Balancing Productivity and Environmental Sustainability in Pomelo Production Through Controlled-Release Fertilizer Optimization

Abstract

In the context of agricultural green transformation, the balance between the environmental footprint and economic return is a key indicator for measuring the synergy of high yields, high efficiency, and environmental friendliness in agricultural systems. However, the pathways and mechanisms for achieving this synergy in orchard systems remain unclear. Based on a three-year field experiment in Pinghe County, Fujian Province, a comprehensive evaluation framework integrating life cycle assessment (LCA) was constructed. This framework was used to systematically analyze the differences in the net ecosystem economic benefit (EEB) and environmental impact of four fertilization regimes: the conventional farming regime with no mulching (A; 1084 kg N ha⁻¹, 914 kg P₂O₅ ha⁻¹, and 906 kg K₂O ha⁻¹), the conventional farming regime with mulching (B), the optimized fertilization regime with water–fertilizer integration (C; 250 kg N ha⁻¹, 200 kg K₂O ha⁻¹, 100 kg MgO ha⁻¹, and 400 kg CaO ha⁻¹), and the optimized fertilization regime with controlled-release fertilizers (D). The results showed that regime D performed best in terms of yield, nutrient-use efficiency, and EEB, which increased by 220.5% and 297.5% compared with regime A, and reduced the input cost by CNY 63,100~69,000 hm⁻². Moreover, compared with regime A, regimes B, C, and D significantly reduced the carbon, nitrogen, and phosphorus footprints, respectively, with the carbon footprint reduced by 6.7~21.7%, 72.4~74.8%, and 71.6~76.5%; the nitrogen footprint reduced by 2.6~19.0%, 80.7~82.2%, and 80.1~83.4%; and the phosphorus footprint reduced by 15.3%, 100%, and 100%. Furthermore, the comprehensive evaluation index (CEI) is D > C > B > A. In total, the three optimized regimes balanced high yield with environmental sustainability, with the D regime showing the best performance, offering scientific support for transitioning to low-carbon, high-value orchards in smallholder systems.

1. Introduction

Amid the global transition toward greener and more sustainable agriculture, balancing environmental footprints and economic returns has become a critical criterion for evaluating whether agricultural systems can achieve synergies between high productivity, resource-use efficiency, and environmental friendliness. Guangxi pomelo (Citrus maxima), a representative citrus crop in southern China, has garnered widespread attention due to its high yield and considerable economic value [1]. In 2021, the annual production of Guangxi pomelo reached 2.105 million tons [2], playing a significant role in driving regional agricultural economic growth. However, under the prevailing smallholder-dominated production system, excessive fertilizer application, particularly of nitrogen, is widespread, primarily driven by short-term economic incentives. This practice has led to a series of environmental issues, including soil acidification, water eutrophication, and greenhouse gas emissions [3,4,5,6]. As a key nutrient influencing both yield and environmental outcomes, nitrogen plays a pivotal role in determining the trade-off between ecological impact and economic return in orchard systems [7]. Therefore, elucidating the effects and mechanisms of optimized fertilization strategies, with a particular focus on nitrogen management, on the net ecosystem economic benefit (EEB) and environmental footprints of Guangxi pomelo production is of critical theoretical and practical importance for promoting the sustainable, efficient, and high-value development of the industry.

Optimizing fertilization practices has been widely demonstrated to enhance the EEB of crop production systems while reducing the environmental footprint, mainly by improving nitrogen-use efficiency, lowering greenhouse gas emissions, and increasing economic returns [8,9,10]. Through scientifically grounded nutrient management, particularly by moderately reducing chemical fertilizer inputs and implementing precision application techniques, it is possible to mitigate potential yield losses while simultaneously enhancing both economic returns and environmental performance. For example, Chen et al. (2022) reported that in pomelo orchard systems, reducing nitrogen, phosphorus, and potassium fertilizers while increasing magnesium supplementation improved economic returns by 15.3%, mainly through a higher net income and income-to-cost ratio (4.72 compared to 4.09 under conventional fertilization), alongside significantly lower environmental costs [8]. Similarly, Qin et al. (2016), based on global citrus production data, found that synchronizing irrigation and nitrogen management increased orchard yields by 10~20% and enhanced nitrogen-use efficiency by 15~40% [9]. Despite these promising outcomes, orchard systems, which are characterized by high input intensity and long production cycles, continue to pose significant challenges in achieving a balance between high yields, environmental sustainability, and economic viability. In recent years, evaluation frameworks such as life cycle assessment (LCA), environmental footprint analysis, and net ecosystem economic benefit have been increasingly applied to staple crop systems (e.g., rice, maize/peanut rotation, and potato) to guide fertilizer optimization and integrated performance assessments [10,11,12,13,14,15]. For instance, Zhang et al. (2024) found that substituting one-third and two-thirds of nitrogen fertilizer with crop straw in a Northeast China rice system increased the EEB by 38.1% and 34.3%, respectively, while markedly reducing greenhouse gas emissions [10]. Han et al. (2024) reported that maize/peanut rotation systems in the Huang-Huai-Hai region, compared with monoculture, reduced indirect greenhouse gas (GHG) emissions by 3.1% and carbon inputs by 18.0%, while substantially improving carbon-use efficiency (69.2%) and economic returns [11]. In contrast, related studies in orchard systems remain limited. Yan et al. (2024), based on a four-year field experiment in Fujian Province, demonstrated that optimized fertilization combined with lime application increased the pomelo EEB by 220.3% and 20.3% relative to conventional and single optimization treatments, respectively, while reducing carbon, nitrogen, and phosphorus footprints by 90.2~91.8%, 85.1~87.2%, and over 99.8%, respectively [16]. Likewise, Zhou et al. (2023) used LCA to show that restructuring fertilization strategies and reducing nitrogen input in pomelo orchards led to a 27.4% reduction in GHG emissions per unit yield and a substantial decline in environmental footprints [17]. Collectively, these studies have laid the foundation for understanding how fertilization practices affect orchard performance through improved nutrient efficiency, reduced environmental losses, and better management of yield–environment trade-offs. However, the specific pathways to achieving a synergy of high yields, high efficiency, and low emissions in orchard systems remain unclear, particularly regarding the trade-offs among agricultural inputs, environmental costs, and economic returns. Further quantitative research focusing on fertilization’s effects on yield stability, fruit quality, soil nutrients, and emissions is urgently needed to identify effective strategies for sustainable orchard intensification, both in subtropical China and globally.

Building on the above background, this study conducted a three-year field experiment (2021–2023) in Pinghe County, a major pomelo-producing region in Fujian Province, China. An LCA framework was established, integrated with the entropy weight method (EWM) and structural equation modeling (SEM), to systematically evaluate the effects and synergies of four fertilization regimes across the 2022 and 2023 growing seasons. The assessment focused on pomelo yield, economic returns, environmental footprints (carbon, nitrogen, and phosphorus), and EEB. Specifically, the objectives of this study were to (1) assess the regulatory effects of different optimized fertilization strategies on orchard-level environmental footprints (carbon, nitrogen, and phosphorus); (2) quantify the direct and indirect impacts of fertilization management on EEB and identify key influencing factors and their underlying mechanisms; and (3) propose a green and efficient fertilization strategy suitable for smallholder farming systems, providing both theoretical guidance and practical insights for achieving sustainable development in low-carbon, high-value subtropical orchard systems.

