Multi-Objective Nitrogen Optimization in Tea Cultivation: A Pathway to Achieve Sustainability in Cash Crop Systems

Abstract

Excessive nitrogen fertilizer application in tea plantations is a common problem that leads to soil acidification and reductions in both yield and economic returns. To evaluate the impacts of varying nitrogen input levels (0–600 kg N ha⁻¹ yr⁻¹) on yield, as well as environmental and economic outcomes, a two-year field experiment was conducted. Results demonstrated that when nitrogen application exceeded 150 kg N ha⁻¹ yr⁻¹, key efficiency indicators—agronomic efficiency (NAE), recovery efficiency (NRE), and partial factor productivity (NPFP) declined markedly. Among all treatments, the 150 kg N ha⁻¹ yr⁻¹ rate achieved the highest NRE at 28.01%. Moreover, environmental burdens including global warming potential, acidification, and eutrophication intensified with increasing nitrogen input. Benefit analysis revealed that yield-based economic returns peaked between 150 and 165 kg N ha⁻¹ yr⁻¹, whereas the highest ecosystem economic benefit, accounting for both profit and environmental cost, occurred at 120 kg N ha⁻¹ yr⁻¹. Thus, 120 kg N ha⁻¹ yr⁻¹ is identified as the optimal application rate for maximizing integrated economic and environmental returns while maintaining yield. These findings provide valuable guidance for promoting sustainable nitrogen management in tea cultivation worldwide.

1. Introduction

Nitrogen (N) plays a vital role in plant growth and has a substantial impact on both crop yield and quality [1]. In China, the consumption of synthetic nitrogen fertilizers increased from 9.43 million tons in 1980 to 25.7 million tons by 2020. Furthermore, the average nitrogen application rate per hectare of arable land has steadily risen over the past decades, reaching 154 kg N ha⁻¹ in 2020. While appropriate nitrogen input supports soil fertility and promotes sustainable agricultural production [2], excessive application—commonly observed in China—often exceeds crop requirements, thereby posing considerable threats to the environment and human health [3,4]. According to national estimates, only 28% of nitrogen fertilizers applied is utilized by crops [5], and the pollution loads of TN increased from 3.64 × 10⁶ t in 1978 to 1.05 × 10⁷ t tons in 2017 [6]. The goal of sustainable agriculture is to minimize adverse impacts to the environment while providing a sustained level of agriculture production [7]. Therefore, investigating the sustainable management strategy for N fertilizers is essential for maintaining food production, improving nitrogen use efficiency, and minimizing environmental effects, all of which are vital for both crop yield and environmental conservation.

Previous research has demonstrated that nitrogen management strategies employing active canopy sensor-based precision techniques have the potential to enhance nitrogen use efficiency while reducing environmental impacts [8]. Similarly, Singh reported that applying nitrogen at fixed times using tools such as leaf meters and leaf color charts not only achieved the highest grain yields but also improved nitrogen utilization efficiency in wheat cultivation [9]. Kumar further observed that determining fertilizer application rates based on the soil nitrogen bank approach could effectively lower environmental risks without compromising crop productivity [10]. Despite the promise of these advanced technologies, their adoption by farmers remains limited primarily due to a lack of access to information and insufficient technical understanding [11]. Therefore, there is a pressing need to develop nitrogen management strategies that address these knowledge gaps and practical constraints.

An effective starting point for nitrogen (N) management is to evaluate the relationship between N application rates and both crop yield and economic returns (i.e., income minus production costs) [12,13]. In maize, recent evidence showed that optimal nitrogen accumulation and remobilization under low-N conditions can synergistically improve yield and nitrogen-use efficiency, highlighting the potential of refining N management strategies [14]. In rice, yield continued to rise with increasing N inputs, with the greatest economic return was observed at an application rate of 263 kg ha⁻¹ [15]. While such practices are widely accepted in cereal crop production [16,17], economic crops still hold substantial potential for reducing N input based on yield and profit optimization. For example, synthetic N fertilizer use in intensive greenhouse vegetable systems could be reduced by approximately 40% without sacrificing productivity [18]. Similarly, recent studies on major cropping systems in China have revealed substantial mitigation potential, showing that net greenhouse gas emissions can be reduced through improved management practices and larger-scale land operations that enhance carbon emission productivity [19,20]. These findings highlight the value and applicability of N management strategies centered on yield and economic benefit optimization, particularly for high-value economic crops.

