stage; BS: Big trumpet stage; TS: Tasseling stage; FS: Filling stage; MT: maturity stage; a, b, c, d represent different levels of significance. a indicates a higher level of significance, such as 0.01 or 0.05, b may indicate a secondary level of significance, and c may indicate a lower level of significance.

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Under the same tillage method, the nitrogen accumulation in the nitrogen application treatment was significantly higher than that in the non-nitrogen application treatment. At the maturity stage, the nitrogen accumulation of UNS2 treatment reached the maximum under the two tillage methods of R and R + S in 2022, which were 275.25 kg hm−2 and 264.88 kg hm−2, respectively. The CK treatment was the lowest, with R CK and R + S CK being 186.85 kg hm−2 and 194.02 kg hm−2, respectively. The treatment of R UNS2 was 3.88% higher than that of R + S UNS2. In 2023, the nitrogen accumulation of the two tillage methods UNS2 was also the best, with R UNS2 and R + S UNS2 being 256.08 kg hm−2 and 260.81 kg hm−2, respectively, which were 39.43% and 47.09% higher than those of R CK and R + S CK treatments, respectively. R + S UNS2 treatment was 1.85% higher than that of R UNS2.

Output and production components

It can be seen from Table 2 that the yield of maize under different tillage methods was UNS > S > U > CK in the two-year experiment, and UNS2 had the highest yield. In 2022, the yield of UNS2 treatment was 14,712.78 kg hm−2, which was 4.15% ~ 56.85% higher than that of the other five treatments. The maximum mass of 100 grains in UNS2 treatment was 44.46 g, which was 3.68% ~ 19.36% higher than that of the other five treatments. The yield of UNS2 treatment was 14,138.3 kg hm−2, which was 2.93% ~ 43.37% higher than that of the other five treatments. The mass of 100 grains in UNS2 treatment was 44.13 g, which was 4.76% ~ 16.10% higher than that of the other 5 treatments. The bald tip length of the two tillage treatments were significantly lower than that of the other treatments, and the average spike length and grain number per spike were significantly better than those of the other treatments. The yield and 100-grain weight of R UNS2 treatment were 4.06% and 0.75% higher than those of R + S UNS2, respectively.

Table 2 Effects of different tillage and fertilizer types on yields and its components of maize.
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In 2023, the yield of UNS2 treatment was the best under R tillage mode, which was 13,602.98 kg hm−2, followed by UNS1 treatment, which was 12,637.85 kg hm−2, and CK was the lowest, which was 9201.24 kg hm−2. The yield of UNS2 treatment was 13,901.88 kg hm−2 under R + S tillage mode, followed by UNS3 treatment at 13,347.81 kg hm−2. The lowest CK was 9101.04 kg hm−2. The 100-grain mass R and R + S tillage methods were the highest in UNS2, which were 43.33 g and 43.58 g, respectively, which corresponded with the yield. The yield and 100-grain weight of R + S UNS2 treatment were 2.20% and 0.58% higher than those of R UNS2, respectively.

Nitrogen uptake and utilization

As shown in Table 3, the nitrogen uptake and utilization of maize plants under the two tillage methods showed UNS > S > U > CK under the two tillage methods, and UNS2 was the highest.In 2022, R UNS2 and R + S UNS2 had the highest nitrogen absorption efficiency, which were 0.92 kg kg−1 and 0.88 kg kg−1, respectively. U was the lowest, and R U and R + S U were 0.74 kg kg−1 and 0.71 kg kg−1, respectively. The nitrogen harvest index R UNS2 and R + S UNS2 were the highest, which were 64.61% and 63.74%, respectively. CK was the smallest, with 56.3% and 60.77% of R CK and R + S CK, respectively. The nitrogen uptake efficiency and nitrogen harvest index of R UNS2 treatment were 4.55% and 1.36% higher than those of R + S UNS2, respectively. The nitrogen use efficiency and nitrogen fertilizer use efficiency of the two tillage methods were significantly better than those of the other treatments.

Table 3 Effects of different tillage and fertilizer types on nitrogen uptake and use efficiency of maize.
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In 2023, the nitrogen uptake efficiency of R and R + S tillage methods was the same as that in 2022, and the UNS2 treatment was the highest, which was 0.85 kg kg−1 and 0.87 kg kg−1, respectively. U was the lowest, with R U and R + S U being 0.70 kg kg−1 and 0.68 kg kg−1, respectively. The nitrogen harvest index UNS2 was the first treatment, with R UNS2 and R + S UNS2 reaching 63.43% and 63.59%, respectively. CK was the smallest, with 56.2% and 55.52% of R’CK and R + S CK, respectively. The nitrogen uptake efficiency and nitrogen harvest index of R + S UNS2 treatment were 2.35% and 0.25% higher than those of RUNS2, respectively.

