Coupling Effects of Organic Fertilizer Substituting Chemical Fertilizer on Potato Yield, Quality and Soil Nitrogen Content in the Erhai Lake Basin of China

Abstract

Rational fertilization boosts crop yields and enhances nutritional value, but over-fertilization is counterproductive. Furthermore, water eutrophication caused by excessive use of nitrogen fertilizers has become a major agricultural non-point source pollution problem in the Erhai Lake Basin of China. This study took high-fertility soil as the research object and set up six treatments: no fertilization (CK), local recommended fertilization (T1), optimized chemical fertilizer (T2), organic fertilizer replacing 20% (T3), 40% (T4), 60% (T5) of chemical fertilizer with equal nitrogen. The results show that replacement of chemical nitrogen fertilizers with organic nitrogen fertilizers at an appropriate ratio can optimize soil nitrogen supply, enhance the activity of soil nitrogen cycle enzymes, thereby promoting the activity of nitrogen metabolism enzymes and nitrogen assimilation capacity in potato plants, and ultimately achieve a synergistic effect of increased yield, improved quality and higher fertilizer use efficiency. Among the treatments, the nitrate reductase (S-NR) activity in potato leaves was 36.74% and 41.66% higher under T3 than T1 and T4, respectively. For potato quality, Vitamin C (VC) content was 17.41% higher under T3 than T2; soluble protein content was 11.44%, 10.63%, and 9.44% higher under T3 than T1, T2, and T4, respectively. The replacement of chemical fertilizers with organic fertilizers mainly enhances the protein content in potato tubers by increasing soil urease (S-URE) activity and leaf relative chlorophyll content (SPAD) value. Based on the comprehensive differential combination evaluation model, considering potato metabolic absorption, yield, quality, and soil nitrogen content, the T3 treatment is the optimal fertilization method in the Erhai Lake Basin of China.

1. Introduction

The potato is one of the important food varieties in many countries around the world, ranking only after rice, corn, and wheat [1]. Dali Bai Autonomous Prefecture in Yunnan Province, China, is suitable for multi-season potato cultivation due to its unique climatic conditions (with an annual average temperature of 15.1 °C, annual rainfall of 1080 mm, annual sunshine duration of 2276.6 h, and a frost-free period of 230 days). As a result, potato cultivation has become one of the important agricultural industries in the region [2]. As the core agricultural area of Dali City, the Erhai Lake Basin has a potato planting area of 1500 hectares, accounting for more than 90% of the total potato planting area in the region, and it serves as an important economic source for local farmers [2]. However, in recent years, in the pursuit of high yields, the overuse of chemical fertilizers has given rise to a host of environmental issues. In particular, the abuse of nitrogen fertilizers has aggravated the eutrophication of Erhai Lake, threatening the safety of local drinking water [3]. In 2018, the government of Dali Prefecture implemented the “Three Bans and Four Promotions” policy, promoting the replacement of chemical fertilizers with organic fertilizers to reduce agricultural non-point source pollution. Although the application of organic fertilizers has increased the content of soil organic matter, long-term excessive input of chemical fertilizers has led to soil nutrient imbalance, reduced fertilizer use efficiency, and may affect crop yield and quality [4]. Therefore, studying how the substitution of chemical nitrogen fertilizers with organic fertilizer nitrogen influences potato nitrogen uptake, soil nitrogen cycling, as well as crop yield and quality is of great significance for optimizing nutrient management in potato cultivation in the Erhai Lake Basin and realizing sustainable agricultural development.

Nitrogen, phosphorus, and potassium are key nutrients for the growth and development of potatoes. The dynamic transformation of soil nitrogen forms directly affects the efficiency of crop nitrogen utilization [5]. The application of organic fertilizers exerts a regulatory effect on soil nitrogen supply capacity, mainly by stimulating microbial activity and reinforcing the processes of nitrogen mineralization and immobilization [6]. Nitrogen, after being absorbed by potato plants, is converted into organic compounds such as amino acids and proteins, thereby promoting the growth and development of potatoes [7]. Nitrogen metabolism is a core process in the growth and development of potatoes, involving the activities of key enzymes such as nitrate reductase (NR) and glutamine synthetase (GS) [8]. Phosphorus is involved in energy metabolism and carbohydrate transport, while potassium regulates enzyme activity and photosynthesis. Both are crucial for starch accumulation in tubers and the formation of quality [9]. Research indicates that in a rubber plantation on Hainan Island, replacing inorganic fertilizer with organic fertilizer could help preserve soil organic carbon stability, nitrogen concentration, and enzyme activity [10]. Research has found that replacing chemical fertilizers with organic fertilizers at an appropriate ratio promotes robust growth and carbon–nitrogen metabolism in barley plants [11]. Yang et al. [12] found that when organic and inorganic fertilizers were applied together, tomato quality improved, yield increased by 12.4%, and dry matter accumulation rose by 41.6%. However, due to the significant differences in climatic factors, crop species, soil conditions, and other aspects among different regions, the comprehensive effects of nitrogen sources such as organic fertilizers (used as substitutes for chemical nitrogen fertilizers) on crop nitrogen uptake and assimilation, nitrogen metabolism, yield, and soil fertility improvement exhibit distinct regional characteristics [13]. The high-fertility soils and unique climatic conditions in the Erhai Lake Basin pose distinctive challenges to potato nutrient management.