2. Materials and Methods

2.1. Experimental Site Description

The experiment was conducted from September 2021 to September 2023 in a pomelo orchard at the Junying Bridge, Renshan Village, Banzai Town, Pinghe County, Zhangzhou City, Fujian Province (24.22° N, 116.18° E). The region is characterized by a subtropical monsoon climate, with an annual average temperature ranging from 17.5 °C to 21.3 °C, annual precipitation between 1600 mm and 2000 mm, and an average frost-free period of 324 days, which is suitable for the growth of honey pomelo. The crop used in the experiment was 16-year-old ‘Sanhong Honey Pomelo’, continuously cultivated without crop rotation. The orchard is located on flat terrain, is well managed, and no records of heavy metal or pesticide contamination have been reported. The soil samples were taken from a 1 m horizontal depth, with the soil type classified as acidic sandy loam. The particle size composition was approximately 50~70% sand (0.05–2 mm), 20~35% silt (0.002–0.05 mm), and less than 15% clay (<0.002 mm). The basic physicochemical properties of the soil are as follows: pH of 4.24, soil organic matter content of 13.95 g kg⁻¹, ammonium nitrogen content of 15.58 mg kg⁻¹, nitrate nitrogen content of 14.43 mg kg⁻¹, available phosphorus content of 229.41 mg kg⁻¹, available potassium content of 144.17 mg kg⁻¹, exchangeable calcium content of 255.58 mg kg⁻¹, and exchangeable magnesium content of 45.70 mg kg⁻¹.

2.2. Experimental Design

The experiment was designed with four regimes (Figure A1): the conventional farming regime with no mulching (A), the conventional farming regime with mulching (B), the optimized fertilization regime with water–fertilizer integration (C), and the optimized fertilization regime with controlled-release fertilizers (D). The optimized regimes involved reduced fertilization based on the fruit tree yield and expert-recommended fertilization rates, with additional applications of oyster shell powder and magnesium sulfate, while maintaining surface grass cover.

The experiment adopted a completely randomized block design with three replicates per treatment. Each replicate contained four pomelo trees, making a total of 12 trees per regime, and 48 trees in the entire experimental plot. Fertilizers were applied as individual components rather than premixed blends. The fertilizers used and their nutrient contents were as follows: urea (N 46%, Sinofert Holdings Limited, Beijing, China), ammonium dihydrogen phosphate (N 15%, P₂O₅ 42%, Hubei Xinyangfeng Fertilizer Co., Ltd., Jingmen, China), potassium sulfate (K₂O 52%, Yunnan Three Circle Chemical Co., Ltd., Kunming, China), magnesium sulfate heptahydrate (MgO 16.4%, Laizhou Zhongda Chemical Co., Ltd., Laizhou, China), and oyster shell powder (CaO ≥ 45%, Quanzhou Shengda Biotechnology Co., Ltd., Quanzhou, China).

Fertilization was conducted at four key growth stages of pomelo: shoot-promoting and flower-enhancing fertilizer (early-to-mid-March), fruit-setting fertilizer (late April), fruit-growing fertilizer (late June), and winter fertilizer (late December). The amounts of fertilizers applied at each time were 29%, 19%, 19%, and 33% of the total annual fertilizer amount, respectively. For all regimes, the shoot-promoting and flower-enhancing fertilizers and the fruit-setting fertilizers were applied beneath the drip line, while the fruit-growing and winter fertilizers were applied as hole fertilizers beneath the drip line.

In regime D, a one-time application of the controlled-release fertilizer (a CRU blend, CU:CRU3:CRU6 = 1:1:8) was carried out during the winter fertilizer stage. This formulation was applied approximately 20 cm from the tree trunk and was designed to release nutrients gradually throughout the growing season, aligning with the nutrient demand of pomelo trees under subtropical acidic soil conditions. The fertilization amounts applied under each regime are detailed in

Table 1. Fertilizer application rates under different regimes.

	N (kg ha−1)	P2O5 (kg ha−1)	K2O (kg ha−1)	MgO (kg ha−1)	CaO (kg ha−1)
A	1084	914	906	0	0
B	1084	914	906	0	0
C	250	0	200	100	400
D	250	0	200	100	400

.

2.3. Sample Collection and Indicator Measurement

2.3.1. Collection of Honey Pomelo Fruit and Soil Samples

During the fruit-ripening period (September 2022 and September 2023), fruit and soil samples were collected from each experimental tree. For fruit sampling, three medium-sized, disease-free fruits were harvested from each tree, and their individual weights were recorded to calculate the average fruit weight. To estimate yield, all fruits on the sampled trees were manually harvested, and the total fruit number per tree was recorded. The yield was then calculated as the product of the number of fruits and the average fruit weight.

For soil sampling, two symmetrical points within 10 cm inside the tree’s drip line were selected, avoiding areas with recent fertilizer application. At each point, soil samples were taken at depths of 0~20 cm and 20~40 cm and then composited by the quartering method to form a single mixed sample per tree. After removing visible roots and debris, each composite soil sample was divided into two portions: one was air-dried and ground for analysis of the basic physicochemical properties, and the other was stored at 4 °C for determination of water-soluble organic carbon and microbial biomass carbon and nitrogen. All analyses were conducted within 7 days of collection to ensure data accuracy.

2.3.2. Determination of Soil Physicochemical Properties

Soil pH was measured using the potentiometric method, with a water-to-soil ratio of 1:2.5 (Thermo Scientific, Orion A215, Waltham, MA, USA). Ammonium nitrogen and nitrate nitrogen were extracted with 2 mol L⁻¹ KCl and measured using a flow analyzer (Flowsys, SYSTEA, Rome, Italy). Available phosphorus was determined using the sodium bicarbonate extraction–molybdenum–antimony anti-colorimetry method [18]. Available potassium was extracted with 1 mol L⁻¹ NH₄OAc (pH = 7.0) and measured using a spectrophotometer (FP6410, INESA, Shanghai, China). Exchangeable calcium and magnesium were extracted with 1 mol L⁻¹ NH₄OAc (pH = 7.0) and analyzed via inductively coupled plasma optical emission spectroscopy (PerkinElmer, Avio 200, Waltham, MA, USA). Water-soluble organic carbon was extracted using water and quantified with the Total Organic Carbon Analyzer (TOC-VCPH/CPN, Shimadzu Corporation, Kyoto, Japan).

2.4. Environmental Footprints and Economic Evaluation

2.4.1. System Boundaries and Functional Unit

This study employs the LCA method, conducted in accordance with the ISO 14040 [19] and ISO 14044 [20] standards, to systematically evaluate both the economic benefits and environmental impacts of the pomelo production process. The system boundaries (Figure 1) are defined from two perspectives: (1) agricultural inputs, including production materials such as fertilizers, pesticides, fruit bags, transportation, and labor, with associated upstream resource consumption and emissions; and (2) environmental impacts arising from nitrogen fertilizer application, including energy loss and GHG emissions (expressed as CO₂-eq). Three environmental footprint indicators are assessed: carbon footprint (CF), nitrogen footprint (NF), and phosphorus footprint (PF). The assessment of agricultural inputs includes both upstream emissions during material production and transport, and downstream emissions related to field application. The functional unit is defined as the production of 1 ton of fresh pomelo fruit per hectare per year, reflecting a typical single-cropping annual cycle for subtropical orchards in southern China. This unit enables standardized comparisons across fertilization regimes.

