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Article

Rubber-Ficus hirta Vahl. Agroforestry System Enhances Productivity and Resource Utilization Efficiency and Reduces Carbon Footprint

by
Jian Pan
1,2,†,
Xiu Zeng
1,†,
Zhengfan Tian
3,
Yan Zhang
1,4,
Yuanran Xian
1,4,
Hanqi Tu
1,
Jianxiong Huang
1,2 and
Xiuquan Wang
1,5,*
1
Rubber Research Institute, Chinese Academy of Tropical Agricultural Sciences, Haikou 571101, China
2
Danzhou Investigation & Experiment Station of Tropical Crops, Ministry of Agriculture and Rural Affairs, Danzhou 571737, China
3
College of Tropical Crops, Yunnan Agricultural University, Pu’er 665099, China
4
Sanya Institute of China Agricultural University, Sanya 572025, China
5
Administrate Office of Danzhou Campus, Chinese Academy of Tropical Agricultural Sciences, Haikou 571101, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Agriculture 2025, 15(16), 1750; https://doi.org/10.3390/agriculture15161750
Submission received: 19 June 2025 / Revised: 10 August 2025 / Accepted: 12 August 2025 / Published: 15 August 2025
(This article belongs to the Special Issue Detection and Management of Agricultural Non-Point Source Pollution)
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Abstract

Developing a more productive, resource-efficient, and climate-smart rubber agroforestry model is essential for the sustainable growth of natural rubber cultivation. In this study, we evaluated whether a double-row rubber plantation intercropped with the medicinal crop Ficus hirta Vahl. (DR-F) could achieve this goal, using a single-row rubber plantation (SR) as the control. We assessed the feasibility of the DR-F system based on productivity, solar utilization efficiency (SUE), partial factor productivity of applied nitrogen (PFPN), carbon efficiency (CE), net ecosystem carbon balance (NECB), and carbon footprint (CF). No significant difference was observed in rubber tree biomass between the DR-F (10.49 t·ha−1) and SR (8.49 t·ha−1) systems. However, the DR-F system exhibited significantly higher total biomass productivity (23.34 t·ha−1) than the SR systems due to the substantial contribution from intercropped Ficus hirta Vahl., which yielded 12.84 t·ha−1(p < 0.05). The root fresh weight yield of Ficus hirta Vahl. reached 17.55 t·ha−1, generating an additional profit of 20,417 CNY ha−1. The DR-F system also exhibited higher solar radiation interception and greater availability of soil nutrients. Notably, the roots of rubber trees and Ficus hirta Vahl. did not overlap at a 4 m distance from the rubber trees. The DR-F system achieved higher SUE (0.64%), PFPN (51.40 kg·kg−1 N), and CE (6.93 kg·kg−1 C) than the SR system, with the SUE and PFPN differences being statistically significant (p < 0.05). Although the NECB remained unaffected, the DR-F system demonstrated significantly higher productivity and a substantially lower CF (0.33 kg CO2·kg−1, a 56% reduction; p < 0.05). In conclusion, the DR-F system represents a more sustainable and beneficial agroforestry approach, offering improved productivity, greater resource use efficiency, and reduced environmental impact.