In recent years, growing attention has been directed toward the ecosystem-level economic benefits of nitrogen management, which incorporate production costs alongside environmental and human health considerations. This integrated approach has become central to advancing sustainable agricultural practices [21,22]. Life cycle assessment (LCA) has emerged as a valuable tool for identifying optimal nitrogen management strategies that minimize ecological impacts [23,24]. Many of those studies concentrated on cereal crops; for example, the experiments conducted by Ma showed that enhancing nitrogen management in wheat production can significantly boost economic benefits for the ecosystem [24], and Yao proposed that N deep placement enhanced net ecosystem economic benefit relative to N broadcasting in rice fields [22]. But observation of the cash crop system is locking or was based on organic substitution management [25]. Additionally, the primary focus was on the impacts of various types of fertilizers, including fast-release and slow-release fertilizers [26]. Tea (Camellia sinensis L.) has a long history as a beverage and holds significant economic value in China and many other countries [27]. Anxi County is a major region for tea cultivation in China, with over 40,000 hectares of land dedicated to tea farming, contributing more than 2.93% of the national total tea production [28]. Tea production has become the main agricultural sector in this region, with more than 70% of farmers involved in the tea industry. However, it has encountered issues such as adverse effects on the environment and health risks for people [29,30]. These problems stem from excessive fertilization practices, particularly the over-application of nitrogen fertilizer, which averages 553 kg per hectare per year, far exceeding what is necessary for optimal yields [31].

To explore the comprehensive effects of nitrogen fertilizer application on tea yield, yield benefit, and ecosystem economic benefits in acidic red soil, we set up different nitrogen application treatments and conducted a systematic evaluation in two growing seasons. We proposed two hypotheses: (1) There is an optimal threshold for nitrogen fertilizer application and tea yield and yield benefit: increasing the application amount will increase yield and benefit within a certain range, but the promotion effect will weaken or turn negative after exceeding the threshold. (2) Ecosystem economic benefits and nitrogen fertilizer application have an inverted “U” relationship: when the application amount is low, the benefit increases with the application amount, and after exceeding the optimal level, the environmental cost caused by excessive nitrogen fertilizer increases, and the benefit begins to decline.

2. Materials and Methods

2.1. Experiment Site

The field study was carried out at a representative oolong tea production site in Anxi County (23.16° N, 118.15° E), Quanzhou City, located in the southwestern region of Fujian Province, China (Figure 1). The selected tea plantations had been under continuous cultivation for over a decade. The experiment spanned two growing seasons (2019–2021) and formed part of a long-term research project initiated in 2017. The site is characterized as a southern subtropical monsoon climate, which corresponds to the Cfa type (humid subtropical climate) in the Köppen–Geiger classification system, with an average annual temperature of 20.3 °C. Total precipitation amounted to 1349 mm and 1313 mm during the 2019–2020 and 2020–2021 growing seasons, respectively (Figure 2). The soil was classified as an acidic red soil in the Chinese Soil Taxonomy, corresponding to Ferralsols in the WRB 2023 system, with a clay loam texture, and the baseline properties of the 0–20 cm topsoil layer included pH 4.02, organic matter content of 23.0 g kg⁻¹, ammonium nitrogen 18.59 mg kg⁻¹, nitrate nitrogen 17.25 mg kg⁻¹, available phosphorus 108.10 mg kg⁻¹, and available potassium 171.22 mg kg⁻¹.

2.2. Experimental Design

The experiment adopted a completely randomized block design with five nitrogen fertilizer treatment groups and three replicates for each treatment. The spacing between tea trees in the tea garden was 0.35 m × 0.85 m (intra-row and inter-row distances). Each plot measured 20 m². Nitrogen, phosphorus, and potassium were supplied using urea (46% N), superphosphate (20% P₂O₅), and potassium sulfate (60% K₂O), respectively, and applied during each tea growing season. Urea is applied as a solid and applied to the soil. The five nitrogen application levels were set at 0, 150, 300, 450, and 600 kg N ha⁻¹ yr⁻¹, while phosphorus and potassium were consistently applied at 100 kg P₂O₅ ha⁻¹ yr⁻¹ and 125 kg K₂O ha⁻¹ yr⁻¹ across all treatments. These treatments were designated as N0, N150, N300, N450 (reflecting local farming practices), and N600.The selected N input levels (0–600 kg N ha⁻¹ yr⁻¹) reflect both local farmer practices (≈450 kg N ha⁻¹ yr⁻¹) and reduction scenarios recommended in regional guidelines and previous studies. Details regarding conventional tea cultivation management, including fertilization timing and rates, pruning, and harvesting schedules, are provided in Figure 3.

2.3. Measurements and Calculation

2.3.1. Tea Sampling and N Analysis

The bud density (tea bud number within 0.1 m²), hundred-bud weight, and fresh weight yield during harvest in each plot were determined for all tea samples. For tea shoots N analysis, dry sample was first digested at 180~250 °C in H₂SO₄-H₂O₂ at 180~250 °C and then was analyzed using a continuous flow analyzer (AA3, Bran+Luebbe, Hamburg, Germany) [32]. Tea N uptake was calculated based on the sum of the dry matter and N concentration of plant parts.