Significant progress has been made in research on the application of multi-index comprehensive evaluation methods in the agricultural field. Single evaluation models, such as Principal Component Analysis (PCA), the membership function method, Grey Relational Analysis (GRA), and the TOPSIS method, have all been widely applied in practice [14,15,16,17]. However, due to differences in the evaluation mechanisms of various models, variations in evaluation perspectives and weights, as well as interference from human factors, the final results of multi-index comprehensive evaluation often lack scientificity and consistency. To address the issue of inconsistent results from single evaluation models, scholars have begun to explore optimizing the evaluation system through algorithm integration. To enhance the scientificity of evaluation outcomes, it is advisable to employ an integrated algorithm that synthesizes results from multiple individual evaluation models [17]. However, the application of such multi-model integrated evaluation methods in agricultural production practices remains relatively limited. This is particularly true in the specific field of potato fertilization management in the Erhai Lake region of China, where relevant research is urgently needed to be strengthened. Therefore, this study aims to (1) quantify the interactive effects of different organic fertilizer substitution ratios on potato growth, yield, plant nutrient uptake and metabolism, quality, soil nitrogen content, soil enzyme activity, and the economic benefits of potatoes under no-fertilizer treatment; (2) use an overall difference combination evaluation model to determine fertilizer management modes characterized by high yield, high quality, high soil fertility, and high economic benefits. This study aims to establish a scientific basis for optimizing potato fertilization practices and to offer technical support for preventing and controlling agricultural non-point source pollution in plateau lake regions.

2. Materials and Methods

2.1. Study Site

The experiment was conducted in a field in Gusheng Village, Wanqiao Town, Dali City, Yunnan Province (100°8′39″ E, 25°48′48″ N) from January to May 2024. The altitude of the site ranges from 1974 m to 3500 m, with an annual average temperature of 15 °C and an annual average precipitation of 1065.7 mm. The rainfall is characterized by distinct dry and wet seasons. And the experimental site has paddy soil, and the main soil properties were as follows: soil pH 6.08, organic carbon 60.65 g·kg⁻¹, total nitrogen 4.92 g·kg⁻¹, total phosphorus 1.21 g·kg⁻¹, total potassium 23.16 g·kg⁻¹, available nitrogen 290.50 mg·kg⁻¹, available phosphorus 68.25 mg·kg⁻¹, available potassium 68.00 mg·kg⁻¹.

2.2. Experimental Design

The experiment included six treatments: no fertilization (CK), local recommended fertilization (T1) with 1350 kg ha⁻¹ (the ratio of N-P₂O₅-K₂O is 15-15-15), optimized chemical fertilization (T2) with 900 kg ha⁻¹ as basal fertilizer (the ratio of N-P₂O₅-K₂O is 13-15-17) + 450 kg ha⁻¹ as topdressing fertilizer (the ratio of N-P₂O₅-K₂O is 15-10-20), 20% nitrogen replacement of T2 with organic fertilizer (T3), 40% nitrogen replacement of T2 with organic fertilizer (T4), 60% nitrogen replacement of chemical fertilizer T2 with organic fertilizer (T5). Tested chemical fertilizers: urea (containing 46% N), calcium superphosphate (containing 16% P₂O₅), potassium sulfate (containing 52% K₂O). The organic fertilizer was a commercial organic produced by Yunnan Shunfeng Erhai Environmental Technology Corp., Ltd. (Dali, China). The contents of organic matter, N, P₂O₅ and K₂O were 37.2%, 1.4%, 1.7% and 2.2%, respectively. The tested crop was potato (Yunnan Potato 1418). Organic fertilizer is applied as basal fertilizer. The insufficient amounts of N, P₂O₅ and K₂O in the organic fertilizer were supplemented with chemical fertilizers to ensure consistent nutrient levels across all treatments (Table S1). All fertilization treatments, including one-third of the local recommended fertilization and others, were applied as topdressing at the tuber formation stage.

A randomized block design was adopted, with three replications for each treatment, totaling 18 plots. Each plot had 4 ridges, with a ridge length of 11.1 m, a ridge width of 0.85 m, and a plant spacing of 0.15 m, covering an area of 34.41 m². Plots were separated by 0.5 m walkways. Cultivation was performed in a single row per ridge, with a sowing density of 78,465 plants per hectare. Fertilizers for each treatment were applied to furrows and holes, and the entire ground was covered with plastic film. One ridge in each plot was reserved for yield measurement. Other field management measures were the same as conventional management methods.

2.3. Measurements and Calculations

2.3.1. Soil Sampling

Before sowing, soil samples were collected from the plow layer (0–20 cm) of the entire experimental field using the five-point sampling method. After air-drying, the basic soil fertility was determined. During the potato tuber bulking stage, rhizosphere soil samples of potatoes in each plot were collected using the five-point sampling method. After air-drying, the activities of soil urease, nitrate reductase, nitrite reductase and protease were determined. Meanwhile, fresh rhizosphere soil samples were preserved to determine the contents of soil nitrate nitrogen and ammonium nitrogen.

2.3.2. Sampling of Plants

During the tuber bulking stage of potatoes, three consecutive representative potato plants with uniform growth were randomly selected from each plot. When sampling, a hoe was used to dig up the plants with their roots intact. The soil clods attached to the roots were gently removed without damaging the roots, and the seed tubers were removed. The plants were taken back to the laboratory for cleaning. After separating the roots, stems, leaves, and tubers and weighing them, they were deactivated at 105 °C for 30 min, then dried to a constant weight at 75 °C, crushed, and the dry matter weight was calculated.

2.3.3. Soil Nutrients and Enzymes

Soil nitrate nitrogen and ammonium nitrogen were determined by the KCl exchange method. The activities of soil nitrate reductase (S-NR), soil nitrite reductase (S-NIR), soil protease (S-ACPT) and soil urease (S-URE) were determined using biochemical assay kits (Suzhou Grace Biotechnology Co., Ltd., Suzhou, China) in accordance with the instructions.