2.4.2. Economic Benefits and Environmental Costs of Pomelo Production

In terms of economic evaluation, this study developed a comprehensive accounting system that includes total revenue, input costs, economic benefits, and EEB, with a focus on fertilizer input. It primarily analyzes the impact of fertilizers, labor, and other production inputs on yield and market revenue. The main cost parameters (e.g., fertilizers, labor, pesticides, bagging materials, and transportation) were based on local market prices during 2022–2023, collected via field surveys from representative pomelo orchards and agricultural suppliers in Pinghe County, Fujian Province. Input costs were recorded separately for each year through field-based bookkeeping by contracted farmers. The same farmers conducted routine operations (weeding, fertilization, pesticide application), ensuring consistency in labor costs. Annual variations in the number of fruit-bearing trees and total yield led to slight differences in bagging and transportation costs, as shown in Figure 2c,d.

Regarding environmental costs, the accounting process comprehensively considered ecological issues, such as greenhouse gas emissions, eutrophication, and soil acidification resulting from the production, transportation, and application of fertilizers. It also estimated the potential economic costs related to human health and ecosystem restoration. The relevant cost factors (e.g., greenhouse gas market prices, water pollution treatment costs caused by NO₃⁻ and NH₄⁺, nitrogen fertilizer emission factors) were primarily taken from the literature, including [21,22,23,24]. The direct and indirect N₂O emissions from nitrogen fertilizers, the loss of NH₄⁺ and NO₃⁻, and their conversion to CO₂-eq were estimated according to the IPCC guidelines and the study by [25].

For the specific calculation of environmental footprints, the CF includes net greenhouse gas emissions (NGE) and carbon sequestration (CS) per unit area. Greenhouse gas emissions come from N₂O emissions during the production of agricultural inputs and the application of nitrogen fertilizers. Carbon sequestration encompasses the carbon absorption capacity of fruit trees, the promotion of soil organic carbon accumulation through grass cover, and soil carbon fixation under no-tillage management. Relevant conversion coefficients, such as 44/28 and 44/12, as well as the global warming potential of N₂O, were taken from [26,27,28].

The NF is based on the amount of nitrogen fertilizer applied, combined with its emission characteristics and related factors during volatilization, leaching, and runoff. The fruit’s nitrogen absorption is based on data from [29] and corrected using a 12% nitrogen leaching emission factor suitable for the study area. The phosphorus footprint (PF) considers phosphorus loss from soil erosion, surface runoff, and leaching following phosphorus fertilizer application. Relevant emission factors are estimated based on regional soil characteristics and existing research, as detailed in Appendix A,

Table A1. Emission factors in agricultural production processes.

Source	Unit	Emission Factor	Reference
Synthetic N fertilizer	t CO2-eq t−1	8.3	[30]
Synthetic P2O5 fertilizer	t CO2-eq t−1	2.33	[31]
Synthetic K2O fertilizer	t CO2-eq t−1	0.66	[31]
Pesticide	t CO2-eq t−1	18	[32]
Diesel fuel	t CO2-eq t−1	3.7	[33]
Fruit bag	t CO2-eq t−1	1.54	[33]
Synthetic N fertilizer	kg NH3 kg−1	0.01110	[34]
Synthetic P2O5 fertilizer	kg NH3 kg−1	0.00100	[34]
Synthetic K2O fertilizer	kg NH3 kg−1	0.00484	[34]
Pesticide	kg NH3 kg−1	0.00119	[34]
Fruit bag	kg NH3 kg−1	0.00001	[35]
Synthetic P2O5 fertilizer	kg P kg−1	0.00001090	[34]
Pesticide	kg P kg−1	0.00059579	[34]
Fruit bag	kg P kg−1	0.00000029	[35]

,

Table A2. Emission factors of fertilizer application.

Source	Unit	Emission Factor	Reference
N2O emission from N fertilizer	%	1.20	[36]
NH3 emission from N fertilizer	%	10.80	[37]
N leaching	%	12.00	[38]
N erosion	%	0.90	[38]
N runoff	%	15.75	[38]
P leaching	%	0.096	[38]
P erosion	%	6.30	[38]
P runoff	%	1.575	[38]

and

Table A3. Proportional factors of carbon sequestration.

Source	Unit	Emission Factor	Reference
C from grass returned	t C ha−1 year−1	0.40	[27]
C absorption by trees	t C ha−1 year−1	0.27	[27]
C increased by no-tillage management	t C ha−1 year−1	0.25	[28]

[27,28,30,31,32,33,34,35,36,37,38].

In summary, this study developed an integrated accounting framework that combines economic benefits with environmental costs, systematically considering the impact of various stages of pomelo agricultural production on resource utilization, environmental emissions, and ecosystem services. The specific calculation formulas and parameter settings are detailed in Appendix A and Appendix C, and the data sources are based on the latest research findings and field data from the study area.

2.5. Comprehensive Evaluation and Structural Equation Modeling

To systematically evaluate the comprehensive performance of different fertilization regimes across multiple indicators, including yield, nutrient efficiency, economic benefits, and environmental footprint, this study employs the EWM to construct a CEI. Additionally, partial least squares structural equation modeling (PLS-SEM) is used to analyze the mechanisms through which various influence pathways affect the EEB. The specific methods are described below.

2.5.1. Construction of the Comprehensive Evaluation Index System and Entropy Weight Method Calculation

This study selects seven representative indicators as evaluation factors, including yield, nitrogen fertilizer partial productivity (PFPN), economic benefits (PE), EEB, CF, NF, and phosphorus footprint (PF). The following steps are used to calculate the CEI. Data normalization: To eliminate the impact of dimensional differences, range normalization is performed based on the attribute of each indicator (positive or negative).

Formula for normalizing positive indicators:

$${\mathrm{X}}_{\mathrm{i}\mathrm{j}}^{\mathrm{\prime }}={\displaystyle \frac{({\mathrm{X}}_{\mathrm{i}\mathrm{j}}-\mathrm{m}\mathrm{i}\mathrm{n}({\mathrm{X}}_{\mathrm{j}}\left)\right)}{\left(\mathrm{m}\mathrm{a}\mathrm{x}\right({\mathrm{X}}_{\mathrm{j}})-\mathrm{m}\mathrm{i}\mathrm{n}({\mathrm{X}}_{\mathrm{j}}\left)\right)}}$$

(1)

Formula for normalizing negative indicators:

$${\mathrm{X}}_{\mathrm{i}\mathrm{j}}^{\mathrm{\prime }}={\displaystyle \frac{\mathrm{m}\mathrm{a}\mathrm{x}\left({\mathrm{X}}_{\mathrm{j}}\right)-{\mathrm{X}}_{\mathrm{i}\mathrm{j}}}{\mathrm{m}\mathrm{a}\mathrm{x}\left({\mathrm{X}}_{\mathrm{j}}\right)-\mathrm{m}\mathrm{i}\mathrm{n}\left({\mathrm{X}}_{\mathrm{j}}\right)}}$$

(2)

where X_ij represents the original value of the j-th indicator in the i-th sample, and X’_ij represents the normalized value.