2.3.2. Estimation of N Use Efficiency

Nitrogen use efficiency is typically assessed through three commonly applied indicators: agronomic efficiency (NAE), recovery efficiency (NRE), and partial factor productivity (NPFP). These indicators were computed using the following equations [33]:

$$\mathrm{N}\mathrm{A}\mathrm{E} (\mathrm{k}\mathrm{g} {\mathrm{k}\mathrm{g}}^{-1})={\displaystyle \frac{\mathrm{T}\mathrm{e}\mathrm{a} \mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d} \mathrm{i}\mathrm{n} \mathrm{N} \mathrm{f}\mathrm{e}\mathrm{r}\mathrm{t}\mathrm{i}\mathrm{l}\mathrm{i}\mathrm{z}\mathrm{e}\mathrm{d}-\mathrm{T}\mathrm{e}\mathrm{a} \mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d} \mathrm{w}\mathrm{i}\mathrm{t}\mathrm{h}\mathrm{o}\mathrm{u}\mathrm{t} \mathrm{N} \mathrm{f}\mathrm{e}\mathrm{r}\mathrm{t}\mathrm{i}\mathrm{l}\mathrm{i}\mathrm{z}\mathrm{e}\mathrm{d}}{\mathrm{T}\mathrm{h}\mathrm{e} \mathrm{a}\mathrm{m}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{t} \mathrm{o}\mathrm{f} \mathrm{N} \mathrm{f}\mathrm{e}\mathrm{r}\mathrm{t}\mathrm{i}\mathrm{l}\mathrm{i}\mathrm{z}\mathrm{e}\mathrm{r} \mathrm{a}\mathrm{p}\mathrm{p}\mathrm{l}\mathrm{i}\mathrm{e}\mathrm{d}}}$$

$$\mathrm{N}\mathrm{R}\mathrm{E} (\mathrm{\%})={\displaystyle \frac{\mathrm{T}\mathrm{e}\mathrm{a} \mathrm{N} \mathrm{u}\mathrm{p}\mathrm{t}\mathrm{a}\mathrm{k}\mathrm{e} \mathrm{i}\mathrm{n} \mathrm{N} \mathrm{f}\mathrm{e}\mathrm{r}\mathrm{t}\mathrm{i}\mathrm{l}\mathrm{i}\mathrm{z}\mathrm{e}\mathrm{d}-\mathrm{T}\mathrm{e}\mathrm{a} \mathrm{N} \mathrm{u}\mathrm{p}\mathrm{t}\mathrm{a}\mathrm{k}\mathrm{e} \mathrm{w}\mathrm{i}\mathrm{t}\mathrm{h}\mathrm{o}\mathrm{u}\mathrm{t} \mathrm{N} \mathrm{f}\mathrm{e}\mathrm{r}\mathrm{t}\mathrm{i}\mathrm{l}\mathrm{i}\mathrm{z}\mathrm{e}\mathrm{d}}{\mathrm{T}\mathrm{h}\mathrm{e} \mathrm{a}\mathrm{m}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{t} \mathrm{o}\mathrm{f} \mathrm{N} \mathrm{f}\mathrm{e}\mathrm{r}\mathrm{t}\mathrm{i}\mathrm{l}\mathrm{i}\mathrm{z}\mathrm{e}\mathrm{r} \mathrm{a}\mathrm{p}\mathrm{p}\mathrm{l}\mathrm{i}\mathrm{e}\mathrm{d}}}\times 100\mathrm{\%}$$

$$\mathrm{N}\mathrm{P}\mathrm{F}\mathrm{P} (\mathrm{k}\mathrm{g} {\mathrm{k}\mathrm{g}}^{-1})={\displaystyle \frac{\mathrm{T}\mathrm{e}\mathrm{a} \mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}}{\mathrm{T}\mathrm{h}\mathrm{e} \mathrm{a}\mathrm{m}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{t} \mathrm{o}\mathrm{f} \mathrm{N} \mathrm{f}\mathrm{e}\mathrm{r}\mathrm{t}\mathrm{i}\mathrm{l}\mathrm{i}\mathrm{z}\mathrm{e}\mathrm{r} \mathrm{a}\mathrm{p}\mathrm{p}\mathrm{l}\mathrm{i}\mathrm{e}\mathrm{d}}}$$

2.3.3. System Boundary and Nitrogen Losses

The system boundary was defined around the tea production phase, encompassing the life cycle stages related to the manufacture and transportation of agricultural inputs such as chemical fertilizers, pesticides, and diesel. Nitrogen losses were categorized into three primary pathways: surface runoff and leaching, ammonia (NH₃) volatilization, and nitrous oxide (N₂O) emissions, with respective emission factors of 10.0%, 10.8%, and 1.2% [20]. The calculation of the carbon footprint of nitrogen input follows the methodology recommended by the IPCC Guidelines (2019), which is widely used in agricultural life cycle assessment studies. Specifically, published emission factors for N₂O emissions, NH₃ volatilization, and nitrate leaching are used [34].