2.3.4. Leaf Area Index (LAI) and Relative Chlorophyll Content (SPAD)

Random samples of ten large, ten medium, and ten small leaves were obtained and pooled. A leaf specimen of area S1 was then punched from the stacked leaves, yielding a fresh weight of W1. The total weight of leaves per plant is counted as W, and the total leaf area of the plant is set as S. S was calculated according to the formula S/W = S1/W1, and then the leaf area index was calculated based on the plot area. In the seedling stage, tuberization stage, tuber bulking stage, and maturity stage of potatoes, the 3rd leaf vein under the top leaf was selected as the object for determination. Three plants were chosen from each plot, and the SPAD values were measured using a portable chlorophyll meter SPAD-502 (Konica Minolta, Inc., Tokyo, Japan) from 9:00 to 12:00 a.m. on sunny and windless days.

2.3.5. Plant N, P, K Accumulation Amounts

Total nitrogen was determined by the Kjeldahl method, total phosphorus by the vanadium-molybdenum yellow colorimetric method, and total potassium by flame photometry. The nutrient uptake of potato plants was calculated as follows [18]:

N (P, K) accumulation (kg·ha⁻¹) = plant biomass × N (P, K) content

(1)

2.3.6. Activities of Nitrogen Metabolism Enzymes in Potato Leaves

During the tuber bulking stage of potatoes, between 8:00 a.m. and 10:00 a.m. on sunny days, three plants with similar growth vigor were selected from each plot. The 3rd and 4th functional leaves from the bottom were taken and stored in an ice box. The activities of nitrate reductase (NR) and glutamine synthetase (GS) were determined using biochemical assay kits (Suzhou Grace Biotechnology Co., Ltd, Suzhou, China) in accordance with the instructions.

2.3.7. Soluble Protein Content in Potato Leaves

Weigh 0.2 g of fresh sample from potato functional leaves, grind it into a homogenate with distilled water, centrifuge at 3000 r·min⁻¹ for 10 min, and reserve the supernatant for use. Pipette 1.0 mL of the sample extract into a test tube, add 5 mL of Coomassie Brilliant Blue reagent, shake well, let it stand for 2 min to allow the reaction to complete, then perform colorimetry at 595 nm to determine the absorbance, and calculate the soluble protein content via a standard curve.

$$\mathrm{Soluble} \mathrm{protein} \mathrm{content} (\mathrm{mg}·{\mathrm{g}}^{-1})={\displaystyle \frac{\mathrm{C} \times \mathrm{Vt}}{\mathrm{WF} \times \mathrm{Vs} \times 1000}}$$

(2)

where C is the soluble protein content obtained from the calibration curve (g), Vt is the volume of the extract from fresh functional leaf samples (mL), Vs is the volume of the extract used for determination (mL), and WF is the weight of the fresh functional leaf sample used (g).

2.3.8. Potato Yield and Nutritional Quality

At the time of potato harvest, all tubers from the undamaged ridges in each plot were harvested separately, graded, counted, and weighed, and the total yield and the yield of commercial tubers (≥50 g) were calculated [19]. The contents of soluble protein, Vitamin C, starch and reducing sugar were determined using biochemical assay kits (Suzhou Grace Biotechnology Co., Ltd, Suzhou, China) in accordance with the instructions.

2.4. Comprehensive Evaluation

2.4.1. Membership Function Method

$$\mathrm{X}\mu 1={\displaystyle \frac{\mathrm{X} - \mathrm{X}\min }{\mathrm{X}\max - \mathrm{X}\min }}$$

(3)

$$\mathrm{X}\mu 2=1 - {\displaystyle \frac{\mathrm{X} - \mathrm{X}\min }{\mathrm{X}\max - \mathrm{X}\min }}$$

(4)

Calculate the membership function values of each comprehensive index, Xμ1 and Xμ2, using Formulas (3) and (4). For positive indicators, the calculation is performed using Formula (3), for negative indicators, the inverse membership function, i.e., Formula (4), is used. X represents the measured value of a certain indicator, Xmax is the maximum value of the measured indicator, and Xmin is the minimum value of the measured indicator.

2.4.2. Principal Component Analysis

After standardizing 17 indicators, including whole-plant dry weight, whole-plant nitrogen accumulation, whole-plant phosphorus accumulation, whole-plant potassium accumulation, potato tuber vitamin C, starch and protein content, yield, leaf nitrate reductase (NR), and leaf glutamine synthetase (GS), soil ammonium nitrogen, soil urease, soil nitrate reductase, soil nitrite reductase, leaf area index, SPAD, and net income, Principal Component Analysis was performed using SPSS 20.0 to obtain the principal component score coefficient matrix. Calculate the score (Y_i) of each principal component according to Formula (5) [20].

Y_i = W_i1X₁ + W_i2X₂……W_inX_n

(5)

In Formula (5), where W_i1 represents the weight of each variable in the principal component, which is calculated according to W_i1 = θ_n/√(λ_n), here, θ_n is the coefficient corresponding to each variable in the component matrix, and √(λ_n) is the square root of the eigenvalue corresponding to the nth principal component.

Calculate the comprehensive evaluation function Y according to Formula (6).

Y = a₁Y₁ + a₂Y₂ + ……+ a_nY_n

(6)

In Formula (6), a_n represents the variance percentage of the nth principal component.

