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Article

Efficient Regulation and Prediction Model Construction for Water and Fertilizer Management Through Resource Utilization of Manure and Urea Co-Application

by
Kaiqi Qi
1,
Xiaofeng Tang
2,
Jianhong Ma
2,*,
Rui Zhao
1,
Junan Bao
1,
Pengshan Tang
1,
Jiaqi Liu
1,
Dandan Pei
1,
Xiaohou Shao
1 and
Xinyu Mao
1,*
1
College of Agricultural Science and Engineering, Hohai University, Nanjing 210098, China
2
Nanjing Agricultural Machinery Extension Center, Nanjing 210019, China
*
Authors to whom correspondence should be addressed.
Water 2025, 17(17), 2594; https://doi.org/10.3390/w17172594
Submission received: 24 July 2025 / Revised: 24 August 2025 / Accepted: 27 August 2025 / Published: 2 September 2025
(This article belongs to the Special Issue Water and Soil Pollution from Agriculture: Mechanisms, Assessments and Mitigation Strategies)
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Abstract

In intensive agriculture, the excessive application of chemical fertilizers leads to approximately 50% nitrogen loss, which exacerbates water pollution and the greenhouse effect. Meanwhile, nitrogen and phosphorus emissions from livestock manure have far exceeded those from chemical fertilizers, becoming the primary source of agricultural non-point-source pollution. This study aims to clarify the comprehensive effects of combining manure with urea application and precision irrigation on the soil environment, lettuce growth, and quality, and to determine the optimal water and fertilizer management strategy. The results indicate that combining manure with urea application and precision irrigation can effectively mitigate non-point-source pollution, enhance soil nutrients, promote lettuce growth, and improve quality. When the irrigation volume reaches 75–78% of field capacity and the ratio of manure to urea nitrogen ranges from 7:3 to 1:1, key indicators for soil health, lettuce growth, and quality can exceed 90% of their respective maximum levels. This study provides a scientific basis and practical guidance for the resource utilization of manure and precise water–fertilizer management in intensive lettuce production.

1. Introduction

In recent years, China’s vegetable industry has experienced rapid development, with continuously expanding cultivation areas. According to statistical data, the national vegetable planting area reached 22,873 thousand hectares in 2023, with a total output exceeding 89.2 million tons [1]. As one of the essential elements for crop growth, the application of nitrogen fertilizer has also continued to increase, alongside the expansion of agricultural production. In 2023, the application rate of nitrogen fertilizer in Chinese agriculture reached 16.0334 million tons [2]. However, the relationship between crop yield and nitrogen fertilizer application is not simply linear. Excessive application of nitrogen fertilizer leads to numerous issues. When the application rate exceeds the absorption and utilization capacity of crops, nitrogen is lost in large quantities through leaching or in gaseous forms [3]. Studies indicate that approximately 50% of nitrogen is lost from farmland into the atmosphere or water bodies, which not only exacerbates global warming and water pollution [4], but may also cause soil degradation, impair soil health, and threaten ecological environments [5]. Therefore, exploring rational fertilization methods that reduce application rates while enhancing efficiency is crucial for achieving sustainable agriculture.
In this context, the partial substitution of chemical fertilizers with manure has gradually emerged as an important strategy of increasing concern. With the rapid development of intensive livestock farming in China, enormous amounts of livestock manure are generated annually, reaching approximately 3.8 billion tons [6]. Livestock manure has become a major source of agricultural non-point-source pollution. Proper treatment and utilization of livestock manure as a resource is not only essential for mitigating its environmental and health risks, but also a key pathway toward achieving the resource-oriented use of agricultural waste. Although manure is rich in nutrients required by crops, its nutrient concentration is relatively low, making it difficult to meet crop demands during peak growth periods [7,8]. Therefore, the combined application of manure and chemical fertilizers is widely adopted and has become an important research direction [9]. Studies have shown that appropriate combined application can increase the content of available nitrogen in soil, thereby improving crop yield and quality [10].
In addition to fertilizer application, water management plays an equally critical role in crop production. On the one hand, excessive irrigation can lead to soil degradation and promote nitrate nitrogen leaching into deeper soil layers, thereby contaminating groundwater [11]. The LEACHM model further demonstrates that both the rate and depth of nitrate nitrogen leaching increase with irrigation volume [12]. On the other hand, moderate water stress can improve crop quality and enhance antioxidant capacity to some extent [13]; however, severe water stress significantly inhibits plant growth, resulting in reduced crop biomass and decreased photosynthetic pigment content [14]. Therefore, quantifying crop growth responses under varying irrigation conditions is essential for achieving both high crop yields and environmental pollution control.
Although the water–fertilizer coupling effect has been extensively studied in grain crops [15,16], research on leafy vegetables has primarily focused on areas such as pest and disease control, heavy metal migration [17,18,19], or the influence of individual factors like irrigation or fertilization on crop growth [20,21,22]. To address this research gap, this study took lettuce as the research object. Based on previous research [23,24,25], five fertilization gradients and three irrigation levels were implemented. Soil nutrients, lettuce biomass, and quality indicators were systematically measured. Through the integration of least squares fitting and multiple regression analysis, this study investigated the combined effects of co-applied manure and urea under precision irrigation on the soil environment, lettuce growth, and quality, with the aim of identifying optimal water–fertilizer management strategies. The findings aim to provide a theoretical foundation and technical support for high-yield, high-quality lettuce production, while mitigating agricultural non-point-source pollution and promoting sustainable agricultural development.

2. Materials and Methods

2.1. Experimental Site

The experiment was conducted from 2022 to 2023 in a plastic greenhouse at the Institute of Animal Husbandry and Poultry Research, located in Jiangning District, Nanjing City, Jiangsu Province (Figure 1). The region experiences a mild climate, classified as a northern subtropical monsoon humid zone. The mean annual temperature and precipitation are 15.7 °C and 1106.5 mm, respectively, with a frost-free period of 237 days. The soil type is yellow-brown loam, and its physicochemical properties are presented in Table 1.