2.3.4. Estimation of Environmental Impacts

Per hectare environmental impacts, including global warming potential (expressed as CO₂ equivalents), acidification potential (as SO₂ equivalents), and eutrophication potential (as PO₄ equivalents), were calculated by summing the equivalent contributions of individual inputs. These impact categories were quantified based on the following calculation formulas [20,35]:

$${\mathrm{E}\mathrm{I}}_{\mathrm{j}}=\sum _{\mathrm{i}=1}^{\mathrm{n}}\left({\mathrm{U}\mathrm{P}}_{\mathrm{i}\mathrm{j}}+{\mathrm{P}\mathrm{S}}_{\mathrm{i}\mathrm{j}}\right)\times {\mathrm{R}\mathrm{a}\mathrm{t}\mathrm{e}}_{\mathrm{i}}$$

Here, EI_j denotes the environmental impact in category j, which includes global warming potential (CO₂ eq), acidification potential (SO₂ eq), and eutrophication potential (PO₄ eq) per hectare. UPij refers to the upstream emission factor of input i contributing to impact category j during the pre-planting stage of tea production. PSij indicates the emission factor associated with the application of input i in impact category j during the tea cultivation stage. Rate_i represents the quantity of each input utilized throughout the tea production process.

$$\mathrm{S}\mathrm{E}{\mathrm{I}}_{\mathrm{j}}={\displaystyle \frac{{\mathrm{E}\mathrm{I}}_{\mathrm{j}}}{\mathrm{T}\mathrm{e}\mathrm{a} \mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}}}$$

where SEI_j represents the impact category which includes global warming (CO₂ eq), acidification (SO₂ eq), and eutrophication (PO₄ eq) potential per unit yield (in tonne, t).

2.3.5. Estimation of Benefits

The estimated of yield benefits (YB) only considered the costs of production. The benefits were estimated using the following equation:

$$\mathrm{Y}\mathrm{B}=\mathrm{T}\mathrm{e}\mathrm{a} \mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}\times \mathrm{P}\mathrm{r}\mathrm{i}\mathrm{c}\mathrm{e}-{\mathrm{P}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}_{\mathrm{c}\mathrm{o}\mathrm{s}\mathrm{t}}$$

The local market prices for spring tea and autumn tea are 1.54 $ kg⁻¹ and 1.85 $ kg⁻¹, respectively, and the price of nitrogen fertilizer (urea) is 0.53 $ kg⁻¹. The production costs include agricultural material cost and the labor cost associated with N fertilizer application.

The estimation of ecosystem economic benefits (EEB) considered the costs of production, environmental, and human health. The benefits were estimated using the following equation:

$$\mathrm{E}\mathrm{E}\mathrm{B}=\mathrm{T}\mathrm{e}\mathrm{a} \mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}\times \mathrm{P}\mathrm{r}\mathrm{i}\mathrm{c}\mathrm{e}-\mathrm{P}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}{\mathrm{n}}_{\mathrm{c}\mathrm{o}\mathrm{s}\mathrm{t}}-\mathrm{E}\mathrm{n}\mathrm{v}\mathrm{i}\mathrm{r}\mathrm{o}\mathrm{n}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{a}{\mathrm{l}}_{\mathrm{c}\mathrm{o}\mathrm{s}\mathrm{t}}-{\mathrm{H}\mathrm{u}\mathrm{m}\mathrm{a}\mathrm{n} \mathrm{h}\mathrm{e}\mathrm{a}\mathrm{l}\mathrm{t}\mathrm{h}}_{\mathrm{c}\mathrm{o}\mathrm{s}\mathrm{t}}$$

$$\begin{gathered} \mathrm{E}\mathrm{n}\mathrm{v}\mathrm{i}\mathrm{r}\mathrm{o}\mathrm{n}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{a}{\mathrm{l}}_{\mathrm{c}\mathrm{o}\mathrm{s}\mathrm{t}}=\mathrm{G}\mathrm{l}\mathrm{o}\mathrm{b}\mathrm{a}\mathrm{l} \mathrm{w}\mathrm{a}\mathrm{r}\mathrm{m}\mathrm{i}\mathrm{n}{\mathrm{g}}_{\mathrm{c}\mathrm{o}\mathrm{s}\mathrm{t}}+\mathrm{A}\mathrm{c}\mathrm{i}\mathrm{d}\mathrm{i}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}{\mathrm{n}}_{\mathrm{c}\mathrm{o}\mathrm{s}\mathrm{t}}+\mathrm{E}\mathrm{u}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{p}\mathrm{h}\mathrm{i}\mathrm{c}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}{\mathrm{n}}_{\mathrm{c}\mathrm{o}\mathrm{s}\mathrm{t}} \\ =(\mathrm{C}{\mathrm{O}}_2\times 0.0204)+(1.87\times \mathrm{N}{\mathrm{H}}_3+0.021\times \mathrm{N})+(1.12\times \mathrm{N}{\mathrm{O}}_3+0.24\times \mathrm{N}{\mathrm{H}}_3+0.0018\times \mathrm{N}) \end{gathered}$$

