The Influence of Microalgae Fertilizer on Soil Water Conservation and Soil Improvement: Yield and Quality of Potted Tomatoes

Abstract

We aim to study the impact of microalgae fertilizer on soil nutrients, water conservation and crop yield and quality while also determining the optimal ratio of microalgae fertilizer to chemical fertilizer. Using “Xinoufen No.9” tomatoes as the test subject, we conducted pot experiments with four different treatments: control with 100% chemical fertilizer (CK), T1 (25% microalgae fertilizer + 75% regular chemical fertilizer), T2 (75% microalgae fertilizer + 25% regular chemical fertilizer) and T3 (100% microalgae fertilizer). The results show that an increased application of microalgae fertilizer enhanced the soil organic matter, ammonium nitrogen, available phosphorus and potassium content. T3 showed the most improvement followed by T2. The co-application of microalgae fertilizer with chemical fertilizer can significantly increase the stem girth, plant height and yield of tomatoes. At the same time, microalgae fertilizer effectively regulates leaf stomatal conductance, promoting tomato leaf respiration. As the stomatal conductance increases, the transpiration rate and net photosynthesis rate of all treatments improve, followed by a decline in intercellular CO₂ concentration, with T2 exhibiting the best performance. Among all treatments, T2 treatment yielded the highest per-plant production (0.630 kg), followed by T3 (0.521 kg). This is because the microalgae fertilizer promotes the distribution of photosynthetic products to the fruit, enhancing the yield and quality of tomatoes. Additionally, the microalgae fertilizer also increases the content of soluble sugars, soluble protein, vitamin C and lycopene in the fruit while reducing the nitrate content. Compared to the control group CK, T2 increases the content of soluble sugars, vitamins and lycopene by 26.74%, 39.29% and 158.31%, respectively. Microalgae fertilizer also helps to improve soil water and thermal conditions, enhancing the water-use efficiency of tomatoes. Compared to CK, the water-use efficiency of T2 treatment increased by 54.05%. Correlation analysis indicates that water and fertilizer factors significantly affect tomato yield, with a correlation exceeding 70%. The net photosynthesis and transpiration rates significantly influence fruit quality, with correlations above 80%. By applying microalgae fertilizer, the efficiency of water and fertilizer use can be effectively improved, thus achieving the goal of water conservation and quality enhancement. Therefore, through comprehensive analysis, using the membership function method of indicators such as soil environment, crop yield, fruit quality and water-use efficiency, it is concluded that T2 is the optimal fertilization treatment. This study provides theoretical support for the application of microalgae biofertilizer technology in the cultivation of tomatoes and other vegetables in the northern, cold and arid regions.

1. Introduction

Fertilizers are instrumental in securing enhanced crop yields and financial returns. However, the long-term and excessive deployment of chemical fertilizers not only undermines their use efficiency but also disrupts soil structure, leading to phenomena such as compaction, salinization, ecological disequilibrium and agricultural pollution. These factors result in a decline in crop production and quality [1,2]. Moreover, the overuse of fertilizers can result in the depletion of soil nutrient content, disturbance of the soil microbial community structure and interference with the soil nutrient cycling process. As a result, it could diminish the soil’s carbon sequestration capacity and exert adverse effects on crop growth and the soil environment [3,4,5]. Microalgae thrive rapidly in controlled cultivation settings, exhibiting formidable carbon fixation abilities [6]. During their growth and propagation, microalgae excrete amino acids, peptides and other nitrogenous compounds, which convert atmospheric nitrogen into nitrogenous organic compounds and augment the concentration of effective nitrogen within the soil [7]. Therefore, microalgae are extensively studied for the development of biofertilizers.

Microalgae fertilizers are capable of enhancing soil microbial structures either by increasing soil organic matter or through indirect action on plants. The symbiotic relationship between microalgae, soil and micro-organisms can furnish crops with an enriched nutrient supply, mitigating nutrient loss through the gradual release of nitrogen, phosphorus and potassium [8]. Enriched with an array of nutrients and trace elements, microalgae fertilizers not only bolster soil fertility but also stimulate crop growth and facilitate the repair of compromised soils [7,9,10]. Since India’s initial report on the use of nitrogen-fixing cyanobacteria as fertilizers in 1939, numerous studies have investigated their efficacy in enhancing agricultural production and nitrogen fixation [11]. Li Shanghao demonstrated that inoculation with cyanobacteria could lead to a 15% increase in the average yield with a maximum of up to 33% [12]. Despite the existence of over 30,000 distinct microalgae species globally, domestic efforts are presently concentrated on identifying strains characterized by high nutritional value, rapid growth rates and robust adaptability, aiming to lay a foundational material basis for the development of microalgae fertilizers [13]. Previous investigations have provided theoretical guidance for the application of microalgae fertilizers. However, environmental factors such as temperature, illumination, pH levels, humidity and salinity could exert significant influence on the growth and proliferation of microalgae. The methodologies and quantities of microalgae fertilizer applications must be customized according to crop and soil conditions. This research delved into the mechanisms through which microalgae fertilizers ameliorate soil physicochemical properties via pot planting trials, assessing their impact on crop growth indicators, yield quality and soil health. It also investigated optimal microalgae fertilizer application rates and techniques, thereby offering theoretical support for the use of microalgae fertilizers in soil quality enhancement, crop yield augmentation and water-saving practices.