2.4.3. Weighted TOPSIS Method

Referring to relevant literature [21], the weighted Euclidean distances between the included indicators and the optimal and worst matrix vectors were calculated in Excel software using Formulas (7) and (8), respectively. Then, the relative closeness (C_i) of each indicator to the ideal solution was computed via Formula (9). The C_i value ranges from 0 to 1, and a C_i value closer to 1 indicates a better treatment effect.

$${\mathrm{D}}^{+}=\sqrt{{\sum }_{\mathrm{j}=1}^{\mathrm{m}}{({\mathrm{z}}_{\mathrm{ij}} - {\mathrm{z}}_{\mathrm{j}}^{+})}^2}$$

(7)

$${\mathrm{D}}^{-}=\sqrt{{\sum }_{\mathrm{j}=1}^{\mathrm{m}}{({\mathrm{z}}_{\mathrm{ij}} - {\mathrm{z}}_{\mathrm{j}}^{-})}^2}$$

(8)

$${\mathrm{C}}_{\mathrm{i}}={\displaystyle \frac{{\mathrm{D}}^{-}}{{\mathrm{D}}^{+}+{\mathrm{D}}^{-}}}$$

(9)

2.4.4. Gray Correlation Degree Analysis Method

Gray correlation degree analysis is a statistical method suitable for quality objectives affected by multiple factors. Its core idea is to first make the original quality objectives dimensionless, then calculate the relational coefficients and relational degrees, and finally analyze whether the influence of each factor is significant and rank the factors based on the magnitude of the relational degrees.

$${∆}_{\mathrm{i}}\left(\mathrm{k}\right)=\left|\mathrm{X}0\left(\mathrm{k}\right) - \mathrm{Xi}\left(\mathrm{k}\right)\right|$$

(10)

Calculate the correlation coefficient Ɛ_i by using Formula (11):

$${Ɛ}_{\mathrm{i}} = {\displaystyle \frac{∆\min + \mathrm{\rho }∆\max }{∆\mathrm{i}\left(\mathrm{k}\right) + \mathrm{\rho }∆\max }}$$

(11)

In the formula, Ɛ_i is the correlation coefficient; X0(k) represents the ideal optimal dimensionless value of the reference sequence at the i-th time; ρ is the distinguishing coefficient, where ρ ∈ [0, 1] and is typically set to 0.5; and Δ_i(k) represents the absolute deviation of each point. This setting is intended to reduce the distortion caused by excessively large values.

2.4.5. Evaluation Model Using Aggregate Differences

With reference to the relevant studies [21], a coupling coordination degree model was constructed by integrating the weighted TOPSIS method, membership function method, Principal Component Analysis, and gray relational analysis. The computational procedure is outlined as follows:

$$\mathrm{C} = 4{\displaystyle \frac{\sqrt[4]{{\mathrm{U}}_1{\mathrm{U}}_2{\mathrm{U}}_3{\mathrm{U}}_4}}{{\mathrm{U}}_1+{\mathrm{U}}_2+{\mathrm{U}}_3+{\mathrm{U}}_4}}$$

(12)

$$\mathrm{T}={\lambda }_1{\mathrm{U}}_1+{\lambda }_2{\mathrm{U}}_2+{\lambda }_3{\mathrm{U}}_3+{\lambda }_4{\mathrm{U}}_4$$

(13)

$$\mathrm{D}=\sqrt{\mathrm{C}\times \mathrm{T}}$$

(14)

where C is the coupling degree; T is the coordination index; D is the coupling coordination degree; λ₁, λ₂, λ₃, and λ₄ are the weights of the four components, respectively, and λ₁ = λ₂ = λ₃ = λ₄ = 1/4.

2.5. Data Analysis

The calculation, collation and statistical analysis of the experimental data were performed using Excel 2016 and SPSS 25.0, respectively. The LSD method was used to test the significance of differences in experimental data between treatments at the p < 0.05 level, and GraphPad Prism 10.1.2 and Origin 8.5 were used for the analysis, processing and plotting of the experimental data. To display the analysis results for the system, the R 4.4.3 package ‘patchwork’ was used to integrate key graphics. All graphics were drawn using the R 4.4.3 ‘ggplot2’ package, and a unified color-coding system was adopted to ensure the consistency and readability of the result display.

3. Results

3.1. Effects of Different Fertilization Treatments on Growth and Metabolic Absorption of Potato

Except for leaf area index at the seedling stage, fertilization treatments had significant effects on tuberization stage, tuber bulking stage, maturity leaf area index, and relative chlorophyll content (SPAD) at different growth stages (Figure 1a,b). The leaf area index and SPAD of the potato increased first, then the leaf area index decreased during the tuber bulking stage, and the SPAD decreased during the tuberization stage. During the tuber bulking stage, the leaf area index of T1 and T3 treatments was significantly increased by 28.69% and 22.98%, respectively, compared with T2 treatment. The leaf area index of the T2 treatment was significantly increased by 28.92–41.49% compared with CK, T4 and T5 treatments. Relative to the CK, fertilization treatment significantly increased SPAD of the potato at each growth stage. During the tuber bulking stage, the potato plants under the T3 treatment had the highest SPAD value. The SPAD of potato treated with T1, T2 and T4 increased by 10.71%, 10.26% and 6.39%, respectively, compared with T5 treatment. Fertilization has a great influence on dry matter accumulation (Figure 1c). In all treatments, dry matter accumulation was ranked tuber > stem > leaf > root. Compared with T1, T2 and T4, the total dry matter accumulation of T3 increased by 12.54%, 18.69% and 18.68%. Fertilization treatment can significantly increase the activity of metabolic enzymes (NR, GS) in leaves, and then increase the soluble protein content in leaves (Figure 1d–f). Among them, the activity of nitrogen metabolism enzyme was the highest in the treatment of organic fertilizer replacing the 30% chemical fertilizer. Fertilization had a great influence on the accumulation of nitrogen, phosphorus and potassium (Figure 1d–i). In all treatments, the order of nutrient accumulation was tuber > stem > leaf > root. The nutrient accumulation was the highest under T3 treatment. Under the T1, T2, and T3 treatments, the average root diameter and total root volume of potato roots are the highest (

Table 1. The effect of different fertilization treatments on potato root system morphology.