Calculation of the indicator weight, p_ij:

$${\mathrm{p}}_{\mathrm{i}\mathrm{j}}={\displaystyle \frac{{\mathrm{X}}_{\mathrm{i}\mathrm{j}}^{\mathrm{\prime }}}{{\sum }_{\mathrm{i}=1}^{\mathrm{n}}{\mathrm{X}}_{\mathrm{i}\mathrm{j}}^{\mathrm{\prime }}}}$$

(3)

The weight reflects the relative distribution of the indicator within the sample.

Information entropy, e_j:

$${\mathrm{e}}_{\mathrm{j}}=-\mathrm{k}{\sum }_{\mathrm{i}=1}^{\mathrm{n}}{\mathrm{p}}_{\mathrm{i}\mathrm{j}}\mathrm{\ast }\mathrm{l}\mathrm{n}\left({\mathrm{p}}_{\mathrm{i}\mathrm{j}}\right)$$

(4)

Here, $\mathrm{k}={\displaystyle \frac{1}{\mathrm{l}\mathrm{n}\left(\mathrm{n}\right)}}$, where n represents the sample size. A smaller entropy value indicates greater variability in the indicator and richer information content.

Calculation of redundancy, d_j, and indicator weight, w_j:

$${\mathrm{d}}_{\mathrm{j}}=1-{\mathrm{e}}_{\mathrm{j}}$$

(5)

$${\mathrm{w}}_{\mathrm{j}}={\displaystyle \frac{{\mathrm{w}}_{\mathrm{j}}}{{\sum }_{\mathrm{j}=1}^{\mathrm{m}}{\mathrm{d}}_{\mathrm{j}}}}$$

(6)

A larger redundancy value indicates a stronger discriminative ability of the indicator and corresponds to a higher weight.

Calculation of the CEI:

$$\mathrm{C}\mathrm{E}{\mathrm{I}}_{\mathrm{i}}={\sum }_{\mathrm{j}=1}^{\mathrm{m}}{\mathrm{w}}_{\mathrm{j}}\times {\mathrm{X}}_{\mathrm{i}\mathrm{j}}^{\mathrm{\prime }}$$

(7)

where CEI_i represents the comprehensive benefit score of the i-th fertilization regime. The CEI serves as a quantitative indicator to comprehensively assess the environmental and economic performance of each regime.

2.5.2. PLS-SEM

To further elucidate the direct and indirect mechanisms through which different fertilization regimes influence the EEB, including pathways such as soil nutrient regulation, nitrogen-use efficiency improvement, and adjustment of agricultural inputs, a PLS-SEM model was constructed. Modeling and path coefficient fitting were conducted using the piecewiseSEM package in R. The model fit quality was evaluated using the goodness-of-fit (GOF) index and the coefficient of determination (R²) for each variable. Significant pathways (p < 0.05) reveal how various optimized fertilization strategies regulate the EEB under the dual objectives of emission reduction and efficiency improvement, providing mechanistic support for the green development of orchards.

2.6. Statistical Analysis

Preliminary data processing was performed using Microsoft Excel 2016 (Microsoft Corp., Redmond, WA, USA). One-way analysis of variance (ANOVA) was conducted to evaluate differences among regimes, followed by Duncan’s multiple range test for post hoc comparisons at a significance level of p < 0.05. This level was considered appropriate given the number of replicates (n = 3) and expected effect sizes. All statistical analyses were carried out using SPSS Statistics version 26.0 (IBM Corp., Armonk, NY, USA). The data in all figures are presented as mean ± standard error (SE) to indicate variability among replicates and support significance interpretation. PLS-SEM was performed using the piecewiseSEM package in R version 4.2.2 (R Foundation for Statistical Computing, Vienna, Austria) to evaluate the direct and indirect effects of fertilization regimes on the EEB through variables including nitrogen input, soil nutrient availability, environmental costs, and yield. Standardized path coefficients were estimated to assess the strength and direction of relationships among these variables. Figures and graphs were generated using Microsoft PowerPoint 2019 (Microsoft Corp., Redmond, WA, USA), GraphPad Prism version 10 (GraphPad Software, San Diego, CA, USA), and Origin version 2021 (OriginLab Corporation, Northampton, MA, USA).

3. Results

3.1. Impact of Different Fertilization Regimes on Pomelo Yield

Over two consecutive years (2022–2023), under different fertilization regimes, compared to the A regime, the B, C, and D regimes all led to improvements in pomelo yield indicators (Figure A2). Specifically, in 2022, the average fruit weight increased from 1.21 kg (A) to 1.39 kg (B), 1.39 kg (C), and 1.43 kg (D), corresponding to increases of 14.9%, 14.9%, and 18.5% (p < 0.05), respectively. Likewise, yield increased from 80.90 t ha⁻¹ (A) to 99.88 (B), 93.87 (C), and 100.19 t ha⁻¹ (D), representing significant improvements of 23.5%, 16.0%, and 23.9% (p < 0.05), respectively. Moreover, significant differences (p < 0.05) were observed between all three optimized fertilization regimes and the A regime. These results indicate that the B, C, and D regimes all contributed to increased pomelo yields, with the D regime showing the greatest yield improvement.

3.2. Effects of Different Fertilization Regimes on Soil Physicochemical Properties

The fertilization regimes exerted significant effects on the soil physicochemical properties in the pomelo orchard over the two-year period (2022–2023) (Figure A3). Compared with regime A, regimes C and D had significantly reduced soil nitrogen (N), phosphorus (P), and potassium (K) contents at both the 0~20 cm and 20~40 cm depths (p < 0.05).

In particular, in 2022, at the 0~20 cm layer, ammonium nitrogen and nitrate nitrogen contents under regime A were 10.82 mg kg⁻¹ and 7.57 mg kg⁻¹, respectively, which significantly declined to 3.30–4.00 mg kg⁻¹ and 1.06–3.11 mg kg⁻¹ under regimes C and D, representing reductions of 63.0~69.5% and 58.9~86.0% (p < 0.05). Available phosphorus decreased from 179.52 mg kg⁻¹ (A) to 138.79 (C) and 132.05 mg kg⁻¹ (D), reductions of 22.7% and 26.4% (p < 0.05), while available potassium dropped from 91.16 mg kg⁻¹ to 42.36 and 40.37 mg kg⁻¹ under the same regimes, reductions of 53.7% and 55.7% (p < 0.05). In 2023, similar trends were observed. At the 0~20 cm layer, available phosphorus declined from 195.33 mg kg⁻¹ (A) to 143.59 mg kg⁻¹ (C) and 126.64 mg kg⁻¹ (D), corresponding to significant reductions of 26.5% and 35.2% (p < 0.05). Available potassium decreased from 103.33 mg kg⁻¹ to 87.33 mg kg⁻¹ (C) and 57.33 mg kg⁻¹ (D), that is, by 15.5% and 44.5%, respectively (p < 0.05). Notably, similar patterns of nutrient reduction were also observed at the 20~40 cm depth across both years (p < 0.05).