Nitrogen transport

It can be seen from Table 4 that in all the treatments of the two-year experiment, the nitrogen uptake stored in the vegetative organs of maize plants at the tasseling stage was greater than that at the maturity stage, which indicated that the nitrogen stored in the vegetative organs was transferred to the grain after the tasseling stage, and it showed a trend of UNS > S > U > CK, in which the UNS2 treatment reached the maximum. In 2022, the nitrogen turnover of UNS2 treatment was 82.64 kg hm−2, which was 7.33–126.03% higher than that of the other five treatments. The nitrogen operation efficiency of UNS2 treatment was 45.9%, which was 3.12–48.39% higher than that of the other five treatments. R + S tillage method the nitrogen flux of UNS2 treatment was 77.83 kg hm−2, which was 5.53–85.70% higher than that of the other five treatments. The nitrogen operation efficiency of UNS2 treatment was 44.76%, which was 2.02–45.75% higher than that of the other five treatments.

Table 4 Effects of different tillage methods and fertilization types on nitrogen accumulation and operation of maize before and after tasseling.
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In 2023, the nitrogen transmutation UNS2 treatment was the maximum under R and R + S tillage methods, and the R UNS2 and R + S UNS2 were 74.58 kg hm−2 and 76.52 kg hm−2, respectively. CK was the smallest, with 35.65 kg hm−2 and 33.71 kg hm−2, respectively. R + S UNS2 treatment was 2.60% higher than R UNS2 treatment. The nitrogen operation efficiency was the highest under the two tillage methods, and the R UNS2 and R + S UNS2 were 44.33% and 44.62%, respectively. CK was the lowest, at 30.71% and 29.94%, respectively. R + S UNS2 treatment was 0.65% higher than R UNS2 treatment.

Water use

Table 5 showed that the water consumption and water use efficiency CK treatment during the growth period were significantly lower than those of other nitrogen application treatments in the two-year experiment, indicating that nitrogen application could significantly improve the water consumption and water use efficiency of maize. In 2022, the water consumption and water use efficiency of UNS2 treatment during the growth period were the highest, which were 453.58 mm and 32.44 kg hm−2 mm−1, respectively. CK was the lowest, at 430.84 mm and 21.77 kg hm−2 mm−1, respectively. The water consumption and water use efficiency of UNS2 treatment during the growth period were also the highest under R + S tillage mode, which were 457.89 mm and 30.88 kg hm−2 mm−1, respectively. CK was the lowest, with 428.88 mm and 22.99 kg hm−2 mm−1, respectively. The water use efficiency of R UNS2 was 5.05% higher than that of R + S UNS3.

Table 5 Effects of different tillage and fertilizer types on yield and water use efficiency (WUE) of maize.
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In 2023, the water consumption of UNS2 treatment was the largest, which was 443.55 mm, which was 1.01%-6.73% higher than that of the other five treatments. The water use efficiency of UNS2 treatment was the highest, which was 30.67 kg hm−2 mm−1, which was 6.34%-38.52% higher than that of the other five treatments. Under the R + S tillage method, the water consumption of UNS3 treatment was the largest, which was 446.65 mm, which was 0.48%-11.84% higher than that of the other five treatments. The UNS2 treatment had the highest water use efficiency of 31.28 kg hm−2 mm−1, which was 4.68–37.25% higher than that of the other five treatments. The water use efficiency of UNS2 was 1.98% higher than that of R UNS2.

Discussion

In this study, the effects of the combination of slow-release nitrogen fertilizer and nitrogen fertilizer on dry matter accumulation, water use efficiency, yield, and nitrogen uptake and utilization of maize under two tillage methods were studied. The results showed that nitrogen application could improve dry matter quality, water use efficiency, yield and nitrogen use efficiency of maize. The overall performance trend under the same tillage method was UNS treatment > S treatment > U treatment > CK treatment. Compared with 2022, the dry matter accumulation, nitrogen uptake and utilization, operation, water use efficiency and yield of maize under the two tillage methods in 2023 were lower than those in 2022, and the subsoiling treatment in 2023 was better than that of rotary tillage.