$$\mathrm{H}\mathrm{u}\mathrm{m}\mathrm{a}\mathrm{n} \mathrm{h}\mathrm{e}\mathrm{a}\mathrm{l}\mathrm{t}{\mathrm{h}}_{\mathrm{c}\mathrm{o}\mathrm{s}\mathrm{t}}=0.30\times {\mathrm{N}}_2\mathrm{O}+0.20\times \mathrm{N}{\mathrm{O}}_3+3.30\times \mathrm{N}{\mathrm{H}}_3$$

The market valuation of CO₂ was estimated at $0.0204 per kilogram. The remediation costs associated with eutrophication caused by NO₃⁻ and NH₃ were $1.12 and $0.24 per kilogram, respectively. For acidification impacts, the cost to mitigate damage induced by NH₃ emissions was $1.87 per kilogram. Additionally, the production of 1 kg of nitrogen fertilizer was associated with environmental damage costs of $0.021 for soil acidification and $0.0018 for eutrophication [15,36]. Regarding human health, the external costs per kilogram of N₂O, NO₃⁻, and NH₃ emissions were $0.30, $0.20, and $3.30, respectively [36,37].

2.4. Statistical Analysis

Data were processed using Microsoft Excel 2016, while statistical analyses were carried out in SPSS 23.0. One-way ANOVA followed by the least significant difference (LSD) test was employed to assess the effects of different nitrogen application rates on tea yield, bud density, hundred-bud weight, nitrogen uptake, and nitrogen use efficiency. SigmaPlot 14.0 was utilized to perform linear regression analyses exploring the relationships between tea yield and nitrogen uptake, as well as nitrogen application rate and associated environmental impacts. In addition, SAS 9.4 was applied to fit linear-plateau models describing the relationship between nitrogen input levels and both yield and economic benefits.

3. Results

3.1. Tea Yield and N Uptake

The results demonstrated that tea yield in Anxi County ranged from 7.83 to 11.22 t ha⁻¹ in 2020, and from 13.16 to 16.19 t ha⁻¹ in 2021. In both years, tea yield increased with nitrogen (N) application rates, with the first fertilized treatment at 150 kg N ha⁻¹ yr⁻¹ showing the most pronounced increase. Beyond this level (N300, N450, and N600), no further significant yield gains were observed. In 2020, the tea yield first rose and then fell as the nitrogen application rate increased, with the peak yield recorded under the N300 treatment. In contrast, in 2021, tea yield increased up to the N150 treatment and then remained relatively stable, with minimal variation observed from N150 to N600. These results suggest that moderate nitrogen input enhances tea yield, while excessive application does not confer additional benefits and may even be counterproductive under certain conditions (Figure 4).

Bud density and hundred-bud weight are key indicators for evaluating tea yield performance. Relative to the N0 treatment, bud density increased significantly under the N150, N300, N450, and N600 treatments by 53.6, 67.4, 94.4, and 58.9 buds per m², respectively (Figure 5A). In contrast, hundred-bud weight showed no significant variation among the five nitrogen treatments, with values ranging from 95.4 to 103.5 g (Figure 5B).

N uptake was significantly positively correlated with tea yield and was increased by 4.65 kg ha⁻¹ for every 1 t ha⁻¹ increase in tea yield (p < 0.05) (Figure 6).

3.2. N Input, Output, and Surplus

In Fengshan Village, Anxi County, annual nitrogen (N) inputs for tea cultivation showed a marked increase, rising from 41 kg ha⁻¹ under the N0 treatment to 641 kg ha⁻¹ with the N600 treatment—representing a 15.63-fold escalation. This rise was predominantly driven by the use of synthetic nitrogen fertilizers, which increased from 0 to 600 kg ha⁻¹ across these treatments. Corresponding to this input, crop nitrogen uptake varied between 89.17 and 130.63 kg ha⁻¹. A net nitrogen surplus was evident in most fertilized treatments (N150–N600), with excess N ranging from 34.86 to 397.66 kg ha⁻¹. Conversely, the N0 treatment displayed a negative nitrogen balance, as plant uptake exceeded input levels, suggesting a reliance on and depletion of existing soil nitrogen reserves (Figure 7).