Meanwhile, soil pH and water-soluble organic carbon (WSOC) levels significantly improved under the optimized regimes. At 0~20 cm in 2022, soil pH increased from 4.48 (A) to 5.23 and 5.30 in regimes C and D, respectively, increases of 16.7% and 18.3% (p < 0.05). WSOC content increased significantly from 163.55 mg kg⁻¹ (A) to 328.50 mg kg⁻¹ (C) and 339.37 mg kg⁻¹ (D), more than doubling under the optimized regimes (p < 0.05). In 2023, at the 0~20 cm depth, regime D had a significantly increased soil pH and WSOC compared with regime A. Specifically, pH increased from 4.38 (A) to 5.16 (D), representing a 17.8% increase, while WSOC rose from 180.54 mg kg⁻¹ to 309.74 mg kg⁻¹, nearly doubling, with a 71.6% increase (p < 0.05). Similar significant enhancements were also observed at the 20~40 cm depth in both years (p < 0.05).

Furthermore, the exchangeable calcium (Ca²⁺) and magnesium (Mg²⁺) contents were significantly elevated under regimes C and D. In 2022, at 0~20 cm, exchangeable Ca increased from 207.0 mg kg⁻¹ (A) to 541.0 (C) and 590.73 mg kg⁻¹ (D), while Mg increased from 48.47 mg kg⁻¹ to 108.93 (C) and 120.6 mg kg⁻¹ (D), significant increases of 124.7% and 148.8%, respectively (p < 0.05). In 2023, regime D further elevated Ca and Mg concentrations to 539.3 and 75.17 mg kg⁻¹, corresponding to increases of 202.2% and 93.9% compared with regime A (p < 0.05). Similar improvements in exchangeable Ca and Mg were observed at the 20~40 cm depth in both years (p < 0.05).

Overall, the reduced and optimized fertilization regimes, especially regime D, effectively mitigated excessive soil N, P, and K accumulation, while significantly enhancing soil pH, exchangeable Ca and Mg concentrations, and WSOC levels across both soil layers and years. These findings highlight the potential of optimized nutrient management to improve soil quality in subtropical orchard systems.

3.3. Comparative Analysis of Economic Benefits and Environmental Costs Under Different Fertilization Regimes

The different fertilization regimes resulted in significant variations in input costs, economic returns, and environmental impacts in the pomelo orchards (Figure 2a–f).

Specifically, in terms of inputs, regime A incurred higher labor costs due to more frequent weeding, while regime C exhibited increased application costs due to the use of fertigation systems. In contrast, regimes C and D achieved cost savings in pesticide inputs, which contributed to their relative advantage in overall input costs. Based on the two-year average input costs (Figure 2c,d), regime A recorded the highest cost at CNY 140,100 ha⁻¹, whereas regime D had the lowest at CNY 74,000 ha⁻¹, representing a reduction of CNY 66,100, CNY 13,300, and CNY 44,700 ha⁻¹ compared to regimes A, B, and C, respectively.

In terms of output (Figure 2a,b), the optimized fertilization regimes significantly improved both pomelo yield and economic performance. Compared with regime A, the average yield over two years increased by 13.9%, 12.1%, and 14.6% under regimes B, C, and D, respectively; while the average economic benefits rose by 186.4%, 103.0%, and 220.5%. Among all regimes, regime D yielded the highest comprehensive benefit, with an average net return of CNY 166,600 ha⁻¹, which was CNY 92,700 ha⁻¹ higher than regime A, and 11.9% and 57.9% greater than regimes B and C, respectively. The benefit–cost ratios were ranked as follows: D (2.81:1) > B (2.37:1) > C (1.72:1) > A (1.30:1), with regime D achieving the highest benefit–cost ratio.

Regarding environmental effects, the ecological costs associated with each fertilization strategy varied significantly (Figure 2f). Due to optimized nitrogen input, regimes C and D notably reduced greenhouse gas emissions, acidification potential, eutrophication potential, and human health risks. Compared to regimes A and B, the average cost reduction from greenhouse gas emissions under regimes C and D was CNY 1800 ha⁻¹; the costs associated with soil acidification remediation were reduced by 76.5%, eutrophication remediation by 81.8%, and human health impacts by 75.9% and 79.3%, respectively. From the perspective of EEB (Figure 2e), the two-year average EEB of regimes B, C, and D increased by CNY 78,300, CNY 49,400, and CNY 100,700 ha⁻¹ relative to regime A, with regime D exhibiting the largest increase of 297.5%.

In summary, regime D achieved the highest economic return while significantly reducing environmental costs, demonstrating strong potential for application in sustainable and efficient pomelo cultivation.

Figure 2. Economic and environmental benefits under different fertilization regimes. (a) Revenue under each fertilization regime in 2022 and 2023; (b) economic benefit under each fertilization regime in 2022 and 2023; (c) agricultural input costs in 2022; (d) agricultural input costs in 2023; (e) net ecosystem economic benefit (EEB) in 2022 and 2023; (f) environmental costs in 2022 and 2023. Note: A refers to the conventional farming regime with no mulching, B refers to the conventional farming regime with mulching, C refers to the optimized fertilization regime with water–fertilizer integration, and D refers to the optimized fertilization regime with controlled-release fertilizers. Different letters indicate significant differences between regimes (p < 0.05, LSD test). Error bars represent the standard error of the three replicates.

Figure 2. Economic and environmental benefits under different fertilization regimes. (a) Revenue under each fertilization regime in 2022 and 2023; (b) economic benefit under each fertilization regime in 2022 and 2023; (c) agricultural input costs in 2022; (d) agricultural input costs in 2023; (e) net ecosystem economic benefit (EEB) in 2022 and 2023; (f) environmental costs in 2022 and 2023. Note: A refers to the conventional farming regime with no mulching, B refers to the conventional farming regime with mulching, C refers to the optimized fertilization regime with water–fertilizer integration, and D refers to the optimized fertilization regime with controlled-release fertilizers. Different letters indicate significant differences between regimes (p < 0.05, LSD test). Error bars represent the standard error of the three replicates.

3.4. Effects of Different Fertilization Regimes on the Environmental Footprint of Pomelo Orchards

The different fertilization regimes exerted significant effects on the CF, NF, and PF of the pomelo orchards (Figure 3a–i). Compared with regime A, the three optimized regimes (regimes B, C, and D) significantly reduced the carbon emissions from orchard production during 2022–2023 (Figure 3a). Specifically, in 2022, the CF under regimes B, C, and D was reduced by 21.7%, 74.8%, and 76.5%, respectively, compared with regime A. In 2023, the corresponding reductions were 6.7%, 72.4%, and 71.6% (p < 0.05). Based on the two-year averages (Figure 3d,g), regime A exhibited the highest annual carbon emissions at 30.85 t CO₂-eq ha⁻¹, while regimes B, C, and D reduced emissions by 1.28, 21.74, and 21.79 t CO₂-eq ha⁻¹, respectively. Among them, regime D achieved the greatest reduction, with a 70.6% decrease (p < 0.05).