2. Materials and Methods

2.1. Overview of the Experimental Site

The experiment was conducted at the Practical Teaching Base of Inner Mongolia Agricultural University, located in Hohhot, Inner Mongolia Autonomous Region (111°42′3″ E, 40°48′19″ N). Before transplantation, samples from the plow layer of the local soil (0–20 cm) were collected for basic physical and chemical property analysis. The soil texture was determined to be sandy loam, with a field water holding capacity of 31%, bulk density of 1.39 g/cm³, pH value of 7.85, organic matter content of 6.66 g/kg, ammonium nitrogen content of 4.6 mg/kg, available phosphorus content of 12.06 mg/kg and available potassium content of 146.98 mg/kg. Based on the comprehensive analysis, the soil fertility level was classified as Level 3.

2.2. Experimental Material

The experiment used the tomato variety “Xinoufen No. 9” as the test material. The tested fertilizer was a compound fertilizer with a N:P₂O₅:K₂O ratio of 15:15:15. The tested microalgae fertilizer was an Algol microalgae active cell biofertilizer with 1 million cells per mL, containing three types of microalgae: Chlorella pyrenoidosa (green algae), Anabaena azotica (cyanobacteria) and Tolypothrix tenuis (cyanobacteria). Each type of algae has a distinct function. Specifically, Chlorella pyrenoidosa accounts for 40% of the total algae content and primarily performs photosynthesis. Anabaena azotica and Tolypothrix tenuis make up 40% and 20% of the total algae content, respectively, and their main function is biological nitrogen fixation (as shown in Figure 1). The product was manufactured and provided by Inner Mongolia Algol Life Science Co., Ltd. in its workshop located in Siziwang Banner, China.

2.3. Experimental Design

The PVC square pots (35 cm in length and 15 cm in height) were used for the experiments, each filled with 16 kg of soil and planted with one tomato plant. Each treatment consisted of six plants, with four treatments in total (

Table 1. Test treatment and fertilizer amount.

Treatment	Treatment Mode	Fertilizing Amount
CK	CF100%	CF: 1.8 g per pot
T1	MF25% + CF75%	MF: 25 mL per pot + CF: 1.35 g per pot
T2	MF75% + CF25%	MF: 75 mL per pot + CF: 0.45 g per pot
T3	MF100%	MF: 100 mL per pot

): 100% chemical fertilizer (CK), 25% microalgae fertilizer + 75% chemical fertilizer (T1), 75% microalgae fertilizer + 25% chemical fertilizer (T2) and 100% microalgae fertilizer (T3). Each experiment was repeated three times. The tomato seedlings were transplanted on 18 May. The water and fertilizer management regime and timing were consistent across all treatments. Irrigation was controlled based on the soil moisture limits, with the upper limit being the field water holding capacity and the lower limit being the moisture content in the 0–15 cm depth of the soil layer within the pot. Irrigation was applied to raise the moisture content from the lower limit to the upper limit as needed. The irrigation quota was calculated using the formula below.

$$I\left(\mathrm{m}\mathrm{m}\right)=({\rho }_0-\rho )Ah$$

(1)

where I represent the irrigation quota (mm), ρ0 represents the field water-holding capacity expressed as volumetric water content (%), ρ represents the lower limit of volumetric water content (%), A represents the surface area of the flowerpot (m²) and h represents the soil depth (m).

For each treatment, fertilization is divided into three stages. After the application of the basal fertilizer, the first fertilization is conducted during the tomato fruit set. The second fertilization takes place 20 days later, followed by the third fertilization after an additional 14 days. The amount of fertilization per plant, inter-row weeding and pest control are consistent across all treatments. All management measures are completed within 1 day.

2.4. Measurement Items and Methods

2.4.1. Measurement Methods

At each growth stage, soil samples were collected from the midpoint between each tomato plant in the pots, with a sampling depth of 10 cm. The samples were stored at 4 °C for the determination of ammonium nitrogen, available phosphorus, available potassium, organic matter content and pH value. The soil organic matter content was determined using the potassium dichromate volumetric method and ammonium nitrogen content was determined using the indophenol blue colorimetric method. The available phosphorus content was determined using the molybdenum–antimony colorimetric method and the available potassium content was determined using the flame photometric method. The soil pH was measured with a Leici PHS-3C pH meter which is produced by Shanghai Laichi Instrument Co., Ltd. in Shanghai, China.

2.4.2. Determination of Soil Moisture

During each growth period, soil was sampled from the midpoint between each tomato plant in the pots, at a depth of 10 cm. The soil moisture content was measured using a WET-2-KIT device which is manufactured by Beijing Yingchi Technology Co., Ltd. in Beijing, China.

2.4.3. Measurement of Tomato Growth Indicators

Three tomato plants were randomly selected at each growth stage to measure plant height and stem thickness.

2.4.4. Measurement of Tomato Yield and Quality Indicators

To measure the tomato yield, the single-harvest method was used. Fruits were weighed and recorded at each harvest until all fruits from the selected plants had been harvested and the average yield was calculated. The content of soluble sugars was determined using Fehling’s solution titration method. Vitamin C content was determined using the 2,6-dichlorophenolindophenol titration method and lycopene content was determined using the toluene extraction-colorimetric method. Soluble protein content was determined using the Coomassie Brilliant Blue G-250 method and nitrate content was determined using the salicylic acid method.

