Environmental degradation poses a significant challenge in the twenty-first century1,2. Over use of chemical fertilizers has led to environmental pollution and soil degradation through leaching and the depletion of soil biodiversity, consequently influencing plant growth3,4. Moreover, prolonged and excessive use of inorganic fertilizers can lead to reduced fertilizer use efficiency, causing soil compaction, acidification, salinization, and disruption of soil nutrient balance5,6,7.Due to the application of chemical fertilizers, it is easy to lead to the aggregation of soluble compounds in the soil, and with the deep soil leakage into the neighbouring rivers and lakes, resulting in the pollution of the groundwater environment8,9,10. In light of the land degradation and ecological consequences associated with the use of chemical fertilizers, it is imperative to advocate for land management practices and enhance soil quality to achieve the objectives of environmentally sustainable crop management and long-term soil stewardship11,12. Currently, an increasing number of scholars are beginning to pursue solutions, the utilization of organic amendments and novel polymer materials represents an approach to enhancing agricultural practices and improving the ecological environment13.

Cake fertilizer is the residue left after oil extraction from the seeds of oil crops, containing nitrogen, phosphorus, potassium and other nutrients, is a better organic fertilizer containing nitrogen and phosphorus14. Sesame, as a major oilseed crop, has a very high phytochemicals15, and it is used extensively in agricultural applications16. As an organic amendment, sesame cake fertilizer exhibits the highest soil enzyme activity in comparison to amino acid ecological fertilizer and fertilizer-like fertilizer17. After 150 days of sesame cake fertilizer treatment18, there was an increase in soil enzyme activity. According to certain studies, although amino acid organic fertilizers have a high rate of absorption and utilization as well as a rapid release of nutrients that can help improve plant resilience and disease resistance19, they are also expensive and require frequent application to maintain long-term fertilizer efficacy20,21. Sesame cake fertilizer has a high organic matter content, a long-lasting fertilizer effect, and a relatively slow rate of nutrient release. It also increases soil fertility and improves soil structure while being kind to the soil environment it is applied to, which can effectively boost soil microbial activity22.Furthermore, sesame seed cake fertilizer has a higher organic matter and trace element content than urea, diammonium phosphate, and other chemical fertilizers23. This increases the soil's nutrient content significantly24,25, and its slow-release effect allows for the application of nutrients to meet crop needs for an extended period of time while preventing soil phosphorus accumulation and nitrogen loss from excessive application26. Furthermore, compared to other biofertilizers and microbial fungicides, the process for creating sesame cake fertilizer is simpler27 and won’t be impacted by the kind of raw materials or processing technology; these raw materials are typically leftovers from the processing of oilseeds and are inexpensive and easily obtained28. The effect of microbial fungicide fertilizer is primarily determined by the soil environment, temperature, humidity, and other factors29, and the effects of these conditions vary greatly. Its large-scale applicability is thus limited by its expensive production and application costs30.

This means that as an organic fertilizer, sesame cake fertilizer offers a lot of benefits for protecting the environment and improving soil31. Long-term improvements to the agroecological environment may be attributed to its high organic matter content and slow-release nutrient features32, which can effectively increase soil fertility and structure, as well as the variety and activity of soil microorganisms33. Meanwhile, sesame seed cake fertilizer is a popular option in sustainable agriculture due to its inexpensive cost and convenient availability16. On the other hand, biofertilizers are expensive, subject to restricted application, and sensitive to environmental circumstances34. Chemical fertilizers, on the other hand, may swiftly supply the nutritional demands of crops, but their prolonged use will degrade the soil and pollute the environment6,35. Thus, the application of sesame seed cake fertilizer management systems in a scientific manner may greatly enhance crop yield potential while also romoting sustainable growth in agricultural output and ecological preservation36.