2.2. Formulation of Organic Fertilizer

First, microbial strains were activated by mixing raw material-grade Bacillus subtilis (100 billion CFU/g) and Bacillus licheniformis (100 billion CFU/g) with 5% brown sugar solution at a 1:10 ratio, followed by incubation at room temperature for 4–6 h.
Second, the bedding material was prepared by spraying the activated bacterial solution onto a mixture of straw and wood chips at a ratio of 1:1000 (v/v).
Lastly, for fermentation and maturation, the mixture was turned thoroughly once or twice daily. When the temperature of the fermentation bed decreased below 50 °C and reached a mature stage, an amount of chicken manure equivalent to that of the previous batch was added to initiate a second fermentation. After maturation, a third fermentation was conducted under the same conditions. Upon completion of all fermentation stages, the final chicken manure organic fertilizer was obtained. The content of organic matter, nitrogen, phosphorus, potassium, and moisture in chicken manure organic fertilizer is shown in Table 2.

2.3. Experimental Design

The experiment utilized lettuce (Lactuca sativa cv. Beishan No. 3), a cultivar widely grown in the Jiangsu region. The experiment included three irrigation levels (70% field capacity (W1), 85% field capacity (W2), and 100% field capacity (W3)) and five nitrogen fertilizer ratios (manure/urea = 1:0 (F1), manure/urea = 7:3 (F2), manure/urea = 1:1 (F3), manure/urea = 3:7 (F4), manure.urea = 0:1 (F5)). The total nitrogen application rate was uniform across all plots at 103.5 kg/ha, with the proportion of manure and urea adjusted according to the designated nitrogen ratio. Irrigation was applied via sprinklers every 7 days. Detailed fertilizer application rates are provided in Table 3.

2.4. Measurement Items and Methods

2.4.1. Determination of Growth Indexes and Yield

The growth indicators measured in this study included leaf number, leaf area index (LAI), and fresh weight. At the seedling stage (18 November), rosette stage (30 December), and heading stage (27 January) of lettuce, ten plants with uniform growth were randomly selected from each plot. The leaf area index was measured using a plant canopy analyzer, and the number of leaves per plant was recorded. Additionally, after harvest, another ten uniformly grown plants were randomly sampled from each plot, and their aboveground fresh weights were determined.

2.4.2. Determination of Quality Indexes

The quality indicators assessed in this study included soluble sugar content, soluble protein content, nitrate content, and chlorophyll content. Following harvest, ten uniformly grown lettuce plants were randomly selected from each plot. Soluble sugar content was quantified using the anthrone colorimetric method [26], soluble protein content was determined via the Coomassie brilliant blue method [27], nitrate content was measured according to the spectrophotometric procedure described by Bian et al. [28], and chlorophyll content was analyzed based on the method established by Arnon [29].

2.4.3. Determination of Soil Indicators

After lettuce was harvested, After the lettuce was harvested, surface soil samples (0–20 cm depth) were collected from an S-shaped pattern. The soil types were then determined following the method described in Soil and Agricultural Chemistry Analysis [30]. NH4+-N, NO3−-N, and organic matter were determined according to the methods specified in Soil Agrochemical Analysis.

2.5. Data Processing

Experimental data were collected and analyzed with Microsoft Excel 2006 and SPSS 22.0. Graphs were plotted with Origin 2024 and Mathematica 9.0. Significant differences between the detected parameters were compared by Duncan’s multiple range test at the 5% significance level.

3. Results

3.1. Effects of Water–Fertilizer Coupling on Soil Nutrients

Figure 2 illustrates the effects of irrigation and fertilization on soil nutrients. For soil organic matter (SOM), significant differences were observed between the manure-only (F1) and urea-only (F5) treatments under the W1 and W2 irrigation levels. The highest SOM values consistently occurred under F1. However, under W3 irrigation, no significant difference was detected between F1 and F5, and the organic matter content in F1 was the lowest among all treatments. When manure was applied alone (F1), SOM content initially increased and then decreased with increasing irrigation volume. In contrast, no significant differences in SOM were observed across irrigation levels under combined manure–urea or urea-only applications.
For soil ammonium nitrogen (NH4+-N), at a given irrigation level, the content exhibited an initial increase followed by a decrease as the proportion of manure decreased. Under both F1 and F5, soil NH4+-N increased and then decreased with higher irrigation, with the lowest values occurring under W1. Significant differences were observed between W1 and W2, while W3 did not differ significantly from either. Under combined fertilizer applications (F2–F4), different irrigation volumes significantly influenced NH4+-N content: in F2 and F4, it gradually decreased with increasing irrigation, whereas in F3, it first decreased and then increased.
For soil nitrate nitrogen (NO3−-N), at the same irrigation level, NO3−-N content initially increased and then decreased across fertilization treatments, peaking consistently under F3. Applications of manure alone (F1) resulted in lower NO3−-N values. When manure was combined with urea, the response of NO3−-N to increasing irrigation varied across treatments. In contrast, under urea-alone applications (F5), NO3−-N content increased progressively with irrigation volume, with significant differences among W1, W2, and W3.

3.2. Effects of Water–Fertilizer Coupling on Lettuce Growth and Development

3.2.1. Leaf Blade Number and Leaf Area Index

Figure 3 shows the leaf number and leaf area index of lettuce at different growth stages. At the seedling stage, under the W1 irrigation level, the leaf number initially increased and then decreased as the proportion of manure decreased, while the leaf area index showed a fluctuating downward trend. The highest leaf number and leaf area index values occurred under the F2 and F3 treatments, respectively; though no significant differences were observed between F2 and the other treatments in terms of leaf number. Under the W2 irrigation level, the leaf number also exhibited an initial increase followed by a decrease with reduced manure application, whereas the leaf area index decreased first and then increased, with peak values recorded under the F2 and F1 treatments, respectively. At the W3 irrigation level, the leaf number fluctuated with decreasing manure input, and the leaf area index again decreased initially and then increased. The maximum leaf number and leaf area index under W3 were observed in the F3 and F1 treatments, respectively. Under the same fertilization treatment, the leaf number tended to increase with higher irrigation volume, while the leaf area index generally decreased initially and then increased. The highest values for both parameters under each fertilization mode occurred under the W3 irrigation condition. In summary, during the seedling stage, the highest leaf number and leaf area index were achieved under the W3F3 and W3F1 treatments, respectively.
At the rosette stage, under the W1 irrigation level, the maximum lettuce leaf number and leaf area index occurred under the F4 and F1 treatments, respectively. At this irrigation level, the application of manure combined with urea did not consistently enhance leaf number or leaf area index. Under W2 irrigation, the highest leaf number and leaf area index were observed in the F5 and F3 treatments, respectively, indicating that manure application did not positively influence leaf number under these conditions. At the W3 irrigation level, the peak values for leaf number and leaf area index were recorded under the F2 and F1 treatments, respectively. In the F1 treatment, both leaf number and leaf area index increased with higher irrigation volume. In contrast, under F5, these parameters initially increased and then decreased with increasing irrigation. Overall, at the rosette stage, the highest leaf number and leaf area index were achieved under the W3F2 and W3F1 treatments, respectively.
At the heading stage, under the W1 irrigation level, the maximum leaf number and leaf area index were observed in the F5 and F4 treatments, respectively. It is noteworthy that leaf number in the F5 treatment differed significantly from all other treatments, while the leaf area index under F4 was significantly different only from that under F2. Under the W2 irrigation level, the highest values for both leaf number and leaf area index were found in the F2 treatment. At the W3 level, the maximum leaf number and leaf area index occurred under the F4 and F1 treatments, respectively. When the fertilization pattern remained constant, leaf number increased with increasing irrigation volume. In both the F1 and F2 treatments, the maximum leaf area index was observed under the W3 irrigation level, whereas under F3, F4, and F5, it occurred under W1. In summary, at the heading stage, the highest leaf number and leaf area index were obtained under the W3F4 and W3F1 treatments, respectively.