2.4.5. Water-Use Efficiency

Water-use efficiency was calculated using the water balance method, with the specific formula as follows:

$$\mathit{E}\mathit{T}\left(\mathbf{m}\mathbf{m}\right)=\mathit{P}+\mathit{I}+∆\mathit{W}-\mathit{R}-\mathit{D}$$

(2)

$$\mathit{W}\mathit{U}\mathit{E}\left(\mathbf{k}\mathbf{g}/{\mathbf{m}}^{\mathbf{3}}\right)={\displaystyle \frac{\mathit{Y}}{\mathit{E}\mathit{T}}}$$

(3)

where ET represents the water consumption by tomato plants (mm), P represents the precipitation (mm), I represents the irrigation quota (mm), ΔW represents the change in soil moisture content (mm), R represents the surface runoff (mm), D represents the pot percolation amount, WUE represents the water-use efficiency of the soil (kg/m³) and Y represents the yield of tomatoes (kg).

Given that this experiment utilizes pot experiments under controlled rainfall conditions, both rainfall and surface runoff can be disregarded, thus R = 0 and P = 0. Consequently, the formula can be simplified as follows:

$$ET\left(\mathrm{m}\mathrm{m}\right)=I+∆W-D$$

(4)

2.4.6. Measurement of Tomato Photosynthetic Indicators

During the peak growth period of tomatoes, the net photosynthetic rate (Pn), transpiration rate (Evap), intercellular CO₂ concentration (Ci) and stomatal conductance (Gs) of plants under different treatments were measured using an LCpro T photosynthesis system produced by ADC of the UK. Measurements were taken from 9:00 to 11:00 on sunny days, focusing on the fourth leaf below the growth point.

2.5. Data Processing and Analysis

The fuzzy evaluation method [14] was employed to analyze the various measured indicators of tomatoes in this experiment. The formula for calculating the membership function value is as follows:

$$R\left(X_I\right)=(X_I-X_{min})/(X_{max}-X_{min})$$

(5)

If the indicator is negatively correlated with the evaluation effect, the membership function is transformed accordingly. The formula for this transformation is as follows:

$$R\left(X_I\right)=1-(X_I-X_{min})/(X_{max}-X_{min})$$

(6)

where R represents the index for comprehensive evaluation, Xi denotes the measured value of each treatment index, X_max represents the maximum value among all measured data and X_min represents the minimum value among all measured data. After calculating the membership function values for each index, these are summed and averaged to calculate the mean membership function value (average degree of membership) for a comprehensive evaluation. A higher average value indicates a better overall effect.

The measured data are processed using Microsoft Excel 2016. Statistical analyses including one-way ANOVA, Pearson correlation analysis, principal component analysis and membership function analysis are conducted using SPSS 27.0 software. Graphs are generated using Origin 2022.

3. Results and Analysis

3.1. Impact of Microalgae Fertilizer on Soil Physicochemical Properties

3.1.1. Soil Nutrients

Figure 2 presents the changes in soil nutrient indicators during various growth stages under different treatments. It is evident that from the seedling to the flowering stage, organic matter in the T1, T2 and T3 treatments significantly increased by 71.5%, 108.4% and 183.1%, respectively, compared to the CK treatment. The soil ammonium nitrogen content also significantly increased, following the increasing order as T2, T3 and T1, with a 90.8%, 79.3% and 15.4% increase, respectively, compared to CK. Following the application of microalgae fertilizer, the contents of available phosphorus and potassium were significantly higher than those in the control and positively correlated with the amount of microalgae fertilizer applied. The treatments (T1, T2 and T3) showed an increase in the available potassium and phosphorus by 71.3% and 73.2%, 124.9% and 185.1%, and 167.2% and 232.8%, respectively. From the flowering to the fruiting peak period, compared to CK, the T2 treatment showed the highest increase in organic matter and ammonium nitrogen, by 376.6% and 78.1%, respectively. However, the contents of available potassium and phosphorus in the soil increased with the increase in microalgae fertilizer, demonstrating the highest increase under the T3 treatment (263.7% and 314.9%). The changes in the physicochemical indicators from the fruiting peak to the later stages showed the same characteristics as the flowering to fruiting peak period. The highest increase in organic matter and ammonium nitrogen was witnessed under the T2 treatment. The most significant increase in the available phosphorus and potassium was observed in the T3 treatment.

Overall, compared to the CK treatment, the combined application of reduced chemical fertilizers and microalgae fertilizer significantly increased the content of soil ammonium nitrogen, available phosphorus, available potassium and organic matter. However, the impact of various treatments on physicochemical indicators differed across different growth stages. During the seedling stage, physicochemical indicators increased with the increase in algae fertilizer. During the flowering to fruiting peak period and the fruiting peak to the later stages, the increase in organic matter and ammonium nitrogen during the T2 treatment was significantly higher than in other treatments.

Table 2. Effect of microalgae fertilizer on soil pH.