γ-polyglutamic acid (γ-PGA) is produced through the condensation of L-type or D-type glutamic acid monomers via their α-amino and γ-carboxyl groups37,38. The material is a novel environmentally friendly polymer that can be manufactured through the large-scale fermentation of microorganisms39. The material exhibits biodegradability, non-immunogenicity, water solubility, and non-toxicity. It serves as a slow-release agent and a matrix to enhance microbial growth, and is extensively utilized in agriculture37,39,40,41. To date, numerous studies have demonstrated that the use of γ-PGA has notable effects on water retention and slow release in soil improvement42. This enhances the soil's water holding capacity and alters the distribution of water within the soil profile, consequently improving water use efficiency (WUE)43,44,45. The utilization of γ-PGA can enhance the uptake of nitrogen (N), phosphorus (P), and potassium (K) by plants, leading to improved efficiency in fertilizer utilization46,47,48. Furthermore, multiple studies have demonstrated that the use of γ-PGA can markedly enhance crop yield45,49,50. However, the aforementioned research findings primarily stem from soil column experiments, indoor experiments, and pot experiments. There is a limited number of experimental studies conducted under field conditions, and the effect of γ-PGA combined with chemical fertilizer on soil nutrients and plant absorption has been addressed51.

In summary, sesame cake fertilizer can serve as a source of nutrients for the soil, while γ-PGA can function as a soil nutrient retention agent, slowing down the leaching process. However, the interaction between the two remains unknown, and attempts have been made to derive the effects on soil fertility and crop yield through field trials with maize as a means of improving soil nutrients and increasing soil WUE. Therefore, in pursuit of sustainable agricultural development, it is proposed to investigate the application of sesame seed cake fertilizer with γ-PGA through a large-scale field trial of summer maize planting to demonstrate the distribution pattern of soil nutrients under different dosage regimes, and to propose a practical plan for the application of sesame seed cake fertilizer with γ-PGA fertilizer.

Materials and methods

Test site and materials

The experiment took place from May 16, 2023, to September 3, 2023, at the Agricultural and Water Experimental Field of North China University of Water Resources and Hydropower in Zhengzhou City, Henan Province, China (34° 78′ N, 113° 79′ E). This location experiences a temperate continental monsoon climate with four dry seasons. The locations of the experimental station are depicted in Fig. 1, while Fig. 2 illustrates the environmental data collected during the experiment. The maize variety chosen was “Zhengdan 958”, developed by Sweet Potato Longyu Seed Science and Technology Co. Ltd. This variety is known for its high yield, consistent performance, and robust resistance to diseases, and it is extensively grown in Huanghuaihai region of China. The soil under examination is Guanzhong loess, which is the predominant soil type in the study area, characterized by small and loose particles. Table 1 Physicochemical properties of the soil under investigation. The sesame cake fertilizer utilized was manufactured by Sheng Yuan Horticulture and consisted of 100% pure sesame sesame cake fertilizer, containing 6.86% N, 2.28% P, and 3.13% K per 100 kg. Poly-γ-glutamic acid (γ-PGA) used in this study was a white powder with a molecular weight of 3.32 × 10.21 kg and was produced by Qingdao Unisys Fertilizer Import and Export Co.

Figure 1
figure 1

Geographic distribution of experimental stations for the study.

Full size image
Figure 2
figure 2

Meteorological date during the experimental period at Zhenzhou Experimental Station, HeNan Province, China.

Full size image
Table 1 Physicochemical properties of the experimental soil.
Full size table

Experimental design and field management

The experiments involved two factors: the quantity of γ-PGA applied and the quantity of sesame cake fertilizer applied. The levels of sesame cake fertilizer applied were designated as B1 = 900 kg/hm2 (low fertility), B2 = 1100 kg/hm2 (medium fertility), and B3 = 1300 kg/hm2 (high fertility). The levels of γ-PGA applied were designated as R1 = 200 kg/hm2, R2 = 400 kg/hm2, and R3 = 600 kg/hm252. A randomised experimental design with 9 treatments was used (Table 2). The experimental area was partitioned into 9 plots, with each plot dimension as 3 m × 3 m. To prevent the interference and mutual effect of experimental plots, they were isolated using protected rows with a width of 1 m. Maize was planted with a row spacing of 40cm and a plant spacing of 30cm, with one plant left per hole. The entire quantity of sesame cake fertilizer was utilized as a basal fertilizer for blending with the soil at a depth ranging from 10 to 20 cm. 5 days after the application of the sesame cake fertilizer, γ-PGA was applied as a mixed application at a depth of 5–10 cm, followed by the planting of corn. Rainfall is the only source of water replenishment during the trial time, and it is sufficient to support the crops' growth needs. Furthermore, there is no top-dressing fertilizer program in place. During the corn's growth phase, hand weeding is done to control the growth of weeds. No additional pests or diseases impacting the maize are present during the experiment, and no pesticides are used; all treatments adhere to the same field management procedures.