3.2.2. Fresh Weight

Figure 4 shows the fresh weight of lettuce at harvest. Under W1 irrigation, no significant differences in fresh weight were observed among the fertilization treatments. Under W2 irrigation, fresh weights in the F2 and F3 treatments were comparatively high and did not differ significantly from each other, but were significantly greater than those in the F1, F4, and F5 treatments. Under W3 irrigation, the F3 treatment yielded the highest fresh weight, though no significant differences were detected among F3, F4, and F5.
Under F1, F2, and F3 fertilization treatments, lettuce fresh weight initially increased and then decreased with increasing irrigation level, peaking under the W2 irrigation treatment. The fresh weight under W2 differed significantly from that under both W1 and W3. In contrast, under the F4 and F5 treatments, fresh weight increased with irrigation volume, reaching its maximum under W3 irrigation; however, no significant differences were observed among the W1, W2, and W3 irrigation levels for these fertilization treatments.

3.3. Effects of Water–Fertilizer Coupling on Lettuce Quality

3.3.1. Nitrate

As shown in Figure 5, under W1 irrigation, the nitrate content of lettuce initially increased and then decreased as the proportion of manure was reduced, with the minimum value occurring under the F1 treatment. The nitrate content under F1 differed significantly from all other fertilization treatments. Under W2 irrigation, the second-lowest nitrate content was observed in the F3 treatment, which also differed significantly from all other treatments. Under W3 irrigation, the lowest nitrate content occurred in the F1 treatment, though no significant differences were detected among the F1, F2, and F5 treatments.
Under both F1 and F2 fertilization treatments, the lowest nitrate content was observed in the W1 irrigation treatment. Notably, under F1, significant differences were found between W1 and both W2 and W3, whereas under F2, no significant difference was observed between W1 and W3. Under F3 fertilization, the minimum nitrate content occurred under W2 irrigation, and significant differences were detected between W2 and both W1 and W3. For the F4 and F5 treatments, the lowest nitrate content was recorded under W3 irrigation; however, under F5, no significant differences were found among the three irrigation levels. Overall, the lowest nitrate content in lettuce was achieved under the W2F3 treatment combination.

3.3.2. Soluble Sugar Content

As shown in Figure 5, under both W1 and W2 irrigation levels, the highest soluble sugar content in lettuce was observed in the F2 treatment, though no significant difference was detected between F1 and F2. Under W3 irrigation, the maximum soluble sugar content occurred in the F1 treatment, followed by F2.
Under the F1 fertilization treatment, soluble sugar content gradually increased with higher irrigation levels, reaching its maximum under W3 irrigation. However, no significant differences were observed among the three irrigation levels. In contrast, under F2, F3, and F4 fertilization treatments, soluble sugar content decreased gradually with increasing irrigation volume, with the highest values occurring under W1 irrigation. Significant differences were observed between W1 and W3 under these fertilization conditions. Under the F5 treatment, soluble sugar content initially decreased and then increased with increasing irrigation, peaking under W1 irrigation, with significant differences among all three irrigation levels. In summary, the highest soluble sugar content in lettuce was achieved under the W1F2 treatment combination.

3.3.3. Soluble Protein Content

As shown in Figure 5, under W1 irrigation, the soluble protein content in the F2, F3, and F4 treatments was higher than that in the F1 and F5 treatments, with no significant differences among F2, F3, and F4. Under W2 irrigation, the soluble protein content in F2, F3, and F4 was lower than that in F1 and F5, and again, no significant differences were detected among F2, F3, and F4. Under W3 irrigation, the soluble protein content decreased initially and then increased with increasing urea application, reaching its maximum under the F1 treatment. It should be noted that no significant differences were observed among the F1, F2, and F3 treatments under this irrigation condition.
Under the F1 and F5 fertilization treatments, soluble protein content increased initially and then decreased with increasing irrigation volume, peaking under W2 irrigation. In the F2 and F3 treatments, soluble protein content decreased first and then increased with higher irrigation, with the highest values observed under W1 irrigation. Under the F4 treatment, soluble protein content gradually decreased with increasing irrigation, also reaching its maximum under W1 irrigation. In summary, the highest soluble protein content in lettuce was obtained under the W2F1 treatment combination.

3.3.4. Chlorophyll Content

As shown in Figure 5, under W1 irrigation, the maximum chlorophyll content was observed in the F2 treatment, although no significant differences were detected among the fertilization treatments. Under W2 irrigation, chlorophyll content decreased initially and then increased with higher urea application rates, reaching its maximum under the F1 treatment. The value in F1 differed significantly from all other fertilization treatments. Under W3 irrigation, the highest chlorophyll content also occurred in the F1 treatment, which again differed significantly from the other treatments.
Under the F1 fertilization treatment, chlorophyll content was highest under W2 irrigation, though no significant difference was observed between W2 and W1. Under F2 fertilization, the maximum chlorophyll content occurred under W1 irrigation, with no significant difference between W1 and W2. For the F3, F4, and F5 fertilization treatments, no significant differences in chlorophyll content were observed across the three irrigation levels. The highest values under these fertilization conditions occurred under W1 for F3 and under W2 for both F4 and F5. In summary, the highest chlorophyll content in lettuce was obtained under the W2F1 treatment combination.