Treatments	Potato Root System Morphology
	Total Root Length (cm)	Total Root Surface Area (cm2)	Mean Diameter of Root System (mm)	Total Root Volume (cm3)
CK	655.88 ± 18.39 b	94.77 ± 4.72 d	0.68 ± 0.05 d	1.03 ± 0.01 c
T1	805.19 ± 24.41 a	143.79 ± 13.14 a	1.1 ± 0.02 ab	2.32 ± 0.1 a
T2	758.02 ± 25 ab	126.13 ± 1.75 b	1.13 ± 0.03 a	2.23 ± 0.04 a
T3	800.07 ± 19.02 a	136.69 ± 2.31 ab	1.17 ± 0.03 a	2.22 ± 0.13 a
T4	758.51 ± 13.45 ab	113.86 ± 1.86 c	1.04 ± 0.06 b	1.47 ± 0.03 b
T5	726.2 ± 27.11 ab	105.44 ± 2.6 c	0.83 ± 0.05 c	1.07 ± 0.05 c

Note: The values shown are mean ± standard error (n = 3). Lowercase letters have been used to indicate significant differences among different fertilization treatments, where the threshold for statistical significance is denoted as p < 0.05.

).

3.2. Effects of Different Fertilization Treatments on Yield and Quality of Potato

The potato yield of different fertilization treatments increased to varying degrees compared with CK treatment. The potato yield of T1 and T3 treatments was 35.55% and 27.59% higher than that of T2 treatment, and 35.46% and 27.50% higher than that of T4 treatment (

Table 2. Potato tuber quality under different fertilization treatments.

Treatments	Tuber Yield (kg·ha−1)	Commodity Tubers (kg·ha−1)
CK	33,384.64 ± 1036.33 c	30,596.18 ± 919.39 c
T1	51,039.26 ± 2775.88 a	48,366.31 ± 2961.37 a
T2	37,653.33 ± 555.59 b	34,584.78 ± 729.9 b
T3	48,041.31 ± 2084.52 a	45,585.07 ± 2139.6 a
T4	37,678 ± 322.66 b	34,850.97 ± 318.23 b
T5	35,259.9 ± 2853.83 bc	31,635.21 ± 2211.19 bc

Note: Values are presented as mean ± standard error (n = 3). Lowercase letters are used to indicate significant differences (p < 0.05) among the values under different fertilization treatments.

). The change trend of commodity potato under different treatments was consistent with potato yield.

Compared with CK treatment, fertilization treatment increased potato quality to varying degrees (Figure 2a–d). The VC content of potato in T1 and T3 treatments was higher than that in T2 (18.21% and 17.0%) and T4 (17.70% and 16.91%) treatments. The contents of starch and soluble protein in potato tubers were the highest under T3 treatment. Compared with T4 treatment, the reducing sugar content of potato in T1, T2 and T3 treatments decreased by 25.08%, 27.67% and 31.59%, respectively. It showed that T3 treatment could increase the content of potato starch, VC and soluble protein, and reduce the content of potato reducing sugar.

3.3. Effects of Different Fertilization Treatments on Soil Enzyme and Soil Nitrogen Content

Compared to the CK treatment, fertilization can significantly increase soil urease, nitrate reductase, nitrite reductase, and acid protease activity (Figure 3a–d). Organic fertilizer replacement treatment can increase urease and nitrite reductase activity. The soil nitrate reductase activity of T1, T2 and T3 treatments was 11.27%, 13.19% and 14.39% higher than that of T4 treatment, respectively. Compared with T1, T3, T4 and T5 treatments, the activity of soil acid protease in T2 treatment increased by 3.79%, 8.72%, 7.38% and 20.69%, respectively. The content of nitrate nitrogen under traditional fertilization and optimized chemical fertilizer was higher than that under organic substitution fertilization, but the content of ammonium nitrogen was opposite (Figure 3e,f).

3.4. Effects of Different Fertilization Treatments on Economic Benefits

The economic benefits of potato under different fertilization treatments are shown in

Table 3. Economic benefits of potato under different fertilization treatments.

Treatment	Input (CNY·ha−2)			Gross Income	Net Income	Income/Input
	Fertilizer Input	Other Inputs	Total Input
CK	0	15,000.00	15,000.00	33,384.64	18,384.64	2.23
T1	4591.968	15,000.00	19,591.97	51,039.26	31,447.29	2.61
T2	4498.84	15,000.00	19,498.84	37,653.33	18,154.49	1.93
T3	5568.523	15,000.00	20,568.52	48,041.31	27,472.78	2.34
T4	6638.203	15,000.00	21,638.2	37,678	16,039.8	1.74
T5	7707.883	15,000.00	22,707.88	35,259.9	12,552.02	1.55

Note: Values are presented as mean ± standard error (n = 3).

. Moreover, compared with the CK, the total income for potatoes under different fertilization treatments all increased to varying degrees. Except for the T1 and T3 treatments, the Income/Input of the other different fertilization treatments all decreased to varying degrees compared with the CK treatment. Only the Net income for potatoes under the T1 and T3 treatments showed an improvement compared with those under the CK treatment. The total income, Net income and Income/Input of potato production reached the highest values under the T1 treatment. The total incomes of T1 and T3 treatments were higher than those of T2 treatment (by 35.55% and 27.59%, respectively), T4 treatment (by 35.46% and 27.50%, respectively), and T5 treatment (by 44.75% and 36.25%, respectively). The net profits of T1 and T3 treatments increased by 73.22% and 51.33%, respectively, compared with that of T2 treatment.

3.5. Coupling and Decision Optimization of Multi-Objective Evaluation Models Based on Minimization of Overall Differences

Based on whole-plant dry weight, nitrogen accumulation, phosphorus accumulation, and potassium accumulation, potato tuber vitamin C, starch and protein content, yield, leaf nitrate reductase (NR), and leaf glutamine synthetase (GS), soil ammonium nitrogen, soil urease, soil nitrate reductase, soil nitrite reductase, leaf area index, SPAD, and Net income, the potato composite index was assessed (

Table 4. Results of multi-objective evaluation based on an integrated difference model.