Nitrogen fertilizer use efficiency was significantly influenced by the rate of nitrogen application. As nitrogen input increased, all three key indicators of nitrogen use efficiency including NAE, NRE, and NPFP declined. Among the four nitrogen application treatments, the highest nitrogen use efficiency was achieved under the N150 treatment, with values significantly greater than those of other treatments (p < 0.05), indicating that a moderate nitrogen input level optimized the balance between crop uptake and fertilizer input. Although the average NAE did not differ significantly between the N150 and N300 treatments, a remarkable difference was observed in NRE, suggesting that the higher N300 application did not result in a proportionate increase in nitrogen uptake by the tea plants. This implies diminishing returns in nitrogen utilization efficiency with excessive fertilization. These findings highlight the importance of applying nitrogen at an optimal rate to maximize fertilizer use efficiency while diminishing potential environmental risks associated with nitrogen surplus (

Table 1. Difference in N use efficiencies under different N treatments.

N Treatment	Agronomic Efficiency NAE (kg kg−1)	Recovery Efficiency NRE (%)	Partial Factor Productivity NPFP (kg kg−1)
N0	/	/	/
N150	26.89 ± 16.31 a	28.01 ± 6.53 a	92.72 ± 24.69 a
N300	10.43 ± 5.00 ab	9.80 ± 1.90 b	43.34 ± 7.26 b
N450	7.60 ± 2.57 b	8.94 ± 4.58 b	29.54 ± 5.55 b
N600	5.03 ± 3.95 b	3.18 ± 2.22 b	21.49 ± 6.91 b

Different letters indicate significant differences between different N rates at p < 0.05 according to LSD.

).

3.3. Environmental Impacts

Among all nitrogen (N) application treatments, the N0 treatment exhibited the lowest environmental impacts, while the N600 treatment showed the highest across all evaluated categories. When environmental burdens were assessed on a per hectare basis, the N150 treatment—representing a moderate nitrogen input—resulted in relatively low impacts, with a global warming potential (GWP) of 2.77 t CO₂ eq ha⁻¹, acidification potential (AP) of 43.23 kg SO₂ eq ha⁻¹, and eutrophication potential (EP) of 13.89 kg PO₄ eq ha⁻¹. In contrast, the N450 treatment, which reflects conventional nitrogen application rates commonly used by local farmers, led to substantially higher environmental pressures, with GWP, AP, and EP reaching 7.00 t CO₂ eq ha⁻¹, 124.75 kg SO₂ eq ha⁻¹, and 40.38 kg PO₄ eq ha⁻¹, respectively (Figure 8).

When normalized to per unit of tea yield (i.e., per metric ton of product), the environmental impacts exhibited a significant positive correlation with nitrogen input levels. Specifically, for every additional 100 kg·ha⁻¹ of nitrogen applied, GWP increased by 0.11 t CO₂ eq t⁻¹, AP by 2.16 kg SO₂ eq t⁻¹, and EP by 0.70 kg PO₄ eq t⁻¹ (Figure 9). These trends indicated that excessive nitrogen fertilization not only led to reduced yield benefits, but also exacerbated environmental risks related to greenhouse gas emissions, acid deposition, and nutrient runoff. Therefore, optimizing nitrogen application—particularly around the N150 level—can play a crucial role in balancing high-yield tea production with environmental sustainability.

3.4. Economic Assessment

Over the two years, the mean tea yield income initially slightly increased and then decreased; meanwhile, the planting, ecological, and health costs all increased with increasing N application rate (Figure 10A). The mean values of yield benefits in N0, N150, N300, N450, and N600 treatments were 12,419, 16,069, 16,577, 16,539, and 16,136 $ ha⁻¹ year⁻¹, respectively. In addition, the ecosystem economic benefits were also significantly improved by N up to the second rate (150 kg ha⁻¹ year⁻¹) but did not significantly increase with higher dose spreading (Figure 10B).

3.5. Optimal N Application Rate

The linear-plateau model described the relationship between N application rate with tea yield and benefits well, and the optimal N application rate can be obtained. The optimal N application rate for tea yield, yield benefits, and ecosystem economic benefits was 204, 165, and 120 kg ha⁻¹ year⁻¹, respectively (Figure 11).

4. Discussion

4.1. Effects of N Rates on Tea Yield and N Balance

Rational application of N fertilizer is one of the most influencing factors for achieving a higher yield [38]. In this study, we observed notable increases in tea yield with nitrogen application rates between 0 and 150 kg per hectare per year; however, no significant yield improvements were seen beyond 150 kg per hectare per year (Figure 4). Based on the practices of local farmers in the study region, the annual N applied rate was over 450 kg ha⁻¹. Hence, they may have great potential for fertilizer saving under rational N fertilization in tea production in China. Further analysis indicated that the affected bud density by N applied rate was higher than that of hundred-bud weight (Figure 5). N application increased the expression of key genes for chlorophyll synthesis in tea leaves and thus improved the bud density that has been advanced to explain this phenomenon [39].