3.4. The Effect of Water–Fertilizer Coupling on Soil Nutrients, Lettuce Growth, and Quality

This study employed irrigation volume and the manure application rate as independent variables, while soil organic matter content, ammonium nitrogen content, nitrate nitrogen content, lettuce fresh weight, and quality indices served as dependent variables. Utilizing the least squares method, data analysis was conducted with Mathematica 14.0 software to establish quadratic regression models, and the optimal values for each dependent variable were determined (Table 4).
The results indicate that achieving optimal values for soil fertility, lettuce fresh weight, and quality simultaneously presents a significant challenge (Table 5). The soil organic matter content reached its maximum value of 1.06% when the irrigation volume was 82.8% of the field capacity and the manure application rate was 4300 kg/ha. The soil nitrate nitrogen content achieved its maximum value of 30.46 mg/kg at an irrigation volume of 70% and a manure application rate of 19,788 kg/ha. Meanwhile, the ammonium nitrogen content peaked at 70 mg/kg when the irrigation volume was 70% and the manure application rate was 19,707 kg/ha. The maximum fresh weight of lettuce was 326.175 g. When the irrigation volume was 70% of the field capacity and the manure application rate was 4300 kg/ha, the nitrate content of lettuce reached a minimum of 181 mg/kg. The soluble sugar and soluble protein content in lettuce both reached 19.27 mg/g when the irrigation volume was 85.5% of the field capacity and the manure application rate was 24,891 kg/ha. The chlorophyll content achieved its maximum value of 0.115 mg/g when the irrigation volume was 88.8% and the manure application rate was 42,946 kg/ha.
Moreover, the 90% confidence intervals for the maximum values of soil organic matter, ammonium nitrogen, nitrate nitrogen, lettuce fresh weight, soluble sugar, soluble protein, and chlorophyll content were defined as the acceptable comprehensive optimization range (Figure 6). The results indicate that when the irrigation volume is maintained at 75–78% of field capacity and the manure-to-urea nitrogen ratio is between 7:3 and 1:1, all measured parameters—including soil organic matter, ammonium nitrogen, nitrate nitrogen, lettuce fresh weight, soluble sugar, soluble protein, and chlorophyll content—can simultaneously reach at least 90% of their respective maximum values. Under these conditions, the nitrate content in lettuce ranges from 223.52 to 238.69 mg/kg, which complies with safety standards for nitrate levels in leafy vegetables and remains well below the hazard threshold [31].

4. Discussion

4.1. Effects of Manure and Chemical Fertilizers on Soil Fertility, Lettuce Growth, and Quality

4.1.1. Effects of Manure and Chemical Fertilizer on Soil Fertility

Soil organic matter plays an important role in maintaining soil fertility and productivity; however, in arable soils, the content of organic matter is particularly low, and in order to increase the content of soil organic matter, manure application is the most effective way [32]. Studies have shown that compared with the application of chemical fertilizers, the application of manure can significantly increase soil organic matter content [33,34,35]. In this study, manure application had a significant effect on soil organic matter content (p < 0.05), and either manure alone or manure with urea increased soil organic matter content compared to urea alone. This may be due to the fact that the sticky organic matter in manure can enhance the aggregation of soil particles and thus effectively increase the soil organic matter content [36].
Lettuce has a high nitrogen requirement, and its yield and quality formation are closely related to soil nitrogen content [37]. The soil N pool consists mainly of organic and inorganic N, and the N absorbed by crops is mainly soil inorganic N, including ammonium N (NH4+-N) and nitrate (NO3−-N) [38]. Some studies have shown that long-term application of manure fertilizer can improve soil fertility and N content [33], and that manure fertilizers help maintain N levels in the soil surface for up to four years [39]. In this study, the effect of fertilizer application on soil ammonium and nitrate nitrogen content was highly significant (p < 0.01), although the nitrogen content applied to the soil was the same for each group of treatments in the experiment, but for the application of manure or urea alone, manure and urea increased the ammonium and nitrate nitrogen content of the soil when applied in combination. This may be due to the fact that urea hydrolyzes too fast and more is lost, resulting in lower soil N content at a later stage, whereas when manure alone is applied, it may result in lower soil N content because of its slower mineralization and slow nutrient release [40], whereas when urea is paired with manure, the manure is rich in a variety of nutrient elements, which can provide sufficient carbon sources for microorganisms to enhance the capacity of soil organic nitrogen mineralization [41,42]. Moreover, the nitrogen in organic fertilizers can be mineralized by microorganisms, thus increasing the amount of accumulated mineralized nitrogen in the soil [43].
Moreover, previous studies have shown that manure application can reduce nitrogen loss, increase soil microbial activity, and enhance soil nitrogen accumulation [44]. Moreover, chicken manure replacing 20% of urea can improve soil nitrogen supply and also increase fertilizer utilization [45]; therefore, manure with urea is conducive to improving soil organic matter, ammonium nitrogen, and nitrate nitrogen content. However, it should also be noted that the large amount of available nitrogen retention at crop harvest may increase the risk of nitrogen leaching.

4.1.2. Effect of Manure and Chemical Fertilizer Dosing on Lettuce Growth

Leaf number and leaf area index are important indicators to respond to crop growth, and previous studies have shown that application of 10% or 20% organic fertilizer can promote crop leaf growth [46]. In this study, fertilizer application had a highly significant effect (p < 0.01) on both number of leaves and leaf area index at the seedling stage of lettuce, when irrigation was uniform, compared to urea. Proper manure paired with urea increased the number of leaves and leaf area index of lettuce. However, at the rosette and nodulation stages, when the irrigation rate was kept at a low level, compared with the application of urea alone, the combination of manure and urea or the application of manure alone did not promote the growth of lettuce leaves very well, which may be due to the fact that a large amount of quick-acting nitrogen is needed at the rosette and nodulation stages, but the nitrogen in the manure needs to be mineralized before it can be absorbed by the crop, which makes it difficult to satisfy the high demand of lettuce [47,48,49]. Therefore, the combination of manure and urea can increase the number of leaves and leaf area index of lettuce, but it is also closely related to the amount of irrigation water.
Studies have shown that manure with urea has a significant effect on crop fresh weight (or yield) [50], which is similar to the results of the present study. In this study, fertilizer application had a highly significant effect on lettuce fresh weight (p < 0.01), and the maximum fresh weight of lettuce under the same irrigation level basically appeared in the ratio of nitrogen application of 7:3 and 1:1 between manure and urea. This may be due to the fact that the matching of manure and urea can better meet the demand of the crop growth, and in the early stage of the growth of the crop, the crop requires more nitrogen, and the urea can supply nitrogen rapidly to guarantee the nitrogen supply of the crop in the early stage of the growth, while the slow release of nitrogen from manure secures nitrogen supply in the middle and late stages of the crop [51]. This is in line with previous reports, where it was shown that the highest fresh weight of the crop was achieved when the ratio of manure to urea N application was 1:1 [52]. Ratios of manure to urea nitrogen application of 7:3 and 1:1 increased the fresh weight of the crop.