Treatment	Membership Function		Principal Component Analysis		Weighted TOPSIS		Gray Relation Analysis		Combined Evaluation Model
	Value	Rank	Value	Rank	Value	Rank	Value	Rank	Value	Rank
CK	0.110	6	−3.620	6	0.149	6	0.576	6	0.142	6
T1	0.662	2	1.393	2	0.561	2	0.793	2	0.808	2
T2	0.542	3	0.277	4	0.492	3	0.723	4	0.710	4
T3	0.866	1	3.297	1	0.841	1	0.900	1	0.976	1
T4	0.550	4	0.567	3	0.484	4	0.734	3	0.722	3
T5	0.294	5	−1.913	5	0.324	5	0.642	5	0.492	5

Note: Values are presented as mean ± standard error (n = 3).

). The outcomes and rankings of identical treatments vary across different evaluation methods. A correlation analysis was performed on the evaluation values obtained from each treatment across the four individual evaluation models (Table S2). The average correlation coefficient between each single evaluation model and the other three models ranges from 0.758 to 0.921. A comprehensive evaluation was performed by integrating the results from the four individual models using an overall difference combination model, with the outcomes presented in Table S2. The results indicate that treatments T3 and T1 ranked first and second, respectively.

3.6. Factors Influencing Soil Fertility and Potato Metabolism and Growth to Promote Tuber Protein Content

Potato tuber yield, starch content, protein content, and vitamin C content all show a significant positive correlation with potato biomass, SPAD, NR, GS, leaf soluble protein, total nitrogen and total potassium uptake by plants, soil nitrate reductase, and acid protease. However, the reducing sugar content in potato tubers is negatively correlated with soil fertility indicators, as well as plant growth and metabolic absorption indicators (Figure 4). The effect of organic fertilizer substitution on potato tuber protein content is mainly related to S-URE and SPAD (Figure 5). All soil fertility factors and the model of plant growth, metabolism, and absorption explain 96% of the total variation observed in the effect of applying different proportions of organic fertilizer substitution on the soluble protein content in potato tubers.

4. Discussion

4.1. Effects of Equal Replacement of Chemical Fertilizer Nitrogen with Organic Fertilizer Nitrogen in the Rhizosphere Soil of Potatoes by Increasing Soil Enzyme Activity

The content of the active nitrogen pool, as the main component of soil available nitrogen, directly affects the nitrogen absorption efficiency of crops [22]. Studies have shown that replacing chemical nitrogen fertilizer with organic fertilizer can increase the content of soil mineral nitrogen and significantly improve the nitrogen supply for crops [23]. The results of this study demonstrated that all fertilization treatments led to higher soil nitrate and ammonium nitrogen contents compared to the CK. Meanwhile, replacing chemical nitrogen with organic fertilizer nitrogen significantly changes the content of soil active nitrogen pools. Among them, compared with the optimized chemical fertilizer treatment, the treatment with 20% replacement of chemical nitrogen by organic fertilizer nitrogen increases the ammonium nitrogen content in potato tubers during the expansion, while reducing the nitrate nitrogen content. On the one hand, this is beneficial to increasing the inorganic nitrogen content in the surface soil, providing a sufficient nitrogen source for crop growth; on the other hand, it reduces the accumulation of NO₃⁻-N in deep soil and lowers the risk of leaching. The mechanism lies in that humic acid in organic fertilizers forms stable complexes with mineral nutrients, which reduces nitrogen loss and thereby increases the content of mineral nitrogen [24]. Moreover, replacing chemical fertilizers with organic fertilizers at an appropriate nitrogen-equivalent ratio can promote the conversion of the inert nitrogen pool to the active organic nitrogen pool and simultaneously induce the priming effect of soil nitrogen. However, a high concentration of ammonium nitrogen will inhibit the reproduction of nitrifying bacteria, which in turn will hinder the nitrification process [25].

Soil enzyme activity is an important indicator reflecting the intensity and direction of soil nutrient transformation as well as the level of soil fertility, and it is susceptible to soil disturbances [26]. The application of organic fertilizers into soil can enhance soil enzyme activity and soil fertility [27]. Soil urease and soil nitrate reductase are directly involved in nitrogen transformation, and their activity levels directly reflect the amount of nitrogen in the soil. Urease plays an important role in the transformation of nitrogen in soil and fertilizers; it is the only enzyme that catalyzes the hydrolysis of urea and is one of the indicators for evaluating soil fertility [28]. The results of this study demonstrate that all fertilization treatments increased the activities of soil urease, nitrate reductase, nitrite reductase, and acid protease compared to the CK treatment. Notably, compared with conventional fertilization, the combined application of organic fertilizer and nitrogen fertilizer can increase soil urease activity. This suggests that the slow decomposition of organic fertilizer is more conducive to enhancing urease activity, promoting the mineralization of organic nitrogen into ammonium nitrogen, and facilitating soil nitrification [29]. Soil nitrate reductase and nitrite reductase are key enzymes affecting soil denitrification efficiency, which can reduce nitrogen loss in the soil and enable better utilization of nitrogen by plants [30]. In our study, the treatment with 20% replacement of chemical nitrogen by organic fertilizer nitrogen could increase the activities of soil nitrate reductase and nitrite reductase. The reason may be that the application of organic fertilizer increases soil organic matter content, improves soil structure, regulates soil aeration conditions, facilitates the combination of ammonium nitrogen (NH₄⁺-N) with oxygen to form nitrate nitrogen (NO₃⁻-N), provides substrates for nitrate reductase, and ultimately increases the activities of soil nitrate reductase and nitrite reductase [31]. Soil protease activity is closely related to soil nitrogen. Various protein-containing and peptide compounds in the soil are converted into inorganic nitrogen under the catalysis of protease, which can be absorbed and utilized by plants, and it is an important component that promotes soil nitrogen cycling [32]. Our study demonstrates that both the application of chemical fertilizer and the partial substitution of chemical fertilizer with organic manure significantly enhance soil protease activity compared to the no-fertilizer control. The possible reasons are as follows: on the one hand, organic fertilizers provide abundant carbon sources, supplying soil microorganisms with essential substrates and energy; on the other hand, chemical fertilizers provide readily available nutrients to microorganisms, addressing the nutritional bottlenecks in their growth and metabolism [33]. Soil enzymes play a crucial role in soil nutrient cycling and metabolic processes. The results of this study show that soil acid protease (S-ACPT), soil urease (S-URE), and nitrate reductase (S-NR) are the main factors affecting NH₄⁺-N and NO₃⁻-N in potato soil. The reason may be that S-ACPT, S-URE, and S-NR are involved in the hydrolysis of proteins, nucleic acids, etc., degrading complex nitrogen-containing compounds into small molecular forms that can be absorbed and utilized by plants, thus playing a role in the nitrogen cycle [34].