Treatment	Seedling Stage to Flowering Stage	Flowering Stage to Fruitful Stage	Fruitful Stage to Later Stage
CK	8.3 ± 0.2 a	8.4 ± 0.02 b	7.7 ± 0.12 c
T1	8.1 ± 0.09 b	8.5 ± 0.02 a	7.6 ± 0.03 b
T2	8.1 ± 0.06 b	8.4 ± 0.04 b	7.5 ± 0.08 a
T3	8.1 ± 0.05 b	8.3 ± 0.03 b	7.5 ± 0.06 a

Note: lowercase letters (a,b,c) in the figure represent significant differences among different fertilizer treatment groups at the p < 0.05 level.

presents the changes in soil pH values during various growth stages under different treatments. Throughout the entire growing period, the soil pH first increased and then decreased. Compared to the CK treatment, the application of microalgae fertilizer during the seedling to flowering stage resulted in a decline (3%) in soil pH values for all treatments. During the flowering to fruiting peak period, soil pH values slightly increased during all three treatments compared to CK. From the fruiting peak to later stages, the soil pH declined with the T3 treatment, exhibiting the most significant decrease, which was about 2.6% lower than CK.

3.1.2. Soil Moisture Content

Figure 3 displays the changes in soil moisture content during the crop growth period under different treatments. Throughout the growth period, the soil moisture content initially increased and then decreased. Compared to the CK treatment, microalgae fertilizer significantly increased soil moisture content, which was 20%, 67% and 75% for T1, T2 and T3 treatments, respectively. The moisture content increased with the increase in algae fertilizer, particularly for the T3 treatment, where the rate of increase was fast and the rate of decrease was slow, indicating a significant water retention effect. This suggests that algae fertilizer can effectively maintain soil moisture content, thereby significantly promoting water conservation.

3.2. Impact of Microalgae Fertilizer on Tomato Plant Height and Stem Thickness

Figure 4 illustrates the changes in tomato plant height and stem thickness during the transplantation period under different treatments. Throughout the tomato growth period, the plant height and stem thickness gradually increased with the extension of transplantation days. At the beginning of tomato transplantation, there were no significant differences in plant height and stem thickness. However, 30 days after transplantation, both plant height and stem thickness started to increase significantly. Compared to the CK treatment, the T1, T2 and T3 treatments all showed plant height and stem thickness improvements, with the T1 treatment exhibiting the most significant increase (15.3% and 29.3%). This indicates that the combined application of algae fertilizer and chemical fertilizer can significantly promote crop growth.

3.3. Effect of Microalgae Fertilizer on Tomato Leaf Photosynthetic Parameters

The application of microalgae fertilizer significantly improved the photosynthetic capacity of tomato leaves. Figure 5 shows the changes in photosynthetic parameters of crops under different treatments. It is evident that all photosynthetic parameters under T1, T2 and T3 treatments were significantly different from those under CK treatment. With the increase in stomatal conductance, the intercellular CO₂ concentration continuously increased under all treatments. The least increase was observed for T2 treatment compared to CK. The transpiration rate and net photosynthesis rate continued to improve under all treatments, with T2 treatment achieving the highest. With the increase in microalgae fertilizer application, a good nonlinear relationship was established among leaf stomatal conductance, intercellular CO₂ concentration, transpiration rate and net photosynthesis rate. The correlation coefficients were all above 0.89.

3.4. Impact of Microalgae Fertilizer on Tomato Yield and Water-Use Efficiency

Table 3. Effects of microalgae fertilizer on yield and water-use efficiency of tomato.

Treatment	Water Consumption/mm	Individual Plant Yield/kg	Water-Use Efficiency/kg·m3
CK	324 ± 9.07 a	0.485 ± 0.005 c	12.21 ± 0.31 c
T1	295 ± 6.56 b	0.477 ± 0.007 c	13.21 ± 0.46 b
T2	273 ± 6.02 c	0.630 ± 0.011 a	18.81 ± 0.41 ab
T3	266 ± 6.11 c	0.521 ± 0.008 b	15.98 ± 0.24 a

Note: lowercase letters (a,b,c) in the figure represent significant differences among different fertilizer treatment groups at the p < 0.05 level.

presents the effect of microalgae fertilizer on tomato yield and water-use efficiency. Microalgae fertilizer significantly affected tomato water consumption, yield and water-use efficiency. The CK treatment showed the highest and lowest water-use efficiency. The comprehensive analysis showed that water consumption under treatments from lowest to highest followed T3 < T2 < T1 < CK, yield from highest to lowest was T2 > T3 > CK > T1 and water-use efficiency from highest to lowest was T2 > T3 > T1 > CK. The T2 treatment had the highest yield per plant and water-use efficiency (18.81 kg/m³), which demonstrated a 54.05% increase compared to the CK treatment. The T3 treatment had a slightly lower yield per plant than T2, but its water consumption was the lowest, with a 17.9% reduction compared to CK. It resulted in relatively high water-use efficiency (15.98 kg/m³), with a significant increase (30.88%) compared to CK. The overall water-use efficiency of the T1 treatment was not significantly higher (8.2% increase) than CK.