4.1.3. Effect of Manure–Fertilizer Combination on Lettuce Quality

Vegetable quality has become an agronomic indicator as important as yield in facility-based cultivation systems. It is worth noting that leafy crops such as lettuce exhibit significant nitrate enrichment capacity due to their morphophysiological characteristics, presenting a potential food safety risk [53]. Previous research studies have shown that manure application can reduce the accumulation of nitrate levels in crops [54], and Fan et al., in their study, noted that organic fertilizers improved crop quality and reduced nitrate levels in crops by 13.02% [55]. In the present study, there was a highly significant effect of manure application on the nitrate content of lettuce (p < 0.01). It should be noted that in this study, nitrate content in the crop was reduced when manure alone was applied compared to urea alone, which is in agreement with the study of Abd-Elrahman et al. (2022), whose study showed that mineral fertilizers induced about 1.24 times more nitrate accumulation than organic fertilizers alone [56]. However, the combination of manure and urea does not necessarily reduce the accumulation of nitrate in the crop, which may be related to the crop growth characteristics. The nitrogen in the manure is mainly in the organic state, and the release is slow but continuous; if the crop still absorbs a large amount of nitrogen at the late stage of growth, the nitrate nitrogen released from mineralization of the organic fertilizers may lead to an increase in nitrate accumulation, especially at high temperatures, which can lead to an increase in nitrate accumulation. Nitrate accumulation increases, especially at high temperatures or under well-aerated soil conditions [57]. Sugars are essential for plant growth and development, and sugar not only promotes cell proliferation and leaf neogenesis, but also effectively slows down leaf senescence, which is one of the important indicators of crop quality [58]. Previous studies have shown that organic fertilizers can increase the soluble sugar content of tomato by 24% compared to conventional fertilizers [59]. This is similar to the results of the present study.
In this study, fertilizer application had a highly significant effect on soluble sugar content (p < 0.01), with the highest values yielded by the manure-alone (F1) and the 7:3 manure-to-urea (F2) treatments across all three irrigation levels, indicating that increased application of manure increased the soluble sugar content of lettuce. This is consistent with one of the studies by Li et al. which showed that crop soluble sugar content increased linearly with the amount of manure [60]. This may be due to the fact that the manure contains the crop-growth-required micro quantitative elements and active substances, but is also rich in humic acids, active enzymes, and growth factors, which promote crop root development and nutrient uptake and improve the efficiency of crop utilization of carbon sources, thus increasing soluble sugar content [61]. Thus, application of manure can increase the soluble sugar content of the crop. Soluble proteins are important organic permeation regulators in plants and play an important role in mitigating damage [62]. Previous studies have shown that the application of bio-organic fertilizers can significantly promote the growth of crops and increase the content of soluble proteins [63] and that manure application increased soluble protein content by 124% compared to no manure application [64].
In this study, manure application had a highly significant effect on lettuce soluble protein content (p < 0.01). The highest soluble protein content in lettuce across the three irrigation levels was consistently observed with the application of manure alone or in combination with urea, aligning with findings from earlier studies. Crop protein synthesis requires an adequate source of nitrogen, and the ammonium nitrogen absorbed by the crop is converted to amino acid precursors by ammonia assimilation and is further synergistically catalyzed by key enzymes (to ultimately synthesize the various types of proteins required by the crop [65], whereas the slow and steady mineralization of organic nitrogen in manure reduces the risk of nitrogen loss and can provide adequate nitrogen supply to the crop, so the application of manure can increase the soluble protein content in the crop. Chlorophyll is vital in plants, and it plays a central role in crop growth, development and yield formation. The amount of total chlorophyll content is closely related to the rate of crop photosynthesis, and the ability to photosynthesize directly affects the yield and quality of lettuce, as well as chlorophyll being an important measure of plant leaf senescence [66]. Some studies have shown that organic fertilizers can increase crop chlorophyll and carotenoid content and improve the photosynthetic efficiency of crops [67].
In the present study, fertilizer application had a highly significant effect on the chlorophyll content of lettuce (p < 0.01). The highest chlorophyll content in lettuce was consistently observed between the F1 and F2 fertilization treatments across all three irrigation levels. This result may be attributed to the ability of manure to enhance soil water retention and improve soil structure [68], thereby supplying more available nutrients to the plants. Additionally, manure supports root development, facilitating improved nutrient uptake and leading to higher chlorophyll levels [69]. Furthermore, previous studies have shown that organic fertilizers can upregulate genes involved in chlorophyll biosynthesis, further increasing chlorophyll content in crops [70]. Thus, application of manure helps to increase chlorophyll content in lettuce.

4.2. Effects of Irrigation on Soil Fertility, Lettuce Growth, and Quality

4.2.1. Effect of Irrigation on Soil Fertility

Efficient irrigation and fertilizer management have been increasingly used for crop production; moderate irrigation can achieve better soil nutrient balance, and inappropriate water supply not only affects soil biological activities and soil properties but also crop growth [71,72]. In this study, irrigation had a significant effect on soil organic matter content (p < 0.05), and had a highly significant effect on soil ammonium and nitrate nitrogen (p < 0.01). In this study, application of manure could increase ammonium and nitrate nitrogen content, but it was also related to the amount of irrigation; moderate irrigation could increase soil nutrients, but excessive irrigation could instead decrease soil nutrients. This may be due to the fact that when irrigated at 70% and 85% of field holding capacity, soil aeration is better, which is conducive to the hydrolysis and mineralization of manure and urea, but when irrigated at 100% of field holding capacity, the soil tends to be anaerobic, microbial activity is enhanced, and the nutrients in the manure are rapidly mineralized, leading to losses, and thus over-irrigation may lead to nutrient losses [73].