4.2. Effects of Equal Replacement of Chemical Fertilizer Nitrogen with Organic Fertilizer Nitrogen on Nutrient Uptake and Yield of Potato

Research findings indicate that when organic fertilizers replace chemical fertilizers at an equal nitrogen application rate, the stability and long-term sustainability of crop production are significantly improved [35]. It has also been found that replacing 36–46% of chemical with organic fertilizer nitrogen at equal rates can significantly increase potato yields in the semi-arid regions of Northwest China [36]. In this study, under the same nutrient input, the yields of treatments where part of chemical nitrogen was replaced by organic fertilizer nitrogen in high-fertility soil were all higher than that of the CK treatment, and different proportions of organic fertilizer nitrogen replacing chemical nitrogen had different effects on yield. Compared with the optimized chemical fertilizer (T2) treatment, the treatment with 20% chemical fertilizer replaced by organic fertilizer at equal nitrogen rate (T3) could significantly increase potato yield. However, compared with other regions, the optimal proportion of organic fertilizer replacing chemical nitrogen fertilizer is lower in high-fertility soil. This occurs because the local soil already has a relatively high organic matter content; applying an excessive amount of organic fertilizer will consequently lead to an overaccumulation of organic matter in the soil. Due to the slow release of nutrients from organic fertilizers, insufficient nutrient supply in the later stage leads to a reduction in potato tuber yield. In agricultural production practice, economic benefits have always been the ultimate goal agricultural producers were concerned with [37]. The results of this study show that the total income, net profit and income/input of potato production all increase first and then decrease with the increase in the proportion of organic fertilizer replacing chemical fertilizer. This indicates that a higher proportion of organic fertilizer replacing chemical fertilizer does not necessarily significantly increase the input in agricultural production; on the contrary, it may lead to resource waste, which is consistent with the conclusions of many previous studies [38]. This study found that in potato cultivation, all fertilization treatments resulted in a higher gross income than the CK control. However, only the T1 and T3 treatments achieved a net income greater than that of the CK. The increase in income is mainly due to the fact that the economic increase brought by the increase in potato yield is greater than the increase in the cost of replacing chemical fertilizer with organic fertilizer. However, a higher proportion of organic fertilizer replacing chemical fertilizer not only reduces potato yield but also significantly increases the cost of fertilizer application, resulting in losses. The results showed that the higher the proportion of organic nitrogen replacement from various organic fertilizers, the higher the fertilizer production cost and the lower the economic benefit, which is consistent with the conclusion of this study. The combined application of organic and inorganic fertilizers is beneficial to increasing the nutrient accumulation of crops [39]. Crop biomass is closely related to nutrient accumulation, and nutrient accumulation is the basis for biomass accumulation and yield formation. Studies by Geng Wei et al. [40] have shown that the application of organic fertilizers can increase soil nutrient content, promote the absorption of N, P, and K by crops, and thereby promote their growth. Research has confirmed that the application of organic fertilizers can increase the plant height, number of main stems, leaf area index, and dry matter content of potatoes [41]. The results demonstrated that fertilization significantly enhanced nitrogen, phosphorus, and potassium accumulation in the entire plant and their subsequent uptake in tubers compared to CK. Furthermore, the T3 treatment promoted both processes more effectively than T1, which aligns with previous research [42]. The reason is that the appropriate replacement of chemical fertilizer with organic fertilizer provides a large amount of energy substances for soil microorganisms, promotes the increase in microbial biomass nitrogen, affects the soil quality of farmland and the microecological environment for root growth, facilitates the coordinated supply of soil available nutrients, makes their release more gradual and maintains them at a high level, thus ensuring the nutrients required for the growth of potatoes in the later stage.

4.3. Effects of Equal Replacement of Chemical Fertilizer Nitrogen with Organic Fertilizer Nitrogen on Nitrogen Metabolism and Potato Quality