Table 2 Experimental design of sesame seed cake fertilizer with γ-PGA application.
Full size table

Sampling and measurements

Plant sampling and analysis

The developmental stages of maize, from seedling to maturity, were documented and quantified at V6 (6-leaf stage), V12 (12-leaf stage), VT (tasseling stage), R2 (filling stage), and R6 (maturity) on the following dates: 30 May, 15 June, 15 July, 5 August, and 3 September, respectively. Three maize plants were randomly selected from the central strip of each plot at every growth stage. A tape measure and vernier caliper were employed to measure the plant height, stem thickness, and leaf area of the maize plants. Subsequently, the aboveground parts of the maize plants were severed from the roots and transported to the laboratory for treatment at 105 °C for 30 min to induce mortality, followed by drying at 75 °C for 12 h to determine the aboveground biomass. Following the harvest of the maize and its subsequent sun-drying, the maize was ultimately processed to determine the overall maize yield (kg/hm2). Harvest index (HI) was calculated by dividing the kernel yield at physiological maturity by the total aboveground biomass53. The crop's nitrogen use efficiency (NUE) was quantified using the partial factor productivity of N fertilizer (PFPn, kg/kg). The partial factor productivity of N fertilizer (PFPn) was determined using Eq. (2) as follows54:

$$\text{PFPn}=\frac{\text{Y}}{{\text{N}}_{A}}$$
(2)

where Y is grain yield (kg/ha); and NA is the applied N fertilizer (kg/(N ha)).

Soil sampling and analysis

The soil auger method was utilized to conduct soil profile sampling. Initially, after removing all visible plant residue, five sampling points were taken at the four apexes as well as in the middle of each plot, with a spacing of 1m or more between adjacent sampling points. After that, samples were taken at the sampling points at 0–20, 20–40, and 40–60 cm using an earth auger with a diameter of 50 mm. The five samples from the same depth were then mixed to create a composite sample55. Finally, the composite samples at various depths were measured three times. Every test plot used the same sampling locations and techniques, and all soil samples were kept in storage at 4 °C. The sampling period coincided with the determination of the maize plant stage. Within two days after the soil sample collection, the drying process was used to determine the soil water content (SWC). Soil was dried and ground separately and sieved through a 1 mm sieve for the determination of total soil nitrogen, and samples were decocted in concentrated sulfuric acid until colourless and transparent, then fixed and filtered, and the Kjeldahl nitrogen meter was used for the determination of total nitrogen (TN)56. Soil was leached with 0.5 mol L-1 sodium bicarbonate solution for the determination of available phosphorus (AP) by the molybdenum-antimony colourimetric method using an ultraviolet spectrophotometer57.The ammonium acetate leaching method was employed, and atomic absorption spectrophotometer (WASP) was utilized58. The concentration of available potassium (AK) was determined using an atomic absorption spectrophotometer (WFX-100), and the specific analytical procedure was conducted in accordance with the ISO instrument certification method.

The approach for determining water usage through soil moisture content relies on the water consumption equation as follows59:

$$ {\text{ET}}\, = \,{\text{W}}_{{\text{T}}} \, + \,{\text{P}} - \Delta {\text{W}} - {\text{R}} - {\text{D}}\, + \,{\text{K}} $$
(3)

where ET is the crop water consumption (mm); WT is the irrigation size (mm); P is the effective precipitation at the reproductive stage (mm); ∆W is the change in water storage within the wet layer of the soil profile from the initial to the final time period (mm); R is the quantity of the surface runoff (mm); D is the amount of deep seepage (mm); and K is the quantity of the groundwater recharge (mm).

This experiment was carried out under rainfed conditions with WT = 0. The test plots were flat and separated by ridges, with R = 0. Additionally, there was minimal variation in SWC between 60 and 80 cm, indicating D = 0. Furthermore, there was no observed groundwater recharge, and K = 0.

In this case, the soil water storage can be determined as follows60:

$$\text{W}=\frac{{\theta }_{\text{v}}{\rho }_{\text{b}}\text{h}}{{\rho }_{\text{w}}}$$
(4)

where θv is the volumetric water content (%) of the 0–60 cm soil; ρb is the average volumetric soil mass (g/cm) of the 0–60 cm soil layer; and ρw is the density of water (g/cm); and h is the thickness of the soil layer (cm).