3.5. Impact of Microalgae Fertilizer on Tomato Nutritional Quality

In the quality index of tomato quality indicators, the contents of soluble sugars, soluble proteins, vitamin C, nitrates and lycopene have consistently been focal points of research, particularly lycopene, renowned for its cancer-fighting properties and its role in enhancing the immune system [15,16]. Figure 6 illustrates the variations in soluble sugars, soluble proteins, vitamin C, nitrates and lycopene in tomatoes under different treatments. Compared to the sole application of chemical fertilizers, the inclusion of microalgae fertilizer in various treatments can elevate the levels of soluble sugars, soluble proteins, vitamin C and lycopene by 19.19~26.74%, 99.99~383.33%, 21.43~39.29% and 69.49~158.31%, respectively. Moreover, microalgae fertilizers can also reduce the nitrate content in tomatoes. Comprehensive data analysis revealed that the highest increments in soluble sugars, vitamin C and lycopene content were observed under the T2 treatment while showing 26.74%, 39.29% and 158.31% increase compared to the CK treatment. The highest content of soluble proteins was recorded for the T3 treatment, marking a 383.33% increase over CK, while the nitrate content was reduced by 20.55%.

3.6. Factors Affecting Tomato Yield and Quality and Correlation Analysis

3.6.1. Correlation Analysis

Figure 7 shows the correlation between tomato yield and soil physicochemical properties. Except for pH value, tomato yield showed a significant positive correlation with soil organic matter, available phosphorus, ammonium nitrogen, available potassium and moisture content. The correlation coefficients all exceeded 0.7. The yield and organic matter correlation was the highest (0.94). The soil pH value showed a significant negative correlation with yield and other physicochemical indicators, with a correlation coefficient of 0.91. It indicated that high pH values are detrimental to crop growth and yield. However, the application of algae fertilizer can reduce soil pH value, thereby increasing crop yield.

Photosynthesis is the fundamental pathway for physiological metabolism and substance accumulation during plant growth. Figure 8 shows the correlation between tomato quality indicators and photosynthesis. Soluble sugars, soluble proteins, vitamin C and lycopene content all showed a positive correlation with net photosynthesis rate. The correlation coefficients for soluble protein and lycopene content exceeded 0.9. The crop transpiration rate also showed a positive correlation with quality indicators but to a lesser extent. Only lycopene content and transpiration rate had a correlation coefficient of 0.80. All quality indicators showed a negative correlation with intercellular CO₂ concentration.

3.6.2. Principal Component Analysis

The crop yield, soil physicochemical properties and water-use efficiency are crucial indicators for evaluating crop quality and selecting optimal fertilization treatments. Therefore, a principal component analysis (PCA) was conducted based on the nine factors affecting tomato yield and water-use efficiency: organic matter content, available phosphorus content, available potassium content, ammonium nitrogen content, soil moisture content, leaf net photosynthesis rate, transpiration rate and intercellular CO₂ concentration. The results are presented in

Table 4. Principal component analysis results.

PC	Eigenvalue	Variance Contribution Rate/%	Cumulative Variance Contribution Rate/%
1	6.461	71.785	71.785
2	2.274	25.266	97.051

.

The eigenvalues and contribution rates from the PCA serve as the basis for selecting principal components. The cumulative variance contribution rate of the first two principal components exceeded 95%, indicating that these two components essentially encapsulate all the variability information of the nine influencing factors. The variance contribution rate of the first principal component reached 71.785% while incorporating the most significant variability information. Its expression is as follows.

The expression for the first principal component is

$$Y_1=0.380x_1+0.325x_2+0.303x_3+0.278x_4+0.378x_5+0.319x_6+0.127x_7-0.302x_8-0.319x_9$$

(7)

where x₁, x₂, x₃, x₄, x₅, x₆, x₇, x₈ and x₉ represent water-use efficiency, soil organic matter content, available phosphorus content, available potassium content, ammonium nitrogen content, soil moisture content, leaf net photosynthesis rate, transpiration rate and intercellular CO₂ concentration, respectively. In PC1, x₁, x₂, x₃, x₄, x₅ and x₆ have relatively large positive coefficients, indicating that the first principal component primarily reflects how soil physicochemical properties can directly affect crop yield.

The expression for the second principal component is

$$Y_1=-0.058x_1-0.361x_2+0.223x_3+0.169x_4-0.170x_5+0.189x_6+0.381x_7+0.379x_8+0.354x_9$$

(8)

In PC2, x₇, x₈ and x₉ have larger coefficients, indicating that leaf photosynthesis is a necessary factor which could affect the yield. Based on the factor score, the principal component values for different treatments are calculated, and a comprehensive score is determined based on the principal component values and their contribution rates. The comprehensive scores from

Table 5. Principal component values and comprehensive evaluation scores for different microalgae fertilizer treatments.

Treatment	Principal Component Values		Comprehensive Score	Ranking
	1	2
CK	−2.13	−0.71	−1.71	4
T1	−0.97	0.04	−0.69	3
T2	1.87	−0.38	1.25	1
T3	1.23	1.05	1.15	2

indicate that the rankings from CK to T3 are as follows: −1.71, −0.69, 1.25 and 1.15. The impact of microalgae fertilizer on tomato yield, ranked from highest to lowest is T2 > T3 > T1 > CK, with T2 having the greatest effect on tomato yield, followed by T3.

3.6.3. Membership Function Analysis

The impact of fertilizers on plant growth is complex, resulting from the combined effects of multiple factors. This study employed the fuzzy mathematics membership function method to conduct a comprehensive evaluation of fifteen indicators related to tomato growth, quality, photosynthesis and soil physicochemical properties under different fertilization treatments.