Nitrogen metabolism is one of the important processes in potato physiological metabolism. The level of nitrogen metabolism is a key indicator of the potato’s ability to synthesize proteins and form yields. The nitrogen metabolism process is regulated by multiple factors, among which the activity of key enzymes is a decisive factor. The main regulatory enzymes include nitrate reductase, glutamine synthetase, glutamate synthase, glutamate dehydrogenase, and transaminase, whose activity changes directly affect nitrogen assimilation efficiency. Nitrate reductase (NR), a key rate-limiting enzyme in the NO₃⁻-N assimilation process, is mainly localized in the cytoplasm of crop roots and leaves, and its activity is a major determinant of nitrate nitrogen assimilation efficiency [43]. In higher plants, approximately 95% of ammonium ions (NH₄⁺) are assimilated into amino acids via the glutamine synthetase/glutamate synthase (GS/GOGAT) cycle [44]. Amino acids are the main form of nitrogen storage and transport in plants. Studies have shown that there is a certain relationship between leaf nitrate reductase (NR) activity and nitrogen accumulation, and it is positively correlated with protein content [45]. The results of this study also indicate that nitrate reductase activity in potato leaves is extremely significantly positively correlated with the protein content in leaves and tubers. As a nitrate-responsive enzyme, soil NR exhibits high sensitivity to soil nitrate nitrogen concentrations, thereby playing a pivotal role in modulating nitrogen fertilizer uptake efficiency and assimilation utilization rates in agricultural plants [46]. Studies have shown that the appropriate combined application of organic fertilizers and chemical nitrogen fertilizers can significantly increase the nitrate reductase activity in rice, and the optimal proportion of combined application varies with soil types and climatic conditions [47]. In this experiment, the CK treatment showed the lowest leaf nitrate reductase activity, while the T3 treatment resulted in the highest; meanwhile, the soluble protein content, nitrate reductase (NR) activity, and glutamine synthetase (GS) activity in potato leaves all exhibited a positive correlation with soil nitrate nitrogen. This indicates that replacing 20% of chemical fertilizer nitrogen with organic fertilizer nitrogen affects soil nitrogen supply by reducing the application of chemical fertilizer to a certain extent, and this serves as the main pathway for regulating potato nitrogen metabolism and uptake, thereby better providing the nutrients required for potato growth and development.

As the end product of nitrogen metabolism, protein content is a key indicator for evaluating crop quality. Studies have shown that increasing the activity of key enzymes in nitrogen metabolism is beneficial to increasing the protein content in potato tubers [48]. The findings demonstrate that fertilization consistently enhanced potato tuber protein content compared to the CK. Moreover, at an equivalent nitrogen application rate, the T3 treatment was more effective in increasing protein content than the T2 treatment. In addition to protein content, the levels of starch, VC (vitamin C), and reducing sugar are also key indicators determining the nutritional quality of crops, and their content levels directly affect processing characteristics and edible value. The reducing sugar content in potato tubers is of great significance to potato processing enterprises. This is because potato chips tend to turn black during the frying process, and the lower the reducing sugar content, the better the quality of the processed potato chips [49]. Starch, as an important polysaccharide carbohydrate synthesized in higher plants, plays a key physiological role in the growth, development and reproduction of plants. Starch is also one of the most important energy sources for humans and animals, and occupies an irreplaceable position in maintaining the energy flow and material cycling within ecosystems [50]. Vitamin C, also known as ascorbic acid, is an indispensable water-soluble vitamin in the human body. The results indicated that the CK treatment exhibited the lowest protein and starch content in the tubers. In contrast, compared with the application of chemical fertilizer alone, replacing 20% of chemical fertilizer nitrogen with organic fertilizer nitrogen increased the contents of VC, starch and soluble protein in potatoes and decreased the content of reducing sugar. Replacing 40% of chemical fertilizer with organic fertilizer at an equal nitrogen rate increased the contents of starch and reducing sugar in potatoes. It indicated that the treatment of replacing 20% of chemical fertilizer with organic fertilizer at an equal nitrogen rate could better prolong the storage period of tubers compared with other treatments. The research by Wu Minjuan et al. [51] on the impact of organic fertilizer on potato yield and storage quality demonstrated that organic fertilizer application notably improved potato quality and increased its shelf life. As a core process of plant basic metabolism, nitrogen metabolism directly affects crop yield and quality formation by regulating the accumulation of photosynthates and protein synthesis [52]. Meanwhile, the level of nitrogen metabolism also determines the absorption efficiency of crops for mineral nutrients and plays a key role in nutrient regulation during the growth period [53]. The results of this study are consistent with previous research. Through correlation analysis between the nitrogen metabolism activity in potato leaves and their yield and quality under the condition of replacing chemical fertilizers with organic fertilizers at equal nitrogen rates, it was found that nitrate reductase, glutamine synthetase, and glutamate synthase in potato leaves were all significantly positively correlated with yield, as well as the contents of potato starch, VC, and soluble protein. Nitrate reductase, glutamine synthetase, nitrate reductase, and soluble protein in potato leaves play a leading role in improving potato yield and quality.

A single management practice rarely achieves multiple objectives simultaneously, including high crop yields, superior product quality, enhanced soil fertility, and maximized economic returns. Principal Component Analysis evaluated the growth of potatoes under different fertilization treatments [54]. The optimal fertilization management strategy was derived using the TOPSIS method [55]. However, in agricultural practice, relying solely on a single evaluation method may introduce certain errors into the results. Therefore, it is necessary to apply multiple evaluation models for comprehensive assessment. Further, when organic fertilizer nitrogen replaced 20% of the chemical nitrogen fertilizer, the results obtained from gray relational analysis, membership function analysis, Principal Component Analysis, weighted TOPSIS analysis, and the comprehensive evaluation value of potato were all the highest.

5. Conclusions

Organic substitution treatment can increase soil nitrogen cycle enzyme activity and ammonium nitrogen content, promote nutrient absorption, and improve the quality and yield of tubers. Potato biomass, SPAD, NR, GS, leaf soluble protein, total nitrogen and total potassium uptake by plants, S-NR, and S-ACPT have significant effects on potato tuber yield, starch content, protein content, and vitamin C content. The soluble protein content in potato tubers is closely related to soil urease and SPAD. Based on the overall differentiation evaluation model, replacing 20% of chemical fertilizer nitrogen with organic fertilizer nitrogen is the optimal treatment method considering potato growth, yield, quality, soil enzymes, nitrogen content, and fertilizers. This study can provide the optimal fertilization method for efficient potato cultivation in the Erhai Lake Basin of China.