1. Introduction

There is a broad international consensus that rising concentrations of greenhouse gases (GHGs) are driving global warming [1,2], with direct emissions from agricultural systems ranking among the major contributors, comparable with emissions from the transportation sector [3]. Additionally, the use of fertilizers, pesticides, diesel, and other inputs in agricultural production leads to substantial indirect GHG emissions [4]. China made a landmark climate commitment in 2020, pledging to reach peak CO2 emissions before 2030 and achieve carbon neutrality by 2060 [5]. Agriculture, despite being a significant source of GHG emissions, also serves as a potential carbon sink. With appropriate management, agricultural systems can reduce emissions while sequestering atmospheric CO2 [6]. Metrics such as NECB and CF are key indicators for assessing how agricultural systems contribute to carbon sequestration, emission reduction, and overall environmental impact. These metrics also provide insights and policy guidance for achieving carbon neutrality in the agricultural sector [4,5,7].
Agricultural production relies heavily on natural and artificial resources, such as light, N fertilizer, water, and land. Excessive or inefficient use of these resources can lead to waste and environmental degradation, undermining the long-term viability of farming systems [8,9,10,11]. Strategies such as crop selection, optimized fertilization, intercropping, planting density design, and crop rotation can significantly enhance the use efficiency of nitrogen, carbon, and water, contributing to more sustainable agriculture [11,12,13,14].
Natural rubber, derived from rubber tree (Hevea brasiliensis) (originally from Brazil), is one of China’s four strategic materials. It is cultivated primarily in Hainan, Yunnan, and Guangdong provinces. Rubber trees have a growth cycle of more than 30 years (with a juvenile phase of 7–8 years) and are an important economic forest crop in China’s tropical regions, with a planting area of about 1.20 × 106 ha [15]. However, in the past decade, prolonged low rubber prices have highlighted the inefficiency of monoculture rubber plantations. These systems also suffer from underutilized land and light resources, low resource recycling rates, and limited studies on GHG emissions, making their role in carbon sequestration unclear [16,17]. Therefore, rubber cultivation faces challenges in increasing output, improving resource efficiency, and achieving climate goals.
Agroforestry systems, which integrate trees with crops or livestock, offer a promising solution. They provide diverse ecosystem services and are typically associated with higher economic returns, improved carbon sequestration, and greater resource utilization, making them a more sustainable land management strategy [18,19,20]. In rubber-based agroforestry systems, commonly used intercrops include banana, corn, pineapple, arrowroot, pepper, alpinia oxyphylla, and amomum villosum [15]. However, due to limitations in market capacity, there is a need to explore additional economically viable intercrops.
Ficus hirta Vahl. is a medicinal plant in the Moraceae family, and it is widely found across Southeast Asian forest ecosystems. Its roots are rich in bioactive compounds, such as psoralen (0.42 mg·g−1), flavonoids (9.76 mg·g−1), and polyphenols (4.55 mg·g−1), giving it high economic value [21]. As wild populations can no longer meet market demand, cultivation has become essential to prevent resource depletion and meet growing needs [22]. Ficus hirta Vahl. thrives in sparse forests and demonstrates a degree of shade tolerance, making it suitable for agroforestry. Previous studies have shown that it can yield 10.80–18.22 t·ha−1 when intercropped in Masson pine forests [23,24]. Notably, wild Ficus hirta Vahl. is often found growing in rubber plantations, indicating its potential as a compatible intercrop. Its inclusion may enhance benefits, resource use efficiency, and climate resilience. However, further research is needed to validate the feasibility of rubber-Ficus hirta Vahl. intercropping systems.
Therefore, this study aims to evaluate the viability of a rubber-Ficus hirta Vahl. agroforestry model by examining productivity, resource utilization efficiency, and GHG emissions, providing theoretical and technical support for the sustainable development of both rubber and Ficus hirta Vahl. industries.

2. Materials and Methods

2.1. Overview of the Test Site

This experiment was conducted at the Danzhou base of the Rubber Research Institute, Chinese Academy of Tropical Agricultural Sciences. This site is 142.8 m above sea level, with relatively flat terrain and a typical tropical marine monsoon climate characterized by distinct dry and rainy seasons. Annual rainfall is approximately 1044 mm, with about 66.0% falling during the rainy season from May to October, mostly concentrated between July and September. The average annual temperature ranges from 20.7 to 29.8 °C. Figure 1 shows the precipitation and air temperature during the experimental period. Two planting patterns were used. Equal row spacing used 3 m × 7 m spacing, with a planting density of 480 plants·ha−1 (single-row rubber plantation). Alternating wide and narrow rows had wide row spacing of 20 m and narrow row spacing of 2 m × 4 m, with a planting density of 420 plants·ha−1 (double-row rubber plantation). As described in our previous study [20], to avoid slantwise growth, which results in heavy shading within 7–10 years after planting in the double-row rubber plantation, a vertical cordon rubber tree variety (Reyan 7-20-59) was used, planted in an east–west direction in 2019. By 2023, the trees had reached a crown diameter of approximately 3 m and a height of about 5 m. The initial soil nutrient status was determined as follows: organic matter, 8.7 g·kg−1 [measured by a total organic carbon analyzer (Multi N/C 3100, Analytik Jena AG, Jena, Germany)]; total nitrogen, 0.6 g·kg−1 [determined using an automatic Kjeldahl apparatus (K9860, Haineng Future Technology Group Co., Ltd, Jinan, China)]; available phosphorus (P), 20.4 mg·kg−1 [analyzed by spectrophotometry after extraction with sodium bicarbonate (Evolution 201,Thermo Fisher Scientific Inc., Madison, USA)]; available potassium (K), 66.1 mg·kg−1 [measured via flame photometry (FP6410, Shanghai Xinyi Precision Instruments Co., Ltd., Shanghai, China) after extraction with ammonium acetate]; cation exchange capacity, 3.83 cmol·kg−1 [determined by spectrophotometry (Evolution 201, Thermo Fisher Scientific Inc., Madison, USA) after extraction with hexamminecobalt(III) chloride)]; and pH = 4.8 [measured with a pH meter (FE28-FIVE Easy Plus, Mettler-Toledo Instruments (Shanghai) Co., Ltd., Shanghai, China)]. The soil was classified as silty clay loam texture.