Table 6. Membership degree and ranking results of tomato indexes and soil indexes under different treatments.

Participating Indicators	CK	T1	T2	T3
Organic matter	0.462	0.471	0.692	0.571
Ammonium nitrogen	0.412	0.359	0.303	0.051
Available phosphorus	0.153	0.029	0.096	0.079
Available potassium	0.054	0.088	0.079	0.123
Soil moisture content	0.248	0.219	0.217	0.177
Plant height	0.594	0.577	0.574	0.644
Stem diameter	0.689	0.667	0.653	0.638
Net photosynthetic rate	0.485	0.538	0.519	0.532
Transpiration rate	0.544	0.577	0.578	0.55
Intercellular CO2 concentration	0.476	0.474	0.508	0.527
Soluble protein	0.5	0.667	0.75	0.667
Soluble sugar	0.045	0.231	0.389	0.5
Vitamin C	0.5	0.5	0.5	0.5
Nitrate	0.715	0.734	0.652	0.429
Lycopene	0.144	0.396	0.288	0.805
Average membership degree	0.401	0.435	0.453	0.452
Ranking	4	3	1	2

presents the membership degrees and ranking results of tomato and soil indicators under different treatments. The membership function values for different treatments are 0.401, 0.435, 0.453 and 0.452. Following the principle that a higher membership function value indicates a stronger promoting effect on tomato growth, the ranking of the promoting effects of different microalgae fertilizer treatments from strongest to weakest is as follows: T2 > T3 > T1 > CK. It demonstrates that the T2 treatment has the strongest promoting effect on tomato growth, followed by T3. This is consistent with the conclusions drawn from the principal component analysis (Table 6).

4. Discussion

4.1. Impact of Microalgae Fertilizer on Soil Quality and Soil Moisture Content

A substantial body of research indicates that microalgae fertilizer can effectively increase the content of nitrogen, phosphorus, potassium and organic matter in the soil (as shown in Figure 2) and can also secrete trace elements (such as iron, zinc, selenium) to promote plant growth [17,18,19,20]. This is attributed to the nitrogen-fixing, carbon-fixing, sugar-secreting, and phosphorus-solubilizing functions within microalgae. Nitrogen fixation can absorb atmospheric nitrogen and enhance the nitrogen content of soil algae, carbon fixation can absorb atmospheric carbon dioxide through photosynthesis to increase soil carbon content, sugar secretion can secrete extracellular polysaccharides to improve soil aggregation and adsorb harmful heavy metals, and phosphorus solubilization can dissolve insoluble phosphorus in the soil algae and convert it into phosphorus that can be directly absorbed by plants. It is through these functions that microalgae can effectively improve soil quality [21].

Soil quality is pivotal for sustaining the biological productivity of ecosystems, safeguarding environmental quality and promoting the health of flora and fauna. Therefore, enhancing soil quality is crucial for advancing the green and sustainable development of agriculture. The application of microalgae could significantly increase soil organic matter content [22,23,24,25]. This could be attributed to the strong carbon fixation ability of microalgae, which can absorb carbon dioxide from the air through photosynthesis and sequester it in the soil, with a carbon fixation efficiency 10–50 times higher than that of terrestrial plants [6]. Microalgae cells are typically about 3–5 μm in size. They make up the largest group of primary producers in the world, accounting for more than 32% of global photosynthesis. The primary life characteristic of microalgae is that they are photoautotrophic. The basic concept of photosynthesis is the biochemical process of green algae cells using their chlorophyll and their cells acting in bright and dark light to convert carbon dioxide and water into organic matters and release oxygen. Microalgae are photoautotrophs. During the dark reaction phase, some three-carbon compounds change to form glucose, and others are recombined to form five-carbon compounds, thus making the dark reaction continue. The process of carbon fixation and organic matter transformation by chlorophyll in microalgae fertilizer is shown in Figure 9.