Similarly, notable reductions were observed in nitrogen footprint under optimized fertilization (Figure 3b). In 2022, the NF under regimes B, C, and D decreased by 19.0%, 82.2%, and 83.4%, respectively, compared with regime A. In 2023, the reductions were 2.6%, 80.7%, and 80.1% (p < 0.05). Based on annual average data (Figure 3e,h), regime A showed the highest nitrogen emissions at 416.84 kg N ha⁻¹. In contrast, regimes C and D reduced nitrogen discharge by 330.57 and 330.79 kg N ha⁻¹, respectively, both exceeding a 79% reduction rate, with regime D showing the most significant effect (p < 0.05).

The phosphorus footprint also exhibited a consistent downward trend (Figure 3c). In 2022, the PF under regimes B, C, and D decreased by 15.3%, 100%, and 100%, respectively, compared with regime A. In 2023, regimes C and D continued to achieve complete phosphorus emission elimination (p < 0.05). In terms of annual average PF (Figure 3f,i), regime A had the highest emissions at 73.87 kg P ha⁻¹, while both regimes C and D reduced emissions by 73.80 kg P ha⁻¹, achieving near-total mitigation.

In summary, the optimized fertilization regimes, particularly the regime designated as D, not only significantly improved nitrogen-use efficiency but also effectively reduced the environmental footprint across the carbon, nitrogen, and phosphorus dimensions. These findings highlight their potential in facilitating low-carbon and sustainable transformation in pomelo orchard systems.

3.5. Responses of PFPN and Environmental Footprints to Different Fertilization Regimes in Pomelo Orchards

The different fertilization regimes had significant effects on the PFPN in the pomelo orchards (Figure A4a). Compared with the conventional fertilization regime (A), the optimized regimes (C and D) significantly enhanced nitrogen-use efficiency. In 2022, the PFPN under regimes C and D increased by 403.1% and 437.0%, respectively, relative to regime A (p < 0.05). In 2023, the corresponding increases were 365.6% and 349.2% (p < 0.05). These findings suggest that optimized fertilization strategies can substantially reduce nitrogen input per unit yield while maintaining high productivity, thereby improving resource-use efficiency.

Regarding environmental performance, regression analyses revealed that the CF, NF, and PF all declined significantly with increasing PFPN (Figure A4b–d, p < 0.001). Notably, the regression slopes in 2023 were generally steeper than those in 2022, indicating that higher nitrogen-use efficiency led to more substantial reductions in environmental burdens per unit of yield. Specifically, for each unit increase in PFPN, the marginal decreases in the CF, NF, and PF became more pronounced, underscoring the vital role of efficient nitrogen management in mitigating greenhouse gas emissions and nutrient losses.

In conclusion, the optimized fertilization regimes significantly enhanced the PFPN and reduced environmental footprints, providing strong scientific support for the sustainable and efficient development of pomelo orchards.

3.6. Mechanisms of Different Fertilization Regimes on the EEB of Pomelo Orchards

PLS-SEM was employed to evaluate the EEB of pomelo orchards under the different fertilization regimes. The model exhibited a high goodness of fit (GOF > 0.70), indicating satisfactory model performance (Figure 4). The results identified economic benefit (PE) and environmental cost (ECE) as the two primary determinants of pomelo EEB. Specifically, reduced fertilization regimes showed significant negative correlations with soil nutrient availability (nutrients), nitrogen input (N input), and agricultural input cost (EC) (p < 0.001). A significant positive relationship was observed between nitrogen input and environmental cost (p < 0.001), suggesting that nitrogen application substantially increases environmental burdens, thereby exerting a negative impact on EEB. In contrast, economic benefit was positively correlated with yield (p < 0.001) and negatively correlated with agricultural input cost (p < 0.001), indicating that yield improvement enhances economic return, while reduced input costs further contribute to increased profitability. The pomelo EEB was jointly influenced by the environmental cost associated with nitrogen application and the economic return. The optimized fertilization regimes, by effectively regulating soil nutrient availability, yield, and agricultural input levels, significantly improved economic benefit while simultaneously reducing environmental costs, ultimately leading to an enhanced EEB. Overall, optimized fertilization not only increased pomelo yield but also contributed to sustainable orchard development by lowering input costs and minimizing environmental impacts.

3.7. Comprehensive Evaluation of Economic and Ecological Benefits of Different Fertilization Regimes in Pomelo Orchards

To comprehensively evaluate the impact of different fertilization regimes on pomelo production, this study establishes a comprehensive assessment system based on seven key indicators: Yield, PFPN, EEB, PE, CF, NF, and PF. The results demonstrate that different fertilization regimes have significant effects on pomelo production efficiency and ecological performance across multiple dimensions (Figure 5a,c). While the A regime shows a certain advantage in yield, its PFPN, PE, and EEB are relatively low, with higher carbon, nitrogen, and phosphorus footprints, indicating considerable environmental pressure. The B regime shows improvements in environmental footprints compared to the A regime, but its resource-use efficiency and economic returns remain suboptimal, yielding moderate performance overall. In contrast, both the C and D regimes exhibit superior resource utilization efficiency and ecological benefits. Both significantly reduce CF, NF, and PF while substantially increasing PFPN and PE levels. Notably, the D regime shows the lowest environmental footprints and the highest PFPN and PE values in both 2022 and 2023, with its EEB also significantly surpassing the other regimes, reflecting strong ecological and economic synergies. The CEI, calculated using the EWM, further confirms these trends (Figure 5b,d). The D regime consistently receives the highest CEI values in both years, followed by the C regime, and the A regime ranks the lowest. This indicates that the D fertilization regime is the most superior in terms of overall performance.

These results highlight that optimized fertilization strategies, especially the promotion of the D regime, not only effectively improve nitrogen-use efficiency and economic returns but also significantly reduce greenhouse gas emissions and nutrient loss, facilitating the high-yield, high-efficiency, and environmentally friendly development of the pomelo industry.

4. Discussion

4.1. Impact of Optimized Fertilization on Pomelo Yield

The results of this study indicate that all three optimized fertilization regimes effectively increased pomelo yield, with regime D demonstrating the most significant improvement. Fertilization management plays a crucial role in enhancing fruit yield and fertilizer-use efficiency in pomelo orchards. By tailoring nutrient supply structures to match both soil fertility conditions and pomelo nutrient uptake characteristics, optimized fertilization can ensure stable and increased production [39]. The superior yield performance observed under regime D is primarily attributed to the combined application of conventional and controlled-release nitrogen fertilizers. This strategy reduced nutrient losses and enhanced root nutrient uptake capacity, thereby improving nitrogen-use efficiency. Similar findings have been reported by Xiao et al. (2019), who found that the use of controlled-release nitrogen fertilizers promoted root system development in peach trees, resulting in denser and more concentrated root distribution, prolonged root lifespan, and ultimately improved fruit yield [40]. A meta-analysis by Liu et al. (2023) further confirmed that controlled-release fertilizers improved nitrogen-use efficiency by 10.0%, contributing to a 6% increase in rice yield [41]. These findings support the effectiveness of regime D in improving both crop yield and fertilizer efficiency. Regime B exhibited the second-highest yield increase. This may be attributed to the beneficial effects of grass cover, which reduces water and nutrient losses, improves soil structure, and increases soil organic matter and microbial biomass [42,43]. By improving the soil ecological environment, grass cover creates favorable conditions for crop growth and fruit production [44]. In contrast, the yield improvement under regime C was relatively limited. Irrigation and fertilization ratios are known to significantly influence crop yield, with optimal yield and nutrient–water-use efficiency observed only under full irrigation (1.0 ETc) and specific fertilization rates (300–120–60 kg ha⁻¹) [45]. It is speculated that the limited yield response observed in regime C may be attributed to the specific soil characteristics of the experimental region, which is an acidic sandy loam located near a river. These conditions may have weakened the nutrient–water coupling efficiency and increased nutrient loss, thereby constraining yield improvements. Overall, the three optimized fertilization regimes promoted stable pomelo production and increased fruit yield, providing scientific support for more efficient orchard management.