Nitrogen application had a significant effect on the dry matter accumulation, yield and constituent factors of maize, and the combined application of slow-release nitrogen fertilizer and urea could ensure the nutrient supply of maize during the growth period. Studies have shown that the effect of one-time basal application of slow-release nitrogen fertilizer is similar to or significant that of fractional urea in terms of yield and its components35. In this experiment, all treatments were treated with one-time fertilization 15 days after emergence, and the results showed that the dry matter accumulation, yield and constituent factors of UNS treatment were significantly higher than those of S treatment and U treatment under R and R + S tillage methods, and the UNS2 treatment had the best effect, which was consistent with the results of Guo et al.36 and Ji et al.37. The accumulation of dry matter during the growth period of crops is the basis for yield formation, and the level of dry matter accumulation determines the final grain yield, which is one of the fundamental ways to increase crop grain yield38. In this experiment, the dry matter accumulation, spike length, grain number per spike, 100-grain quality and yield of UNS treatment were better than those of S treatment and U treatment, because the dry matter accumulation of maize showed an increasing trend of “S” curve with time, and the dry matter accumulation of ordinary urea treatment was the fastest at jointing stage, followed by large flare stage. However, after the tasseling stage, it was significantly lower than that of the slow-release nitrogen fertilizer combined treatment, and although the common urea had a fast fertilizer effect in the early growth stage, it could provide good fertility, promote the nutrient absorption of maize roots, and improve the dry matter accumulation of maize plants (Fig. 2 R–U treatment in the JT period), but due to nitrogen loss, it could not effectively provide fertility in the late growth stage, which led to the weakening of the nutrient absorption capacity of maize roots, and the dry matter accumulation and yield were lower than those of the combined application of slow-release nitrogen fertilizer and urea. However, due to the time required to release fertility by slow-release nitrogen fertilizer alone, the supply of early growth fertility was insufficient, resulting in defertilization (Fig. 3 R–S treatment in JT period). The combination of slow-release nitrogen fertilizer and urea can well solve the shortcomings of slow nitrogen release and insufficient nutrients in the early stage of crop growth, and ensure the fertility supply of crops in the whole growth season, which is similar to the conclusion of Wang et al.39 that slow-release nitrogen fertilizer can improve the dry matter accumulation of summer maize, which is mainly reflected in the conclusion after the silking period.

Studies have shown that nitrogen fertilization has a significant effect on water uptake and utilization of maize, and can improve water use efficiency40. In this experiment, the water use efficiency of UNS treatment > S treatment > U treatment > CK treatment under the two tillage methods, which is consistent with the conclusion that controlled-release urea can improve the water use efficiency of crops by Zheng et al.41 and Hu et al.42, and the water use efficiency of UNS2 treatment reached the highest level. The reason is that the application of slow-release nitrogen fertilizer can ensure that maize has sufficient nutrients in the later growth stage, and can still maintain high root volume and metabolic enzyme activity in the later stage, which can improve the ability of maize to absorb water, thereby significantly improving its water use efficiency, while urea single application treatment can lead to denitrification in the late growth stage of maize under the one-time fertilization mode, thereby limiting its growth and development and reducing water use efficiency, which is consistent with the results of Lv et al.43

For a long time, with the increase of crop yields, the use of nitrogen fertilizers has increased year by year, which has exacerbated nitrogen loss and environmental pollution, and caused an increase in costs44. Studies have shown that the combination of slow-release nitrogen fertilizer and urea can alleviate this situation, reduce nitrogen loss, improve nitrogen accumulation, utilization, and operation, and save costs 45. In this experiment, the nitrogen accumulation and nitrogen uptake and utilization of aboveground parts of maize plants were treated by UNS treatment > S treatment > U treatment, among which UNS2 treatment was the best, which was consistent with the results of Zhou et al.46 and Wang et al.47. Compared with the S treatment and U treatment, UNS treatment can solve the problem of insufficient nutrient supply in the early or late stage of maize growing season, promote nitrogen accumulation in shoots, improve nitrogen use efficiency of maize, and improve nitrogen harvest index, which is consistent with the results of Guo et al.48. At the same time, in this study, the nitrogen uptake of vegetative organs stored in the vegetative organs of maize plants at the tasseling stage was greater than that at the maturity stage, indicating that the nitrogen stored in the vegetative organs was transferred to the grain after the tasseling stage, and the nitrogen in the UNS treatment had the highest amount of nitrogen to the grain and the highest efficiency to the grain, and the UNS2 treatment was the best. This is due to the fact that the nitrogen in the grain comes from two parts, one part is the nitrogen absorbed and stored in the vegetative organs before the tasseling stage and transferred to the grain after the tasseling stage, and the other part is the nitrogen absorbed and assimilated by the plant after the tasseling stage. Therefore, the combination of urea and slow-release nitrogen fertilizer can improve nitrogen accumulation and nitrogen use efficiency, so that more nitrogen can be transferred to the grain, thereby increasing yield, which is similar to the results of Wang et al.49