The water utilisation efficiency can be determined as follows59:

$$ {\text{WUE}} = \frac{{\text{Y}}}{{{\text{ET}}}} $$
(5)

where WUE is the water use efficiency (kg/(hm2·mm)); and Y is the crop yield (kg/hm2).

Economic efficiency

Economic efficiency can be determined as economic efficiency = production value—production inputs. The production value refers to the yield of maize kernels, while production inputs encompass expenses related to fertilizer, seed cost, and inputs during ploughing and harvesting. Fertilizer inputs were sesame cake fertilizer and γ-PGA, which were calculated by multiplying the weight of fertilizer applied by the unit price of fertilizer; the price of sesame cake fertilizer used in this experiment was 5.82 yuan/kg, and the price of γ-PGA was 13.76 yuan /kg. Other inputs included seed costs and inputs for ploughing and harvesting. According to local standards, machinery was used for both ploughing and harvesting, with a mechanical operating cost of 3000 yuan /hm2 (100 yuan/day × 30 days/hm2) and a seed cost of 1100 yuan /hm2; the value of production was calculated by multiplying the measured yields by the local annual market price of maize, with a unit price of maize grain of 2.74 yuan/kg (https://lswz.henan.gov.cn/2023/09-21/2819632.html). Those cost inputs were factored in along with maize yield to calculate the economic benefits.

$$ {\text{Net gain}} = {\text{Output}} - {\text{Fertilizer input}} - {\text{Other input}}, $$
(6)

Statistical analysis

The raw data was subjected to normality verification using Kolmogorov–Smirnov test and to variance homogeneity verification using Levene’s test. Statistical analyses were conducted using SPSS25. Initially, the main and interaction effects were examined through a two-way analysis of variance (ANOVA). The significance levels were denoted as ns, *, and **, where * represents a significant difference (P < 0.05), ** represents a highly significant difference (P < 0.01), and ns indicates no significant difference (P > 0.05). Subsequently, within-group analyses were employed to assess differences between sesame cake fertilizer dosing with different γ-PGAs on the measured variables under the same sesame cake fertilizer treatment at a significance level of P < 0.05, indicated by the use of capital letters. For conducting multiple comparisons of mean annual values, the study employed Duncan method (P < 0.05) to compare different treatments. Lower case letters were used to indicate significant differences at P < 0.05, specifically for sesame cake fertilizer with different γ-PGA on the measured variables. The variables exhibited differences. The entropy weight-TOPSIS method was employed for a comprehensive evaluation of the available options to identify the optimal solution. The results are presented as the mean ± standard deviation (SD) and denoted by capital and lowercase letters, based on three replicates per treatment. Graphs were generated using Origin Pro 2021, while tables and data analysis were conducted using WPS Office 2019.

Results

Water use efficienc (WUE) and N use efficiency(NUE)

Table 3 shows the results of the effects and significance analysis of sesame cake fertilizer, γ-PGA, and their interaction on maize water and N use efficiency. The findings indicated that while WUE and NUE both increased with increasing γ-PGA under the B1 treatment, WUE and NUE decreased with rising γ-PGA under the B2 and B3 treatments. However, there were no significant variations in WUE between B2R1 and B2R2 and between B3R1 and B3R2. Furthermore, for WUE, the B3R1 treatment was significantly better than the others, while for NUE, the B1R3 treatment was significantly better. These results suggest that under conditions of low fertilizer availability, augmenting γ-PGA can enhance NUE.

Table 3 Effects and significance of sesame cake fertilizer, γ-PGA, and their interaction on maize WUE and NUE.
Full size table

Total nitrogen (TN)

The results of the two-way ANOVA (Table 4) indicated that with the exception of the R6 period in the 20–40 cm soil layer and the R2 period in the 40–60 cm soil layer, when γ-PGA treatments had no significant effect, sesame seed cake fertilizer and γ-PGA treatments as well as sesame seed cake fertilizer and γ-PGA interactions had a significant impact on TN under different treatments.