Wang Mingyi et al. [26] also demonstrated that inoculating barren soil with microalgae and cultivating it under appropriate light and temperature conditions could multiply the soil’s organic matter content within just 40 days. This significant increase is mainly due to the rapid and mass reproduction of microalgae under favorable conditions, which fixes carbon through photosynthesis while effectively enhancing soil organic matter content. However, the increase in soil organic matter content is not directly proportional to the amount of microalgae fertilizer applied [27]. The increase in organic matter with the full application of microalgae fertilizer (T3) was lower than with moderate microalgae applications (T1, T2), with soil organic matter content increasing by 17.3% and 33.4%, respectively. At the same time, biological nitrogen fixation is a physiological function unique to the active cells of nitrogen-fixing microalgae, which is carried out under the catalysis of nitrogenase. The heterocysts in the cell chains of nitrogen-fixing microalgae are composed of nitrogenase, and under the catalysis of nitrogenase, they can fix free nitrogen molecules from the atmosphere into nitrogenous compounds. The experiments showed a significant increase in the soil’s ammonium nitrogen content after applying algae fertilizer. The main reason is that the microalgae fertilizer contains one type of green algae and two types of blue-green algae, among which cyanobacteria in microalgae have a notable nitrogen fixation effect, second only to the symbiotic combination of leguminous plants and rhizobia [28]. The increase in available phosphorus content in the soil treated with microalgae is due to the mineralization of organic phosphorus in the soil by microalgae while releasing available phosphate ions and thus enhancing the soil’s available phosphorus content. Additionally, microalgae contain abundant bioactive substances like polysaccharides and polyphenols, which can also promote the release of potassium ions in the soil, leading to a significant increase in available potassium content [29,30]. The reason is that the cyanobacteria in microalgae form a symbiotic relationship with a higher plant. Both the symbiotic and associative or endophytic microbe systems also help mobilize phosphorous and other soil nutrients (e.g., K, Mg, Ca, Zn, Na and Mo) that plants might otherwise have difficulty in accessing at an adequate rate. Microalgae living cell biofertilizer can effectively degrade and solidify chemical residues through its adsorption and metabolism mechanism. The main components of the microalgae cell wall are polysaccharides, proteins and lipids that chelated with metal ions and adsorbed to the cell surface. Therefore, applying microalgae fertilizer increases soil elements such as nitrogen, phosphorus and potassium while potentially reducing the need for chemical fertilizer [31]. The soil pH decreased after applying algae fertilizer due to the interaction between algae fertilizer and chemical fertilizer, which generates a large amount of strong acid ion NO₃⁻, thereby lowering the soil’s pH value. The experiments showed that soil moisture content increased with the increase in microalgae fertilizer. The soil water-holding capacity is usually affected by factors like porosity connectivity, pore distribution, soil particle size, organic matter content and soil structure characteristics [28]. Microalgae fertilizer contains a large amount of organic matter, which can bind mineral particles in the soil to form stable aggregates, thereby increasing the soil porosity and water storage capacity [32,33]. The organic matter in microalgae fertilizer decomposes to release humus, which can combine with soil minerals to form a protective film, effectively reducing water evaporation and enhancing the soil’s water retention capacity. Issa et al. [32] found that the extracellular polysaccharides produced by green algae and cyanobacteria have adhesive properties, which help in the aggregation of soil particles, improve soil structure and enhance the stability of soil aggregates. Stable soil aggregates are an important factor in maintaining soil fertility. Soil aggregates provide the minimum pores needed for plant growth and root penetration into the soil. A good soil aggregate structure can increase the oxygen content in the soil and improve its water retention capacity.

Thus, microalgae fertilizers can significantly improve soil nutrient content, reduce soil pH and decrease water loss [22,23,24,25]. The impact of microalgae fertilizer on soil quality and moisture content varies with the type of microalgae, soil quality and the amount of fertilizer applied. To determine the optimal ratio of chemical and microalgae fertilizers, it is essential to consider the specific soil conditions of the region, the crops being cultivated and local fertilization practices to find the fertilization scheme that best suits the local soil and crops.

4.2. Impact of Microalgae Fertilizer on Crop Growth and Fruit Quality

Wuang et al. [34] cultivated sesame plants in soil enriched with Spirulina, which resulted in a 55.3% increase in plant height compared to the control group that did not use any fertilizer and a 71.8% increase compared to plants that used a commercial chemical fertilizer. In this study, the T1 treatment showed the best performance, with a 15.3% increase in plant height and a 29.3% increase in stem thickness compared to the CK treatment, which was consistent with other research findings. Compared to the CK treatment, the T2 treatment showed a 29.8% increase in the net photosynthesis rate and a 9.3% increase in the transpiration rate, with a 13.6% decrease in the intercellular CO₂ concentration. This is because plant photosynthesis is closely related to nitrogen supply levels and a moderate increase in nitrogen and organic fertilizers can enhance photosynthesis and delay leaf senescence [35,36]. Fertilizers can secrete a large amount of macronutrients suitable for rapid plant growth, while microalgae can secrete micronutrients that can regulate the soil. The rational combination of the two is beneficial for crop growth and the improvement of soil quality. The application of microalgae bio-organic fertilizer along with chemical fertilizers can effectively regulate leaf stomatal conductance. When the stomatal conductance is high, the evaporation rate of water within the plant also increases, leading to an increase in the transpiration rate. However, the closure of the stomata also restricts the crop’s absorption and assimilation of CO₂, which is not conducive to the increase in carbohydrates and the improvement of water-use efficiency [37]. In summary, the moderate application of chemical and microalgae fertilizers plays a crucial role in regulating the stomatal conductance, intercellular CO₂ concentration and transpiration rate while further affecting plant photosynthesis. Previous studies have only involved the co-application of chemical fertilizers and organic manure or pure biological microalgae fertilizers, and have pointed out the role and advantages of microalgae as a plant fertilizer [28]. However, previous research did not consider the situation of co-applying biological microalgae fertilizer with chemical fertilizers. Therefore, the purpose of this study is to explore the impact of co-applying biological microalgae fertilizer with chemical fertilizers on crop growth and soil physicochemical properties in order to determine the optimal ratio for their co-application.