Our findings demonstrated a significant positive correlation between nitrogen uptake and tea yield (p < 0.05), with annual N uptake ranging from 89.17 to 130.63 kg ha⁻¹ yr⁻¹. Meanwhile, nitrogen surplus increased substantially with higher N application rates, spanning from −48.10 to 397.66 kg ha⁻¹ yr⁻¹ (Figure 6 and Figure 7). The magnitude of N surplus was strongly influenced by both fertilizer input and plant uptake, aligning with earlier research conducted in China [40]. In the N150 treatment, the annual surplus was approximately 35 kg ha⁻¹, closely matching the national average of 40 kg ha⁻¹. However, under the N450 treatment, which reflects local farmer practices, the surplus exceeded 260 kg ha⁻¹—far surpassing figures reported for other regions using the same estimation method, including the United States (27 kg ha⁻¹ in 2017), the European Union (47 kg ha⁻¹ in 2014), and India (134 kg ha⁻¹ in 2018). Previous studies have indicated that when nitrogen surplus exceeds 20% of crop N uptake, the risk of nitrogen-related pollution increases markedly [41]. Therefore, based on the uptake values observed from N150 to N600 treatments, maintaining the surplus below 25 kg ha⁻¹ yr⁻¹ in this region would be advisable to reduce both environmental risks and fertilizer expenditure.

Nitrogen use efficiency (NUE) is a multifaceted process involving both nitrogen uptake and internal utilization mechanisms. In this study, the N150 treatment consistently exhibited significantly higher efficiency metrics across both years: agronomic efficiency (NAE), recovery efficiency (NRE), and partial factor productivity (NPFP) increased by 158–434%, 186–781%, and 114–331%, respectively, when compared to the N300, N450, and N600 treatments (Table 1). This enhanced efficiency under the N150 treatment may be attributed to the lower nitrogen input, which did not compromise tea yield or nitrogen uptake relative to the higher-rate treatments. These findings are in line with previous studies on cotton, wheat, and tomato, where nitrogen use efficiency varied across application rates and informed best management practices [42,43,44]. Moreover, our results indicate that nitrogen input had a more pronounced effect on tea yield than on nitrogen uptake (Table 1), likely due to the perennial nature of tea plants, where a portion of absorbed nitrogen is stored in woody tissues such as roots and stems rather than contributing immediately to harvestable biomass.

4.2. Effects of N Rates on Environmental and Benefits Impacts

Extensive research has demonstrated that excessive nitrogen fertilization in agriculture contributes to growing nitrogen surpluses, thereby accelerating environmental degradation [3,45]. It is therefore essential to consider both nitrogen uptake efficiency and associated environmental impacts when developing optimal nitrogen management strategies. In the present study, environmental consequences—including global warming, acidification, and eutrophication—were quantified using life cycle assessment (LCA). The results indicated a linear increase in environmental burdens with rising nitrogen application rates (Figure 8 and Figure 9), a pattern also observed in maize production across different genotypes [46]. As a leaf-harvested economic crop, tea demands relatively high nitrogen inputs, and previous meta-analyses have evaluated the influence of nitrogen rates on soil characteristics, yield, and shoot quality [47]. However, limited research has assessed the environmental consequences of varying nitrogen levels in tea production systems, despite evidence that even recommended application rates may lead to significant ecological risks [48]. This study further contributes by examining the relationship between nitrogen input and ecosystem economic benefits in acidic tea garden soils.

When expressed per ha of tea production area, compared with local farmers’ practices (N450 treatment), N150 treatment significantly reduced the global warming by 60%, acidification potential by 65%, and eutrophication potential by 66%, and such reductions were mainly attributed to the reduced N runoff and leaching, NH₃ volatilization, and N₂O emission during the tea production stage (Figure 8). The same situation was observed when environmental impacts were expressed per metric ton of tea yield production (Figure 9). Chen also highlights the importance of strengthening N management in pomelo production, based on agricultural practices that could achieve higher yields with lower environmental impacts and thereby avoid tradeoffs between productivity and ecosystem economic costs [20]. In addition, the precipitation amounts and intensity have a strong negative influence on N retention in soil [49], so it is necessary to effectively supply N to tea garden and minimizing N losses, especially in the subtropical monsoon climate zones (Figure 2).