Table 4 Analysis summary for soil TN content.
Full size table

Figure 3 depicts the intra-group analysis chart of soil TN distribution throughout the entire maize growth period. The results indicated that in the 0–20 cm soil distribution, B1 treatment during the V6 period and B1 treatment during the R2 period indicated an rise in TN content with the rise of γ-PGA. B2 and B3 treatments during the V12 period, B2 treatment during the R2 period, and B2 treatment during the R6 period demonstrated a decrease in TN content with the increase of γ-PGA.In the 20–40 cm soil layer distribution, B1 treatment during the R2 period indicated a boost in TN content with the rise in γ-PGA. B2 treatment during V12 period, B2 treatment during the R2 period, and B2 treatment in the R6 period, TN content decreased with the rise in γ-PGA. Therefore, the rise in γ-PGA resulted in a synergistic effect on TN when treated with B1, and an antagonistic effect when treated with B2 and B3.

Figure 3
figure 3

Intra-group analysis of soil TN vertical distribution in 0–60 cm soil layer throughout the entire maize growth period.

Full size image

Figure 4 shows the soil total nitrogen (TN) distribution in 0-60cm soil horizons at different growth stages for different treatments. The findings demonstrated that soil TN in the 0–40 cm soil layer had a tendency to reduce, increase and decrease at different periods, whereas at different periods in the 40–60 cm soil layer, it shown a quick decrease in the V12 period (Fig. 4(c)), after which it remained steady throughout all periods.Overall, the distribution of soil TN in the soil horizons of 0–40 cm was more pronounced and exhibited greater stability in the soil horizons of 40–60 cm. The soil TN content was higher under the treatments of B2R2 and B3R2 compared to the other treatments.

Figure 4
figure 4

Distribution of soil TN across different soil horizons at various growth stages.

Full size image

Available phosphorus (AP)

The results of the two-way ANOVA (Table 5) indicated that in addition to γ-PGA treatment at R2-stage in 0-20cm soil layer, sesame cake fertilizer treatment and γ-PGA treatment at R2-stage in 20–40 cm soil layer, and sesame cake fertilizer treatment at V12 stage and γ-PGA treatment at R6 stage in the 40–60 cm soil layer had no significant effect on AP. In other different therapies, sesame cake fertilizer and γ-PGA treatment and the interaction of sesame cake fertilizer and γ-PGA had significant effects on AP.

Table 5 Analysis summary for soil AP content.
Full size table

Figure 5 depicts the intra-group analysis chart of the distribution of AP in the soil throughout the entire growth period of maize. The findings indicated that in the 0–20 cm soil layers, B3 treatment during the VT period demonstrated a rise in AP content with the increase in γ-PGA.B2 and B3 treatments during the V12 period, B1 treatment during the VT period, and B1 treatment during the R6 period revealed a decline in AP content with the increase of γ-PGA. In the 20–40 cm soil layer, B1 treatment during the R2 period indicated a rise during AP content with the increase during γ-PGA. B3 treatment during the V6 period, B2 treatment during the V12 period and B1 treatment during the VT period displayed a subsequent decrease during AP content with the increase of γ-PGA. In the 40–60 cm soil layer distribution, B1 treatment during the R2 period, AP content climbing with the increase of γ-PGA. B3 treatment during the V6 period, B1 treatment during the VT period and B3 treatment during the R6 period exhibited a trend of diminishing AP content with the increase of γ-PGA. Thus, the application of γ-PGA resulted in antagonistic effects of AP in V6, V12, and VT periods. However, in R6 and R2 periods, AP of B1 and B2 treatments at different depths partially transformed into synergistic effects.

Figure 5
figure 5

Intra-group analysis of AP vertical distribution in 0–60 cm soil layer throughout the entire maize growth period.

Full size image

Figure 6 illustrates the distribution of soil AP in 0-60cm soil horizons at different growth stages for different treatments. The results indicated that the variability of soil AP distribution was more pronounced in the 0–20 cm and 40–60 cm soil layers, while the distribution was relatively stable in the 20–40 cm soil layers. Additionally, soil AP content was significantly higher under B1R1 and B2R1 treatments than other treatments.

Figure 6
figure 6

Distribution of AP in different soil horizons at various growth stages.

Full size image

Available potassium (AK)

The results of the two-way ANOVA (Table 6) indicated that the sesame cake fertilizer treatments, γ-PGA treatments, and their interaction had an effect on AK levels at various depths throughout the entire maize fertility cycle.