The application of microalgae not only promotes crop growth but also improves fruit quality. The outcome of this study shows that with the increase in microalgae fertilizer application, the contents of soluble sugars, soluble proteins, vitamin C and lycopene increase while nitrate content decreases. The highest contents of soluble sugars (26.74%), vitamin C (39.29%) and lycopene (158.31%) were observed in the T2 treatment compared to CK. The highest soluble protein content was observed under the T3 treatment, with an increase of 383.33%, while nitrate content decreased by 20.55%. COPPENS et al. demonstrated that applying a microalgae fertilizer significantly increased the glucose concentration in tomato fruits by 18% compared to organic fertilizers alone and by 33% compared to inorganic fertilizers [8]. Hao Jiangwei et al. found that the nitrate content in tomatoes decreased under a rational application of nitrogen fertilizer after investigating the impact of nitrogen fertilizer on tomato quality [38]. Their report aligns with the findings of this study.

The application of active microalgae-based fertilizer has, to varying degrees, improved the quality of vegetables such as tomatoes, eggplants and onions, and there is already a substantial amount of research on this subject [38]. Zhang et al. explored the effects of different fertilization methods on the quality of Chinese cabbage using the following five different fertilization approaches: (1) NPK 15:15:15 inorganic fertilizer (IF); (2) organic fertilizer (with organic matter ≥ 45%, N ≥ 6%, OF); (3) microalgae-based biofertilizer (comprising a biomass ratio of Chlorella sp. to Anabaena sp. of 2:1, MF); (4) the combination of inorganic fertilizer and microalgae-based biofertilizer (IM); (5) and the combination of organic fertilizer and microalgae-based biofertilizer (OM). It was pointed out that Chinese cabbage fertilized solely with microalgae-based fertilizer had lower levels of protein and pigment content compared to those fertilized with conventional fertilizers. Using microalgae as the only source of fertilizer appears to be less optimal for the cultivation of Chinese cabbage than when using conventional fertilizers. It is recommended to use microalgae-based biofertilizers in conjunction with conventional fertilizers to maximize crop yield and quality. Furthermore, more research is needed to optimize the proportion of microalgae-based biofertilizers with organic and/or inorganic fertilizers [39].

4.3. Impact of Microalgae Fertilizer on Yield and Water-Use Efficiency

This study indicates that reducing chemical fertilizers and supplementing with microalgae fertilizers can significantly increase tomato plant height and stem thickness at various growth stages, promote the allocation of photosynthetic products to fruits, regulate yield components and increase tomato yield to varying degrees. In this study, the T2 treatment achieved the highest yield per plant and water-use efficiency. The yield was 0.630 kg per plant, water consumption was as low as 273 mm and water-use efficiency reached 18.81 kg/m³, which significantly increased by 54.05% compared to the CK treatment. Thus, as the amount of microalgae fertilizer increases, tomato water consumption gradually decreases while improving water-use efficiency. Similar studies by Pang Mei and Xi Huanfang indicated consistent outcomes [40,41]. This is because microalgae can directly promote plant respiration and photosynthesis, nucleic acid and chlorophyll synthesis, and nutrient absorption through the secretion of substances such as indole acetic acid, auxins, cytokinins, vitamins and extracellular polysaccharides, thereby promoting the growth and development of crops. In particular, the extracellular polysaccharides secreted by microalgae bind soil particles together, improving soil structure. This, in turn, reduces soil salinity and retains moisture [42,43,44,45]. The T3 treatment had the lowest water consumption among all treatments, which was reduced by 17.9% compared to CK and 2.6% compared to T2. Applying microalgae fertilizers can affect changes in the soil bulk density and porosity during the crop growth period, regulate soil plow layer water storage and reduce ineffective evaporation of soil capillary water, thereby decreasing crop water consumption and improving water-use efficiency. However, the yield with a full application of algae fertilizer (T3) was not as high as with reduced chemical fertilizer supplemented with microalgae fertilizer (T2) because the rational combination of chemical and microalgae fertilizers throughout the growth period can better meet the nutrient needs of crops. Chemical fertilizers can provide the nutrients required for rapid crop growth due to their high nutrient content while organic matter and microbes in microalgae fertilizers can improve soil structure and enhance soil fertility and water retention capacity while providing a better environment for crop growth.

5. Conclusions

In this paper, through a pot experiment, four different ratios of microalgae fertilizer and chemical fertilizer were set up. Through the measurement of tomato growth physiological indicators, soil physical and chemical indicators, fruit quality indicators, tomato yield and the water and fertilizer utilization efficiency, along with theoretical analysis of the above measured data, the following conclusions were drawn: The soil is the key to crop growth and fruit quality. Microalgae living cells give life back to the barrier soil. Microalgae fertilizer provides nutrients to plants in an autotrophic manner, improving the efficiency of nutrient use and reducing the dependence of crops on chemical fertilizers. The life function of microalgae absorbing CO₂ and releasing oxygen creates an environment for the aerobic micro-organism in the soil surface to produce and reproduce, and then form vigorous and fertile soil life. Acidic substances such as folic acid, nucleic acid, and fatty and rotten acid produced and transformed by algae can neutralize the alkali to reduce the pH value of the soil. Algae produce indole acetic acid, auxins, cytokinins, vitamins, extracellular polysaccharides, and other organic matter that can promote the formation of the soil aggregate structure, reduce compaction, and improve soil fertility activation.

The application of 75% microalgae fertilizer + 25% fertilizer can not only promote the nutrient absorption of tomatoes, but improve the yield, fruit quality and water and fertilizer utilization efficiency of tomatoes while also improving the soil surface environment. This paper provides theoretical support for the application of microalgae fertilizer technology in the cultivation of tomatoes and other vegetables.