Compared with rotary tillage, subsoiling is a commonly used conservation tillage measure, which has less disturbance to the soil in the tillage layer than traditional rotary tillage, can increase the depth of the tillage layer, break the bottom layer of the plough, reduce the soil bulk density, promote root growth, improve water use efficiency, increase the dry matter accumulation of crops, and promote the accumulation, absorption, utilization and transport of nitrogen in the aboveground parts, thereby increasing the yield50,51. The results of Yin et al.52 and Liu et al.53 showed that the combined application of slow-release nitrogen fertilizer and urea could reduce the maximum depth of soil nitrate nitrogen, increase the nitrate nitrogen content in the 0–40 cm soil layer, and decrease the nitrate nitrogen content in the 40–100 cm soil layer, reducing the nitrogen loss, which was conducive to the absorption, utilization and operation of nitrogen, and improved the nitrogen utilization efficiency, and concluded that the comprehensive effect was the best when the ratio was 7:3 (slow-release nitrogen fertilizer: urea). In this experiment, the two tillage methods of subsoiling and rotary tillage were interacted with slow-release nitrogen fertilizer and urea, and the subsoiling was 50 cm, which not only made the spatial distribution of roots reasonable, but also met the spatial requirements of nitrogen absorption in maize. Due to the slow release of nitrogen, slow-release nitrogen fertilizer significantly improves soil nitrogen supply in the late growth stage and meets the time requirement of crop nitrogen demand. The results showed that the dry matter accumulation, water use efficiency, yield, nitrogen uptake and utilization and operation of the subsoiling treatment in 2022 were slightly lower than those of the rotary tillage treatment, which was slightly different from the results of Cao et al.54 and Ji et al.55, because the plots were not all disturbed in the first year of subsoiling, and the bottom layer of the soil plough was not completely broken, so the effect of subsoiling treatment in 2022 was slightly lower than that of rotary tillage treatment. In 2023, after two consecutive years of subsoiling, 80% of the soil in the plot was disturbed, the breaking effect of the plough bottom layer was significantly better than that in 2022, the soil compactness and soil bulk density were reduced, the spatial distribution of roots was more reasonable, the biomass of roots in the soil was improved, the absorption of nutrients and water was promoted, the soil water use efficiency was improved, the ear length, the number of grains per spike and the weight of 100 grains were increased, and the nitrogen absorption, utilization and operation were promoted, thereby promoting the increase of corn yield, and the subsoiling treatment was better than the rotary tillage treatment. Therefore, on the basis of subsoiling, the combination of slow-release nitrogen fertilizer and urea can better meet the space and time requirements of root absorption, and can improve the dry matter accumulation, water use efficiency, nitrogen accumulation, absorption, utilization and transport of spring maize, thereby increasing yield, which is consistent with the research results of Zhou et al.56 and Hu et al.57. However, the rainfall during the growth period in 2023 was significantly lower than that in 2022, which limited the growth and development of maize roots, and reduced the ability of roots to absorb nutrients and water in the soil, which led to dry matter accumulation, water use efficiency, nitrogen uptake, utilization and transport, yield and its components under the two tillage methods of R and R + S in 2023 were slightly lower than those in 2022.

Conclusion

This study showed that after two years of subsoiling to break the bottom layer of the plough, the subsoiling effect was significant, and the subsoiling treatment could effectively improve the yield, nitrogen uptake and utilization, water use and dry matter accumulation of maize compared with rotary tillage treatment. UNS can effectively ensure the nutrient supply of maize during the growth period, so as to achieve the effect of increasing yield. Among them, UNS2 has the best treatment effect and the highest yield, which is the most suitable fertilizer ratio, and should be demonstrated and promoted in a large area. After many years of long-term tracking and positioning experiments, the effect of breaking the bottom layer of the plough will be better, and the next research will continue to focus on multi-year subsoiling and rotary tillage, in order to provide a theoretical and scientific basis for selecting suitable tillage mode and optimal fertilization ratio in areas with similar ecological types.