Figure 7 depicts the intra-group analysis chart of the distribution of AK in the soil throughout the entire growth period of maize. The results indicated that in the 0–20 cm soil layers, B2 treatment in the V6 period, B1 treatment in the V12 period and VT period, AK content decreased with the increase of γ-PGA. In the 20–40 cm soil layer distribution, B2 treatment during the R2 period, with the increase of γ-PGA, the AK content increased. B2 and B3 treatments during V6, and B1 treatments during V12 and R2, with the increase of γ-PGA, the AK content reduced. In the soil distribution of 40–60 cm, B2 treatment during the V6 period increased the AK content with the increase of γ-PGA. B3 treatment during the R2 period, with the increase of γ-PGA, AK content then decreased. In summary, the antagonistic effects of AK were observed at different depths and periods.

Figure 7
figure 7

Intra-group analysis of AK vertical distribution in 0–60 cm soil layer throughout the entire maize growth period.

Full size image

Figure 8 illustrates the distribution of AK in the soil at 0–60 cm soil horizons during various growth stages for different treatments. The findings indicated that the distribution of soil AK in the 0–60 cm soil layer was more pronounced with the application of γ-PGA. Notably, the soil AK content was higher under B3R1 treatment compared to other treatments.

Figure 8
figure 8

Distribution of AK in different soil horizons at various growth stages.

Full size image

Yield and economic benefits

Table 7 shows the effect and significance analysis of sesame cake fertilizer, γ-PGA, and their combined effect on maize yield and economic indicators. The results indicate that the application of sesame cake fertilizer, γ-PGA, and the combined intercropping of sesame cake fertilizer and γ-PGA influenced HI, yield, and net gain. In this study, HI represents the efficiency of assimilatory transfer from straw to seed. The decrease in HI was observed with an increase in γ-PGA under B1 treatment. B2 treatment demonstrated better results compared to B1 and B3 treatments, and reached its peak value under B2R1 treatment. These findings suggest that B2R1 treatment has the potential to improve assimilate transfer and, consequently, seed yield. The results indicated that the increase in γ-PGA did not enhance host immunity, and all samples exhibited varying degrees of antagonistic effects. Upon analyzing the yield results presented in Table 6, under B1 treatment, an increase in γ-PGA led to a corresponding increase in yield. However, under B2 and B3 treatments, an increase in γ-PGA resulted in a decrease in yield. The yield reached its maximum under B3R1 treatment, while there was no significant difference between B3R2 and B2R1. These findings indicate that under low fertilizer conditions, an increase in γ-PGA can enhance maize grain yield. Conversely, under the remaining treatments, it demonstrated a notable antagonistic effect. Upon analyzing the economic benefits presented in Table 6, the net gain decreased as γ-PGA increased, with the highest gain recorded under B3R1.

Entropy weight-TOPSIS analysis

Entropy weight-TOPSIS analysis mitigates and avoids the subjective nature of the decision maker when allocating weights to indicators61. The study aimed to investigate the optimal combination of sesame cake fertilizer with γ-PGA by integrating ten indices including HI, yield, WUE, NUE, and net gain. The treatments were sorted using Entropy Coefficient-TOPSIS analysis, and the results are presented in Table 8. The sorting results were presented in the table. The findings indicated that B2R1 treatment achieved the highest overall evaluation score, while B3R3 treatment obtained the lowest score. Additionally, B2 treatment outperformed B1 and B3 treatments.

Discussion

Effects of water and fertilizer coupling on the soil nutrient (TN, AP, and AK)