4.2. Effects of Optimized Fertilization on Soil Physicochemical Properties in Pomelo Orchards

Rational fertilization is essential for improving soil nutrient availability and promoting healthy crop growth [46]. This study demonstrated that the integration of green yield-enhancing technologies, including grass cover, calcium–magnesium supplementation, water–fertilizer integration, and controlled-release fertilization, significantly improved soil conditions and nutrient availability in pomelo orchards over the course of a two-year field experiment. Similarly, Huang et al. (2021) reported that optimizing NPK fertilizers in combination with lime, mushroom residue, and magnesium fertilizers enhanced root development in pomelo, thereby improving soil nutrient bioavailability [47].

Our findings revealed that regimes C and D resulted in significant reductions in soil nitrogen, phosphorus, and potassium contents. This decline was primarily due to substantial reductions in fertilizer application rates (nitrogen fertilizer decreased by 76.9%, potassium fertilizer by 77.9%, and phosphorus fertilizer by 100%), which effectively prevented the accumulation of mineral nutrients in the soil [48]. Compared with the conventional regime A, regimes C and D significantly increased exchangeable calcium by 95.2~202.2% and magnesium by 18.8~93.9% in the 0~20 cm layer, with similar significant increases in deeper soil layers (p < 0.05). This improvement was attributed to the integration of water–fertilizer integration and controlled-release fertilization with soil amendments (e.g., oyster shell powder and magnesium sulfate), which alleviated soil acidification and minimized nutrient loss [49]. Notably, soil pH values were elevated under all three optimized fertilization regimes relative to regime A, with regimes C and D increasing soil pH by 0.35–0.82 units, indicating a significant increase in soil acidity (p < 0.05). Previous studies have shown that grass cover can alleviate soil acidification by increasing soil organic matter and improving soil structure [50]. In our study, soil pH under regime B increased by approximately 0.02 to 0.12 units relative to regime A, likely due to reduced inputs of acidic or physiologically acidifying fertilizers such as urea and potassium sulfate [51]. Furthermore, the co-application of soil enhancers (e.g., oyster shell powder and magnesium fertilizers) played a synergistic role in raising soil pH. Oyster shell powder contains alkaline substances such as Ca(OH)₂ or CaCO₃, which neutralize excess hydrogen ions (H⁺) in soils that have undergone long-term chemical fertilization, thus mitigating acidification [52]. Magnesium fertilizers further enhance soil structure and promote calcium uptake, thereby increasing the availability of exchangeable Ca and Mg in the soil [53]. These effects support improved water and nutrient uptake, root growth, and acidification resistance [54,55]. In this study, the combination of grass cover, reduced fertilization, and calcium–magnesium supplementation under regimes C and D significantly improved soil pH, with the most pronounced effect observed under regime D. This improvement is likely related to the controlled-release nitrogen fertilizer, which provides a stable and sustained nitrogen supply, better aligned with pomelo nutrient demands [56].

In conclusion, the application of optimized fertilization strategies, including controlled-release fertilizers, water–fertilizer integration, and grass cover, effectively improved soil physicochemical properties and nutrient availability. These findings provide a scientific basis for enhancing pomelo production and fruit quality. It is important to emphasize that fertilization strategies should be dynamically adjusted based on regional soil characteristics and crop nutrient demands.

4.3. Effects of Optimized Fertilization on Economic Benefits

Economic returns and production costs are primary drivers influencing farmers’ decisions on orchard management practices. In pomelo cultivation, traditional management is often associated with high production costs, requiring substantial labor and resource inputs to ensure high yields and fruit quality [57]. In contrast, optimized fertilization strategies rationalize resource allocation, effectively reducing production costs and nitrogen losses while also mitigating environmental impacts, thereby enhancing overall economic profitability.

Among the four fertilization regimes evaluated in this study, regime D exhibited the highest economic benefit. This regime integrated multiple optimized measures, including fertilizer reduction, calcium–magnesium supplementation, grass cover, and the application of controlled-release nitrogen fertilizers. These practices substantially reduced both labor and fertilizer costs, resulting in the lowest overall production inputs. The one-time application of controlled-release nitrogen fertilizer minimized labor requirements and the frequency of fertilization, offering multiple advantages, including labor savings, cost reduction, and increased efficiency [7,58]. Regime B ranked second in terms of economic benefit. This outcome was driven by lower input costs and improved soil conditions. Previous research has shown that cover cropping significantly increases soil microbial biomass and diversity, enhances nutrient cycling, and improves soil health, factors that are closely linked to improved crop performance and economic returns [59]. Moreover, grass cover provided a continuous nutrient source to fruit trees, reducing the need for external inputs while maintaining a favorable economic return. Regime C also demonstrated positive outcomes in stabilizing pomelo yield and reducing agricultural inputs, resulting in higher economic efficiency than the conventional management regime. However, its economic benefit was slightly lower than regimes D and B, largely due to higher technical implementation costs [60] and relatively limited yield improvements.

Overall, regime D achieved the highest economic benefit among all the fertilization strategies, followed by regime B. Future research should further explore the adaptability of regimes D and B under varying ecological conditions and incorporate long-term economic evaluations to refine fertilization strategies for sustainable orchard management.

4.4. Effects of Optimized Fertilization on EEB of Pomelo Orchards

EEB is a key indicator for evaluating the sustainability of agricultural production, reflecting the balance between high yield and environmental friendliness. This study demonstrated that, compared with regime A, all the optimized fertilization regimes significantly improved the pomelo EEB, with regime D showing the most pronounced effect. The superior performance of regime D can be attributed to its use of a one-time application of controlled-release nitrogen fertilizer, which effectively reduced both chemical fertilizer input and labor costs, while also lowering environmental costs. This approach successfully achieved the dual goals of yield improvement and ecological protection. Previous studies have shown that the slow-release characteristics of controlled-release fertilizers can effectively reduce nitrogen losses and greenhouse gas emissions (particularly N₂O), while enhancing root nutrient absorption, thereby minimizing nutrient losses to the environment [40]. Moreover, the application of controlled-release fertilizers helps mitigate water eutrophication and soil acidification, thereby alleviating the negative environmental impacts of agricultural activities [55]. Regime B ranked second in terms of EEB. Studies have indicated that various mulching practices (e.g., straw or plastic mulch) can reduce annual greenhouse gas emission intensity by 20% to 106% [61]. In this study, grass cover not only improved soil fertility and modulated microbial community structure, but also suppressed pests and diseases, creating a favorable ecological environment for pomelo growth. Given its low input costs and environmental co-benefits, this approach exhibits strong potential for wide-scale adoption. Additionally, grass cover reduced soil erosion and promoted nutrient cycling, further enhancing soil ecological function. Regime C also contributed to higher nutrient-use efficiency by reducing fertilizer and water resource waste through precise fertigation. This regime effectively met the crop’s water and nutrient demands while minimizing nutrient loss. However, its relatively higher labor cost resulted in a slightly lower EEB compared to regimes D and B. These findings highlight the importance of considering labor costs as a critical factor in selecting appropriate optimization measures.