Economic return is often a primary factor influencing farmers’ choice of management practices [50]. In this study, yield benefits increased markedly as nitrogen application rose from 0 to 150 kg ha⁻¹ yr⁻¹, but showed a slight decline when rates exceeded this threshold. When environmental and health costs were incorporated, ecosystem economic benefits further declined under higher N inputs (Figure 10). Excessive nitrogen use not only reduced yield-related economic returns but also significantly lowered ecosystem-level benefits. These findings are consistent with results reported by Xia for rice systems in the Taihu Lake region, where both yield and ecosystem economic benefits followed a quadratic relationship with nitrogen input [15]. Furthermore, previous research in Henan province (1987–2010) indicated a positive correlation between N fertilization and cereal aphid outbreaks, suggesting that improved N management may also reduce pest-related production costs [51]. However, our current study did not assess the effects of nitrogen application on pest dynamics, and further investigation is warranted to address this gap.

4.3. Options for Improving N Management

Nitrogen management strategies have primarily focused on maximizing yield and farmer profits, with limited attention paid to associated ecosystem risks [52,53]. In recent years, however, increasing concern has emerged regarding the environmental and human health consequences of nitrogen overuse in agriculture [4]. Optimizing nitrogen input is now recognized as a fundamental approach for achieving high yields while minimizing negative ecological and health impacts [54]. Given the prevalent overapplication of nitrogen in tea production, reducing fertilizer input based on crop yield response is a necessary first step toward enhancing nitrogen use efficiency, reducing nitrogen loss, and lowering input costs [55]. In this study, nitrogen recommendation rates based on yield response and yield economic return were determined to be 204 and 165 kg N ha⁻¹ yr⁻¹, respectively (Figure 10A,B). These values are substantially lower than the average rate of 347 kg N ha⁻¹ yr⁻¹ suggested by the Nutrient Expert system [56], likely due to that system’s reliance on nutrient uptake and efficiency models, which may underestimate the actual nitrogen use efficiency under field conditions [57]. Importantly, excess nitrogen (calculated as the difference between inputs and outputs) accumulates in the environment over time, contributing to long-term ecological damage [58]. Split nitrogen application has been widely recommended as an effective strategy to enhance nitrogen use efficiency and reduce fertilizer loss in crop production [59], a finding that is also supported by our results. Once nitrogen recommendations based on yield and economic return are established, the next step toward sustainable nitrogen management involves incorporating ecosystem economic benefits. Our analysis showed that, when considering environmental costs, the optimal nitrogen rate could be further reduced to 120 kg N ha⁻¹ yr⁻¹ (Figure 11C). Recently, growing interest has been directed toward integrating ecological and economic outcomes in fertilizer management. For instance, Yao demonstrated that urea deep placement significantly improved ecosystem economic benefits by up to 48%, compared to conventional broadcasting methods [22]. Similarly, Xu emphasized the importance of alternative fertilization strategies that balance environmental protection with profitability in intensive vegetable production systems in China [25]. Therefore, nitrogen management frameworks that incorporate ecosystem economic benefits offer promising pathways for promoting sustainable agricultural practices and reducing environmental burdens, with broad implications for global sustainable development [60]. The optimized N range (120–165 kg ha⁻¹ yr⁻¹) provides a practical basis for regional fertilizer recommendations and subsidy programs, enabling farmers to reduce input costs and environmental risks while supporting sustainable tea production.

Despite these contributions, several limitations should be acknowledged. First, although our modeling approach was based on field experimental data, further multi-site and multi-year validations are required to confirm the robustness of the results, as the lack of broader calibration introduces uncertainties in extrapolating our findings to other tea-growing regions. Second, while N–yield response parameters were fitted using a linear-plateau model, more comprehensive parameterization and reporting, including uncertainty ranges, are needed to enhance transparency and reproducibility. Finally, soil type and climate variability, which strongly influence N dynamics and fertilizer use efficiency, were not fully incorporated into the present analysis. Future research should therefore integrate multi-regional soil characteristics and detailed weather variability to improve the reliability and general applicability of the optimization framework.

5. Conclusions

This study systematically evaluated the synergistic effects of nitrogen management on tea yield, environmental impact, and economic performance. The results identified 150 kg N ha⁻¹ yr⁻¹ as an optimal application rate that significantly improved tea yield and nitrogen use efficiency, while maintaining nitrogen surplus below 40 kg ha⁻¹ yr⁻¹—only 15% of that under conventional local practices (450 kg N ha⁻¹ yr⁻¹). Life cycle assessment showed that, compared to the conventional rate, this treatment reduced greenhouse gas emissions, acidification, and eutrophication potentials by 60%, 65%, and 66%, respectively. Significantly, the N rate optimized for ecological-economic benefits (EEB) was decreased to 120 kg ha⁻¹ yr⁻¹, which is 27% to 41% lower than conventional guidelines. This adjustment still resulted in 95% of the maximum yield and improved EEB by more than 35%. Integrating EEB into decision-making was shown to reduce nitrogen loss risk by 52% and enhance resource use efficiency by 40%. The “yield–N uptake–EEB” triadic framework proposed in this study offers a practical approach for the sustainable transformation of tea production and broader green agricultural development.