4.2.2. Effect of Irrigation on Lettuce Growth

Water and fertilizer are also crucial for lettuce growth. Previous studies have shown that lettuce is extremely responsive to water, and that inadequate irrigation may alter the balance of endogenous hormones (e.g., abscisic acid ABA and cytokinins) in the plant, inhibit leaf primordia differentiation, and ultimately affect crop yield [74]. There are reports proving that crop leaf area index increases with increasing irrigation [75,76]. Consistent with previous findings, the maximum values of leaf number and leaf area index of lettuce across the seedling, rosette, and heading stages all occurred under 100% field water holding capacity irrigation. This observation can be attributed to adequate soil moisture facilitating nitrogen transport and increasing nutrient availability in the rhizosphere, thereby significantly enhancing photosynthetic efficiency. Improved rates of CO2 assimilation likely supplied more carbohydrates to support leaf expansion and development, ultimately contributing to greater leaf growth [77]. Moreover, when irrigated at 100% of the field water holding capacity, the manure and urea pairing could increase the number of lettuce leaves and leaf area index compared to urea alone, probably because increased irrigation could increase the soil water content, accelerate the mineralization of manure, alleviate the problem of delayed nutrient supply, and provide sufficient nitrogen for crop growth [49]. Therefore, under the condition of sufficient irrigation, manure with urea can increase the number of leaves and leaf area index of lettuce. Within a certain range, crop fresh weight (yield) tends to increase significantly with increasing irrigation; however, crop yield decreases when the irrigation volume exceeds a threshold [78], which is similar to the results of the present study. In this study, irrigation had highly significant effect on lettuce fresh weight (p < 0.01); the maximum value of lettuce fresh weight basically appeared in the treatment group of W2, although the maximum value of F4 and F5 lettuce fresh weight appeared in the treatment group of W3, but there was no significant difference between the treatments of W3 and W2, so that excessive irrigation leads to the reduction of lettuce fresh weight, which is in agreement with the study of Abd-Elrahman et al. [56], which states that the productivity of lettuce can be improved by irrigating with 80% of crop evapotranspiration rather than 100%. This may be due to the fact that over-irrigation leads to soil nutrient loss and tends to cause deep soil hypoxia and poorer aeration, possibly resulting in greatly reduced nutrient uptake [79]. It should be noted that under W2 and W3 irrigation level conditions, lettuce fresh weight decreased instead of increasing when the proportion of urea application was larger, which may be due to the reduction in the proportion of manure application, which led to the destruction of soil aggregates, the decrease of soil fertilizer retention capacity, and the more serious loss of nutrients by leaching, which affected the growth and development of the crop [80]. Therefore, under appropriate irrigation conditions, manure and urea dosing can increase the fresh weight of lettuce, but over-irrigation will reduce the fresh weight of lettuce.

4.2.3. Effect of Irrigation on Lettuce Quality

In this study, the lowest nitrate content under different manure application rates occurred under different irrigation treatments. When manure application was higher (F1 and F2 treatments), the minimal nitrate content in lettuce was observed under the low irrigation treatment (W1). This may be attributed to the dominant use of manure as the nitrogen source, whose nitrogen conversion requires sufficient moisture. Lower soil moisture not only slowed down nitrogen mineralization from manure but also restricted crop nitrogen uptake, resulting in lower nitrate accumulation in plants under high manure and low irrigation conditions [49]. In contrast, under the F3 fertilization treatment, the lowest nitrate content occurred in the W2 irrigation group, where lettuce also reached its highest fresh weight. This indicates that the W2F3 treatment can simultaneously enhance lettuce growth and reduce nitrate content, demonstrating considerable agronomic and environmental benefits for lettuce production. Conversely, when urea application was higher (F4 and F5), the minimal nitrate content was generally found under the high irrigation treatment (W3). A possible explanation is that under ample irrigation, water-driven leaching led to substantial loss of urea-derived nitrogen, reducing its availability for crop uptake and resulting in lower nitrate accumulation in lettuce [11].
In the present study, irrigation had highly significant effect on the soluble sugar content of lettuce (p < 0.01). In the case of organic fertilizer alone, soluble sugar showed an increasing trend with increasing irrigation water, while in the case of organic–inorganic combination, soluble sugar content showed a decreasing trend with increasing irrigation water. This may be because the single application of organic manure releases nutrients slowly. Higher irrigation levels can promote the mineralization of this manure, facilitating crop nitrogen uptake. This helps maintain a carbon-nitrogen balance within the plant and promotes the formation of photosynthetic products. In contrast, combinations with inorganic fertilizer release a large amount of nitrogen rapidly due to the inorganic component. This leads to an imbalance in the plant’s carbon-to-nitrogen ratio, thereby reducing the conversion of photosynthetic products into sugars [81]. This is in agreement with Xiao et al. who showed that maize improves drought tolerance under water stress by regulating sucrose metabolism and increasing soluble sugar content [82]. Although the application of manure can increase the soluble sugar content of the crop, it is also necessary to control the amount of irrigation, which, in turn, can reduce the soluble sugar content of the crop if it is not properly irrigated.
Irrigation exerted a highly significant effect on the soluble protein content of lettuce (p < 0.01). The maximum soluble protein content under both the manure-only (F1) and urea-only (F5) treatments occurred at 85% of field water holding capacity. This irrigation level likely facilitated the mineralization of manure while minimizing nutrient loss from urea, thereby supplying sufficient nitrogen for protein synthesis in lettuce [49]. In contrast, treatments involving combined manure and urea applications reached peak soluble protein content at 70% field water holding capacity. A possible explanation is that the lower irrigation rate avoided accelerated manure mineralization while still allowing urea to supply adequate nitrogen during early growth stages. As the crop developed, gradual nitrogen release from manure provided a sustained nitrogen source, enabling continuous protein synthesis throughout the growth cycle.
Irrigation also significantly influenced chlorophyll content (p < 0.01). Under the same fertilization treatment, the highest chlorophyll levels were generally observed in the W1 and W2 irrigation groups. This pattern may be attributed to waterlogging conditions under higher irrigation levels, which can result in soil hypoxia, triggering anaerobic respiration in plants. The subsequent accumulation of lactic acid and ethanol may disrupt reactive oxygen species metabolism and accelerate chlorophyll degradation [83].