2.2. Experimental Design

The experiment was carried out from 2022 to 2023 and included two treatments: a single-row rubber plantation (SR) and a double-row rubber plantation intercropped with Ficus hirta Vahl. (DR-F). Each treatment was replicated three times, resulting in six plots in total, each measuring 120 m2. Ficus hirta Vahl. was planted in mid-April 2022 at a distance of 4.75 m from the rubber trees (see Figure 2 for the planting method) and harvested by excavator in mid-December 2023.
Fertilization followed local practices for immature rubber trees, with an application rate of 1.5 kg·tree−1 of a special compound fertilizer (N:P2O5:K2O = 21:11:13). For the SR treatment, the trees received 151 kg N·ha−1·yr−1, 79 kg P2O5·ha−1·yr−1, and 94 kg K2O·ha−1·yr−1. In contrast, the DR-F treatment received 132 kg N·ha−1·yr−1, 69 kg P2O5·ha−1·yr−1, and 82 kg K2O·ha−1·yr−1. In the DR-F system, Ficus hirta Vahl. was established 4.75 m from rubber trees, maintaining clear separation between the two species. Additional fertilization was applied specifically for Ficus hirta Vahl. in the DR-F treatment. During the establishment year (when the plants were smaller), the application rates were 150 kg N·ha−1·yr−1, 90 kg P2O5·ha−1·yr−1, and 90 kg K2O·ha−1·yr−1. These amounts were increased in the second year to 240 kg N·ha−1·yr−1, 120 kg P2O5·ha−1·yr−1, and 120 kg K2O·ha−1·yr−1 as the plants reached mature size. Urea, superphosphate, and potassium sulfate were used as the fertilizer sources. Standard field management practices, such as weeding, irrigation, and soil cultivation, were followed throughout the study.

2.3. Examined Indicators and Methods

2.3.1. Productivity

Rubber Biomass
To assess productivity, total rubber tree biomass was measured in both April 2022 and December 2023. In each plot, 30 rubber trees were randomly selected from both SR and DR-F treatments, and their diameter at breast height (DBH, 1.3 m above ground) was measured using a tape measure. Total biomass (BT) was calculated using an established equation [25]:
BT = 0.136 × DBH2.437
The change in biomass was determined by the difference in measurements between the two time points.
Yield, Biomass, and Economic Benefits of Ficus hirta Vahl.
In December 2023, Ficus hirta Vahl. plants were harvested from all the DR-F plots. The fresh weight of roots and aboveground parts (including stems and leaves) were recorded. Five representative plants were selected to determine moisture content, which was used to calculate total biomass. Because the root is the economically valuable part of Ficus hirta Vahl., its yield was based on the fresh root weight. Net income was calculated by subtracting total input costs, including land preparation, irrigation, seedlings, fertilizers, and labor, from the market value of the harvested fresh roots, which were priced at CNY 8 kg−1. Since the rubber trees were not yet in production, their economic return was not included in the calculation.

2.3.2. Environmental Factors

Available Nutrient Content in Soil and Root Distribution
To evaluate environmental effects, soil samples were collected from the 0–20 cm layer at 3, 12, and 20 months after the planting of Ficus hirta Vahl. Samples from both the SR and DR-F plots were analyzed for soil inorganic nitrogen (nitrate and ammonium), available P, and available K. These measurements were performed using spectrophotometry, flame spectrophotometry, and molybdenum antimony colorimetry, respectively. In the DR-F plots, the area was divided into intercropping (DR-I) and non-intercropping (DR-NI) zones for further comparison.
Just before the harvest of Ficus hirta Vahl., a soil monolith measuring 30 cm × 30 cm × 20 cm (length × width × height) was excavated to study the root distribution in the topsoil (0–20 cm) and determine whether belowground competition occurred between rubber trees and intercrops. In the DR-F plots, sampling locations were positioned 2 m, 3 m (non-intercropping zone), and 4 m (intercropped zone) from the rubber trees. All the roots collected were oven-dried and weighted to calculate the root density (g·m−3).
Light Intensity
To assess differences in light availability between treatments, measurements were taken in September 2023 under clear weather conditions. Light intensity (in lux) was recorded from 8:00 a.m. to 5:00 p.m., at hourly intervals, using an illuminometer (GLZ-C-G, Tuopuyunnong Agricultural Co., Hangzhou, China). In the SR treatment, where the rubber tree rows had formed a nearly continuous canopy, resulting in heavy shade, measurements were taken using an S-shaped multi-point approach between rows. For the DR-F treatment, light intensity was recorded specifically within the area intercropped with Ficus hirta Vahl. to reflect conditions under the mixed planting system.