In summary, regime D emerged as the most effective strategy for improving the EEB in pomelo orchards, owing to its significant advantages in reducing both environmental and labor costs. It exhibits substantial potential for sustainable development in fruit production. Future research should focus on further lowering the cost of controlled-release fertilizers and improving their integration with other green management practices to establish more efficient and environmentally friendly agricultural systems.

4.5. Effects of Optimized Fertilization on the Environmental Footprint of Pomelo Production

Optimized fertilization management is a key strategy for reducing the environmental footprint of pomelo production systems. This study revealed that, compared with regime A, regimes C and D significantly reduced the environmental footprints of carbon, nitrogen, and phosphorus, thereby mitigating the multi-dimensional ecological pressures associated with agricultural production. The core mechanism of this reduction lies in the precise regulation of both total nutrient input and its spatial–temporal distribution, thus breaking away from the traditional paradigm of high input and low efficiency in fertilization.

In tropical and subtropical pomelo-growing regions of China, conventional fertilization practices are often characterized by the excessive application of N, P, and K fertilizers, which typically exceed crop demand by 30~50%. This surplus contributes to nutrient imbalances, greenhouse gas emissions, and nutrient loss, significantly increasing the risk of non-point source pollution [62,63]. Among the four fertilization regimes assessed in this study, regime D exhibited the best environmental performance. By utilizing polymer-coated controlled-release technology, this regime enabled the synchronized release of N, P, and K nutrients to match the dynamic nutrient demands across pomelo growth stages. This significantly improved fertilizer-use efficiency and reduced soil nutrient surplus, which is particularly beneficial in leaching-prone red-soil regions. For instance, Luo et al. (2023) reported that the use of controlled-release fertilizers in citrus orchards in South China reduced the apparent nitrogen loss rate from 42% to 28%, while enhancing P and K utilization efficiency and maintaining stable fruit yields [62]. Regime C relied on integrated fertigation systems to deliver multiple nutrients simultaneously, reducing excessive application and nutrient migration losses. Previous studies have shown that this technique can improve the overall utilization efficiency of N, P, and K to over 60%, while significantly lowering runoff losses and carbon emission intensity [64,65,66]. In contrast, regime B, while capable of reducing surface runoff by 10~15% through vegetation interception, failed to effectively control total nutrient input. As a result, its carbon footprint remained substantially higher than that of the optimized fertilization regimes [67]. Furthermore, excessive fertilization can suppress root development and reduce nutrient allocation efficiency [68], exacerbating nutrient loss fluxes and reinforcing a vicious cycle of high input, low efficiency, and high emissions.

In summary, the optimized fertilization regimes effectively reduced multiple environmental footprints of pomelo production by integrating nutrient input regulation, enhanced utilization efficiency, and loss minimization into a systemic mechanism. These approaches maintained stable yields while reducing environmental impacts, providing replicable and scalable pathways for the green and low-carbon transformation of subtropical orchards, particularly in red-soil regions.

4.6. Comprehensive Performance Evaluation and Optimal Fertilization Strategy Based on CEI

Multi-indicator comprehensive evaluation is a crucial approach for identifying green and efficient orchard management strategies. In this study, we established a CEI system incorporating key metrics including yield, PFPN, economic benefits, and EEB, as well as environmental footprints of carbon, nitrogen, and phosphorus. The EWM was employed to calculate the CEI and systematically assess the overall performance of four fertilization regimes. The results demonstrated a significant trade-off between environmental friendliness and economic efficiency across the different fertilization regimes. Regime D consistently achieved the highest CEI scores over two consecutive years, indicating superior multidimensional performance. By precisely regulating nutrient release and integrating practices such as calcium–magnesium supplementation and living mulch cover, this regime significantly improved fertilizer-use efficiency and yield levels. Simultaneously, it effectively controlled input costs and environmental burdens, achieving a balance between economic viability and ecological sustainability. These advantages underscore regime D’s strong overall competitiveness and long-term development potential. Regime C performed well in enhancing nitrogen-use efficiency, reducing nutrient losses, and alleviating environmental pressures. Relying on integrated fertigation systems, it enabled synchronized water and nutrient delivery, thereby improving resource-use efficiency. However, the high initial investment and maintenance costs associated with this regime limited its improvement in economic returns, resulting in a CEI score lower than that of the D regime. Regime B contributed positively to improving soil physicochemical properties and promoting ecological restoration, thereby supporting stable yield maintenance and partially mitigating environmental risks. However, due to the lack of optimized nutrient formulation, its nitrogen-use efficiency and input–output ratio remained suboptimal, ultimately limiting its overall performance across multiple key evaluation dimensions.

Furthermore, regime D exhibits strong potential for wider applicability across smallholder orchards and other agroecological zones. Its simplified, low-input design—including one-time application of controlled-release nitrogen fertilizer, grass cover, and supplemental use of oyster shell powder and magnesium sulfate—effectively reduces dependence on machinery and labor, making it particularly suitable for regions with limited technical infrastructure or workforce availability. The slow-release nutrient supply better matches the seasonal nutrient demands of pomelo trees, thereby improving fertilizer-use efficiency and yield stability, especially in subtropical acidic soils prone to nutrient leaching [14]. Although the present study did not include treatments isolating the effects of individual components, previous research has demonstrated the respective benefits of reduced fertilization [1], controlled-release nitrogen [14], and calcium–magnesium supplementation [2] in improving nutrient retention and enhancing soil carbon sequestration. Taken together with our findings, it is reasonable to infer that the combined application of these measures produces synergistic effects, particularly in red-soil environments. These characteristics reinforce regime D’ s suitability not only under current smallholder conditions, but also in other orchard systems facing similar agroecological constraints.

In summary, regime D demonstrated the most favorable comprehensive performance under the current smallholder-dominated pomelo orchard system, indicating its high priority for promotion.

5. Conclusions

This study systematically assessed four fertilization regimes (A, B, C, and D) over a three-year field trial, evaluating their impacts on pomelo yield, soil properties, economic returns, environmental footprints, and EEB. All three optimized regimes (B, C, and D) significantly outperformed the conventional regime A, highlighting their potential for sustainable orchard management. Among them, regime D showed the best overall performance by ensuring high yield, enhancing nitrogen-use efficiency, improving soil nutrient availability and economic returns, and markedly reducing the carbon, nitrogen, and phosphorus footprints, thereby maximizing the EEB. This regime, which combined controlled-release fertilization, grass cover, and calcium–magnesium supplementation, offered a balanced approach to resource efficiency and environmental sustainability, with high adaptability for broader application. Regime C also improved fertilizer efficiency and reduced emissions, but its higher input costs constrained its economic benefits. Regime B improved soil quality and yield stability yet remained limited in resource efficiency and emission mitigation. Overall, regime D emerged as the most effective strategy for achieving high yield, low environmental impact, and high efficiency in smallholder pomelo orchards.