4.3. Coupling Effects of Fertilization and Irrigation

Many researchers have established the relationship between water and fertilizer inputs and crop yield and quality through a combination of multiple regression and spatial analysis [76,84]. This study determined the relationship between water and fertilizer inputs and soil nutrients, lettuce growth, and quality by considering a 90% confidence interval. The conclusion was that when irrigation reached 75–78% of field capacity and the ratio of manure to urea nitrogen application was 7:3 to 1:1, soil nutrients and lettuce fresh weight both reached 90% of their maximum values.

5. Conclusions

This study systematically evaluated the comprehensive effects of chicken manure organic fertilizer and urea application at different irrigation levels on soil nutrients, lettuce growth, and quality through water–fertilizer coupling experiments. The main conclusions are as follows:
(1) The synergistic water–fertilizer coupling effect of co-applying chicken manure organic fertilizer and urea not only mitigates agricultural non-point-source pollution and enhances the resource utilization rate of chicken manure but also significantly improves soil fertility and reduces nutrient loss. The combined application of chicken manure organic fertilizer and moderate irrigation significantly increased soil organic matter accumulation. When irrigation reached 85% of field capacity, soil organic matter content increased by 36% compared to the separate application of chicken manure organic fertilizer or urea alone. The combination of compost fertilizer application and moderate irrigation improved nitrogen use efficiency. When irrigation volume reached 75% of field capacity, and the nitrogen ratio of compost to urea nitrogen was 1:1 or 3:7, soil ammonium nitrogen and nitrate nitrogen contents increased by 102% and 141%, respectively. The slow-release characteristics of manure fertilizer enhance soil nitrogen supply capacity while reducing nitrogen leaching and gas losses.
(2) The synergistic water–fertilizer coupling effect of chicken manure organic fertilizer and urea application can optimize lettuce growth and quality. When irrigation volume was 85–100% and the ratio of manure to urea nitrogen was 7:3–3:7, the lettuce leaf number reached an optimal level. When irrigation volume was 70–100% and manure was applied alone or the ratio of manure to urea nitrogen was 7:3, the lettuce leaf area index significantly increased. When irrigation volume is 85% of field capacity and the ratio of manure to urea nitrogen was 7:3 or 1:1, lettuce aboveground fresh weight reached its peak. When manure was applied alone (irrigation volume at 70%), lettuce nitrate content was the lowest. When the irrigation volume was 70% of field capacity and manure was applied alone, nitrate content in lettuce was the lowest. When the irrigation volume was 70–85% and manure was applied alone, soluble sugar, soluble protein, and chlorophyll content significantly increased.
(3) Optimal parameters for water–fertilizer coupling: Through optimization using least squares and multiple regression models, when irrigation volume is 75–78% of field capacity and the nitrogen ratio of manure to urea nitrogen is 7:3 to 1:1, soil nutrients and lettuce growth and quality indicators can simultaneously reach over 90% of their respective maximum values, achieving a balance between high yields, quality, and environmental sustainability.

Author Contributions

Conceptualization, X.M. and X.S.; methodology, K.Q.; software, K.Q.; validation, X.T., R.Z., and J.B.; formal analysis, K.Q.; resources, J.M.; data curation, P.T., J.L., and D.P.; writing—original draft preparation, K.Q.; writing—review and editing, K.Q.; visualization, K.Q. and X.M.; All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by Nanjing Agricultural Machinery Extension Center funded by the 2024 Modern Agricultural Machinery Equipment and Technology Promotion Project of Jiangsu Province (Grant No. NJ2024-32).

Data Availability Statement

Data are contained within the article.