Concurrently, a substantial body of literature has shown significant variations in soil nutrients when using sesame cake fertilizer with different fertilizers62,63,64,65. This study further demonstrated that sesame cake fertilizer with γ-PGA had effects on soil nutrients during the maize reproductive cycle and across various soil horizons, aligning with the findings of previous studies. The observed phenomenon may be attributed to the high nutrient and microbial content present in sesame cake fertilizer, leading to varying soil nutrient levels when applied in conjunction with different fertilizers. In this study, intra-group analyses revealed that the application of γ-PGA had both synergistic and antagonistic effects on soil nutrient levels during the maize reproductive cycle and at all soil depths. However, the specific effects varied among different nutrients. Specifically, in the 0–20 cm soil layer, as γ–PGA grew, so did the TN content of the B1 treatments in the V6, V12, and R2 periods (Fig. 3). This suggests that TN exhibited a synergistic impact when minimal fertilizer was applied. This could be given that γ-PGA absorbs water in the soil to become a hydrogel66,67, and sesame cake fertilizer contains Sesaminol glucosides68, which are hydrophilic compounds15 that bind with each other and retain the N content in the soil, thus contributing to the sesame cake fertilizer with γ-PGA. Meanwhile, in the 0–20 cm soil layer, B2 and B3 treatments during the V12 period, B2 treatment during the R2 period, and B2 treatment during the R6 period demonstrated a subsequent decrease in TN content with the increase of γ-PGA(Fig. 3).This suggests that indicated antagonistic effects under medium and high fertilization. This outcome could be attributed to the excessive utilization of cake fertilization and γ-PGA69. The results of period V6, period V12, and period B1 treatment in the 0–40 cm soil layer revealed a tendency of lowering TN content with rising γ-PGA (Fig. 5), suggesting that AP had antagonistic effects in all three periods. Nonetheless, during the R6 and R2 periods, the B3 treatment in the R2 period and the R6 period in the soil layer 0–20 cm, and the B2 treatment in the R2 period in the soil layer 0–40 cm, demonstrated a trend of a subsequent increase in the AP content with a boost in γ-PGA (Fig. 5), suggesting that the B1 and B2 treatment APs at different depths were partially transformed into synergistic effects. This indicates that the AP was partially converted into a synergistic effect in the B1 and B2 treatments at various depths. This suggests that B1 and B2 treatments led to a reduction in fixation and an increase in the diffusion of AP, thereby enhancing the mobility and effectiveness of soil AP70. During the nodulation stage, the soil TN and AP content decreased rapidly, while the decrease in AK content was less than that of TN. There are two reasons for this phenomenon. Firstly, during the seedling stage, there is a higher demand for soil N and P, which are absorbed more rapidly by the plant71. Conversely, certain studies have indicated that organic fertilizers necessitate a specific quantity of N and P for protein synthesis during their decomposition72. Additionally, γ-PGA possesses a significant quantity of unbound carboxyl groups along its molecular chain, resulting in a high number of reactive sites that readily form complexes with N, P, and K elements73. These fertilizers exhibit high bioefficacy, as the enrichment of N, P, and K in the soil enhances the biomass and activity of soil microorganisms, and expedites the absorption of N, P, and K fertilizers by soil minerals. Therefore, this enrichment plays a crucial role in enhancing plant efficiency, yield, and quality74,75.

In general, there was an increase in soil TN during R2 period (75 days) at a depth of 0–40 cm (Fig. 4). Additionally, soil AP showed an increase during the R6 period (108 days) at a depth of 0–60 cm (Fig. 6), and AK increased in the 0–60 cm soil layer after VT period (60 days). Different treatments exhibited varying trends of increase. It has been demonstrated that the mineralization of sesame cake fertilizer led to an increase in mineral N, available P and K between the 28th and 65th day after application, followed by a subsequent decrease76. However, there was no significant change in effective P or effective K. This pattern may be attributed to the interaction between sesame cake fertilizer and γ-PGA, which delayed the mineralization process of the sesame cake fertilizer. However, the precise rate of mineralization remains unknown.

Effects of water and fertilizer coupling on WUE and NUE, and yield.

Crop yield, as well as WUE and NUE, can be influenced by a range of controllable factors such as the type and application rate of fertilizers, irrigation rate, and fertilizer application strategy, among others, as well as uncontrollable factors including temperature, rainfall, and sudden pests and diseases. Therefore, the regulation of factors to enhance yield along with WUE and NUE is considered an effective approach for attaining sustainable agricultural development77,78. Furthermore, the experimental findings indicated (Tables 3 and 6) that yield and water and nitrogen use efficiency ascends with increasing γ-PGA under the treatment of B1, indicating that the combination of sesame cake fertilizer and γ-PGA intercropping exhibited a synergistic effect in the B1 treatment, leading to a substantial enhancement in WUE and NUE, and yield. These results are consistent with previous findings44,45,46,79,80. However, under the treatments of B2 and B3, yield and water and nitrogen use efficiency diminished with the increase of γ-PGA, suggesting that the interaction between sesame cake fertilizer and γ-PGA resulted in significant antagonistic effects. In terms of WUE, B1R3, B3R1, and B3R2 did not show significant differences (P > 0.05) and were all higher than the other treatments. Regarding NUE, B1R3 exhibited higher values compared to the other treatments. This suggests that the use of γ-PGA could mitigate the negative effects of fertilizer deficiency, but it did not enhance WUE and NUE to improve crop yield. This phenomenon may be attributed to the fact that the ultimate degradation products of γ-PGA consist of glutamic acid monomers, which are benign to the natural environment and can be assimilated and utilized by both crops and soil microorganisms in agricultural practices. Consequently, this promotes the utilization of γ-PGA with sesame cake fertilizer to a certain extent46,81. Meanwhile, pertinent research has demonstrated that excessive fertilization can result in the overgrowth of plant nutrients, leading to the development of plant appendages that are not conducive to fruit formation82. Additionally, high concentrations of γ-PGA may influence the osmotic pressure of the crop root system, subsequently weakening crop root respiration and activity83. As a result, this may have an antagonistic effect on sesame cake fertilizer dosed with γ-PGA.

Table 6 Analysis summary for soil AK content.
Full size table
Table 7 Effects and significance analysis of sesame cake fertilizer, γ-PGA, and their interaction on maize yield and economic benefit indicators.
Full size table
Table 8 Entropy coefficient-TOPSIS analysis results for various treatments.
Full size table

Overall, sesame cake fertilizer with γ-PGA had a significant effect on soil nutrients, yield, and water and nitrogen use efficiency. This evolved from the combination of the numerous phytochemical features of sesame cake fertilize15 and the structural characteristics of γ-PGA84, but it was also a result of the interaction between the fertilizer and soil. The trial findings additionally demonstrated that soil quality and maize production might both benefit from the proper dosages of sesame seed cake fertilizer when paired with eco-friendly ingredients63,85. Thereby, it's crucial to look at the sesame cake fertilizer's fertilization technique using γ-PGA.

Integrated method for application in agricultural production

From an agricultural production perspective, a decrease in the volume of water and kilograms of fertilizer input per unit can lead to an increase in crop yield, thereby achieving greater water and fertilizer productivity. The study findings indicated that the application of sesame cake fertilizer at a rate of 1100 kg/hm2 in combination with γ-PGA at 200 kg/hm2 resulted in the highest overall score, leading to the recommendation of this combination as the optimal choice for sesame cake fertilizer with γ-PGA application. However, the elevated expense of γ-PGA serves as the primary constraint on the utilization of γ-PGA in agricultural production. Meanwhile, the results of this study demonstrated a synergistic effect of γ-PGA on sesame cake fertilizer at lower γ-PGA concentrations, aligning with the outcomes of the comprehensive assessment. Consequently, the utilization of the aforementioned formulation scheme can optimize nutrient uptake efficiency, crop yield, and economic benefits.

Under the co-fertilization conditions of sesame cake fertilizer and γ-PGA, the allocation of soil nutrients will emerge as a critical concern for promoting sustainable and environmentally friendly production. The study illustrated that effective soil nutrient fertilization can be accomplished through the combination of sesame cake fertilizer and γ-PGA. However, this study exclusively addressed alterations in soil nutrient distribution, without conducting a comprehensive analysis of the soil. Therefore, future research should delve deeper into: (1) the effect of sesame cake fertilizer with γ-PGA on soil moisture content; (2) the effect of sesame cake fertilizer with γ-PGA on water and N movement; and (3) the effect of sesame cake fertilizer with γ-PGA on soil bacterial colonies, microbial composition, and soil enzyme activities.

Conclusion

This study demonstrated that the application of sesame cake fertilizer with γ-PGA resulted in significant interaction effects on soil nutrient levels along with WUE and NUE. The inclusion of γ-PGA resulted in an increase in TN, WUE and NUE, and yield when the lower fertilizer treatment was applied. Under the medium (B2) and high (B3) fertilizer treatments, the application of sesame cake fertilizer with γ-PGA demonstrated significant antagonistic effects on soil nutrient levels, WUE and NUE, and crop yield. The inclusion of γ-PGA slowed down the rate of cake mineralisation, consequently delaying the time required to release sesame cake fertilizer.. Through Entropy Coefficient-TOPSIS analysis, it was determined that B2R1 composite score exceeded the other scores. Specifically, the optimal fertilizer application strategy for enhancing soil nutrient distribution, WUE and NUE, and yields was found to be the application of 1100 kg/hm2 of sesame cake fertilizer and 200 kg/hm2 of γ-PGA.