Acknowledgments

We thank all authors for their contributions.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Location of the experimental site.
Figure 1. Location of the experimental site.
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Figure 2. The effects of irrigation and fertilization on soil nutrients. Note: Different uppercase letters indicate significant differences (p < 0.05) among irrigation treatments under the same fertilization condition, and different lowercase letters indicate significant differences (p < 0.05) among fertilization treatments under the same irrigation condition, as determined by Duncan’s test.
Figure 2. The effects of irrigation and fertilization on soil nutrients. Note: Different uppercase letters indicate significant differences (p < 0.05) among irrigation treatments under the same fertilization condition, and different lowercase letters indicate significant differences (p < 0.05) among fertilization treatments under the same irrigation condition, as determined by Duncan’s test.
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Figure 3. Effects of water–fertilizer coupling on leaf number and leaf area index of lettuce at (A) seedling, (B) rosette, and (C) heading stages. Note: Different uppercase letters indicate significant differences (p < 0.05) among irrigation treatments under the same fertilization condition, and different lowercase letters indicate significant differences (p < 0.05) among fertilization treatments under the same irrigation condition, as determined by Duncan’s test.
Figure 3. Effects of water–fertilizer coupling on leaf number and leaf area index of lettuce at (A) seedling, (B) rosette, and (C) heading stages. Note: Different uppercase letters indicate significant differences (p < 0.05) among irrigation treatments under the same fertilization condition, and different lowercase letters indicate significant differences (p < 0.05) among fertilization treatments under the same irrigation condition, as determined by Duncan’s test.
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Figure 4. The effect of water and fertilizer coupling on the fresh weight of lettuce. Note: Different uppercase letters indicate significant differences (p < 0.05) among irrigation treatments under the same fertilization condition, and different lowercase letters indicate significant differences (p < 0.05) among fertilization treatments under the same irrigation condition, as determined by Duncan’s test.
Figure 4. The effect of water and fertilizer coupling on the fresh weight of lettuce. Note: Different uppercase letters indicate significant differences (p < 0.05) among irrigation treatments under the same fertilization condition, and different lowercase letters indicate significant differences (p < 0.05) among fertilization treatments under the same irrigation condition, as determined by Duncan’s test.
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Figure 5. The effect of water and fertilizer coupling on lettuce quality. Note: Different uppercase letters indicate significant differences (p < 0.05) among irrigation treatments under the same fertilization condition, and different lowercase letters indicate significant differences (p < 0.05) among fertilization treatments under the same irrigation condition, as determined by Duncan’s test.
Figure 5. The effect of water and fertilizer coupling on lettuce quality. Note: Different uppercase letters indicate significant differences (p < 0.05) among irrigation treatments under the same fertilization condition, and different lowercase letters indicate significant differences (p < 0.05) among fertilization treatments under the same irrigation condition, as determined by Duncan’s test.
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Figure 6. The effect of water–fertilizer coupling on soil nutrients, lettuce growth, and quality. The colored sections represent 90% of the maximum values.
Figure 6. The effect of water–fertilizer coupling on soil nutrients, lettuce growth, and quality. The colored sections represent 90% of the maximum values.
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Table 1. Soil physical and chemical properties.
Table 1. Soil physical and chemical properties.
pHSOM (%)NH4+-N (mg/kg)NO3-N (mg/kg)TP (mg/kg)Ca2+
(mg/kg)		Mg2+ (mg/kg)
5.910.732238.910.36585343
Note: SOM represents soil organic matter; TP represents total phosphorus in soil.
Table 2. Nutrient content of organic manure from chicken manure.
Table 2. Nutrient content of organic manure from chicken manure.
IndicatorspHMoisture
(%)		SOM
(g/kg)		TN
(g/kg)		TP
(g/kg)		TK
(g/kg)		Effective Number of Live Bacteria
(Billion/g)
Content6.429.8778.962.413.303.530.56
Note: SOM represents soil organic matter; TP represents total phosphorus in soil.
Table 3. Water and fertilizer patterns in each plot of the test site.
Table 3. Water and fertilizer patterns in each plot of the test site.
NumberTreatmentsIrrigationFertilization (kg/ha)
ManureUrea
1W1F170%42,9460
2W1F230,06267.5
3W1F321,473112.5
4W1F412,833157.5
5W1F50225
6W2F185%42,9460
7W2F230,06267.5
8W2F321,473112.5
9W2F412,833157.5
10W2F50225
11W3F1100%42,9460
12W3F230,06267.5
13W3F321,473112.5
14W3F412,833157.5
15W3F50225
Table 4. Regression equations between irrigation and fertilization and soil nutrients, lettuce growth, and quality. Note: x and y represent the amounts of manure and irrigation, respectively.
Table 4. Regression equations between irrigation and fertilization and soil nutrients, lettuce growth, and quality. Note: x and y represent the amounts of manure and irrigation, respectively.
Response VariableRegression EquationR2RMSERMSECV
SOM/Y1 Y 1 = − 3.04082 + 0.0000175006 x + 7.95755 × 10 − 13 x 2 + 0.0874857 y − 1.52051 × 10 − 7 x y − 0.000488889 y 2 0.3720.1090.117
NH4+-N/Y2 Y 2 = 12.5037 + 0.000690017 x − 1.90771 × 10 − 8 x 2 + 0.412142 y + 9.28665 × 10 − 7 x y − 0.00374815 y 2 0.5234.324.68
NO3−-N/Y3 Y 3 = 39.8522 + 0.00219661 x − 3.98774 × 10 − 8 x 2 − 0.0849545 y − 0.00000892643 x y − 0.000407407 y 2 0.6577.197.67
Fresh weight/Y4 Y 4 = − 639.947 + 0.00276832 x − 2.20857 × 10 − 8 x 2 + 21.7300 y − 0.0000194618 x y − 0.123881 y 2 0.36418.119.1
Nitrate content/Y5 Y 5 = 197.657 − 0.00136994 x − 5.86106 × 10 − 8 x 2 + 1.43363 y + 0.0000379245 x y − 0.0130030 y 2 0.22426.928.0
Soluble sugar/Y6 Y 6 = 50.3601 − 0.000217948 x + 1.94311 × 10 − 9 x 2 − 0.726818 y + 0.00000305614 x y + 0.00334370 y 2 0.5102.152.31
Soluble protein/Y7 Y 7 = 25.2353 − 0.000167690 x + 1.05549 × 10 − 9 x 2 − 0.0978119 y + 0.00000222794 x y − 0.000113185 y 2 0.2911.922.09
Chlorophyll/Y8 Y 8 = − 0.230364 − 0.00000313192 x + 5.01652 × 10 − 11 x 2 + 0.00792233 y + 1.84107 × 10 − 8 x y − 0.0000490074 y 2 0.5080.01390.0151
Table 5. Maximum values of soil nutrients, lettuce growth, and quality corresponding to irrigation and fertilization.
Table 5. Maximum values of soil nutrients, lettuce growth, and quality corresponding to irrigation and fertilization.
Response VariableThe Optimal Value of the Response VariableIrrigation Amount (%)Manure
(kg/ha)
SOM (%)1.0682.843,000
NH4+-N (mg/kg)30.470.019,788
NO3−-N (mg/kg)7070.019,707
Fresh weight (g)32685.824,891
Nitrate content (mg/kg)18170.043,000
Soluble sugar (mg/g)19.370.042,946
Soluble protein (mg/g)19.370.042,946
Chlorophyll (mg/g)0.11588.842,946
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Qi, K.; Tang, X.; Ma, J.; Zhao, R.; Bao, J.; Tang, P.; Liu, J.; Pei, D.; Shao, X.; Mao, X. Efficient Regulation and Prediction Model Construction for Water and Fertilizer Management Through Resource Utilization of Manure and Urea Co-Application. Water 2025, 17, 2594. https://doi.org/10.3390/w17172594

AMA Style

Qi K, Tang X, Ma J, Zhao R, Bao J, Tang P, Liu J, Pei D, Shao X, Mao X. Efficient Regulation and Prediction Model Construction for Water and Fertilizer Management Through Resource Utilization of Manure and Urea Co-Application. Water. 2025; 17(17):2594. https://doi.org/10.3390/w17172594

Chicago/Turabian Style

Qi, Kaiqi, Xiaofeng Tang, Jianhong Ma, Rui Zhao, Junan Bao, Pengshan Tang, Jiaqi Liu, Dandan Pei, Xiaohou Shao, and Xinyu Mao. 2025. "Efficient Regulation and Prediction Model Construction for Water and Fertilizer Management Through Resource Utilization of Manure and Urea Co-Application" Water 17, no. 17: 2594. https://doi.org/10.3390/w17172594

APA Style

Qi, K., Tang, X., Ma, J., Zhao, R., Bao, J., Tang, P., Liu, J., Pei, D., Shao, X., & Mao, X. (2025). Efficient Regulation and Prediction Model Construction for Water and Fertilizer Management Through Resource Utilization of Manure and Urea Co-Application. Water, 17(17), 2594. https://doi.org/10.3390/w17172594

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