2.3.3. Resource Utilization Efficiency

Three key indicators were used to evaluate resource utilization efficiency across the SR and DF-F treatments: solar utilization efficiency (SUE), partial factor productivity from applied nitrogen (PFPN), and carbon efficiency (CE).
SUE
SUE was determined using meteorological data on total solar radiation (MJ·m−2) during the experimental period. It was calculated as the percentage of radiation converted into biomass, using the following formula [26]:
SUE = H × B/S × 100%
where B is the change in biomass per unit area (kg·ha−1), H is the combustion heat of dry matter (1779 × 104 J·kg−1), and S is the cumulative solar radiation during the growth period.
PFPN
To assess nitrogen fertilizer efficiency, PFPN (kg·kg−1 N) was calculated as the ratio of biomass gain to nitrogen input. The calculation method is as follows [12]:
PFPN = B/N
where B represents the biomass increase per unit area (kg·ha−1), and N is the nitrogen application rate (kg N·ha−1) for each treatment.
CE
Carbon efficiency, or CE (kg·kg−1 CO2), measures the amount of carbon absorbed per unit of carbon input and reflects the sustainability of the production system. Following the method described by Yang et al. (2024) [11], its calculation formula is as follows:
CE = Bc/Cinput
where Bc is the change in biomass carbon per unit area (kg·ha−1), and Cinput represents the CO2 emissions associated with the production of agricultural inputs. The CO2 emission coefficients for various materials used in this study are listed in Table 1 [4].

2.3.4. NECB and CF

To further assess carbon dynamics, both the net ecosystem carbon balance (NECB; t CO2·ha−1) [7,27] and the carbon footprint (CF; kg CO2·kg−1) were calculated as follows:
NECB = NPP + Cimport − NPPoutput − CH4 − Rh − N2O − Cinput
CF = NECB/B
NECB was defined as the sum of net primary productivity (NPP) and imported carbon inputs, minus the organic matter removed from the system (NPPoutput), methane (CH4) emissions, heterotrophic soil respiration (Rh), nitrous oxide (N2O) emissions, and carbon inputs from production processes. In this study, no external organic matter was added to the system, so Cimport was zero. The output, NPPoutput, referred to the biomass removed with the harvest of Ficus hirta Vahl. The carbon content of biomass was assumed to be 40%, and the CO2 conversion factor used was 44/12.
Emissions of CH4 were considered negligible due to the dry land conditions of the study site. Rh was estimated based on the methods outlined in Wu’s research [28], while N2O emissions were calculated based on the results of Zhang et al. [29] using the IPCC-recommended parameter method, assuming that 1.05% of the applied nitrogen was emitted as N2O. The corresponding CO2-equivalent conversion factor for N2O emissions was 44/28 × 296. The Cinput included CO2 emissions from the production and transport of all agricultural inputs. CF was calculated as the ratio of NECB to biomass change (B) per unit area during the experimental period.

2.4. Data Analysis

Data processing and statistical analysis were performed using Microsoft Excel 2016 and DPS software (V9.50). Analysis of variance was performed using the Tukey–Kramer test, with statistical significance defined as p < 0.05.

3. Results

3.1. Productivity and Influencing Factors

3.1.1. Biomass

As presented in Table 2, the DR-F treatment demonstrated significantly higher productivity than the SR treatment (p < 0.05). In the fourth year of growth, there was no notable difference in the DBH of rubber trees between the two systems. However, as the trees matured, the DBH in the DR-F treatment reached 9.12 cm, significantly higher than the 8.11 cm observed in the SR system. This indicates that the DR-F planting system provided a more favorable environment for rubber tree growth (p < 0.05).
While the growth advantage of rubber trees in the DR-F system over those in the SR system did not reach statistical significance throughout the experimental period, the inclusion of Ficus hirta Vahl. in the DR-F treatment contributed an additional biomass of 12.84 t·ha−1. This brought the total biomass in the DR-F system to 23.34 t·ha−1, which was significantly higher than that recorded in the SR treatment (p < 0.05).

3.1.2. Economic Benefits of Intercropping

Figure 3 illustrates the economic returns from intercropping Ficus hirta Vahl. The fresh root yield reached 17.55 t·ha−1, with a corresponding additional input cost of CNY 49,763 ha−1. Notable seedling acquisition and harvesting costs accounted for 30% and 38% of this total investment, respectively. Despite the added inputs, the intercropping approach yielded a substantial output value of CNY 70,180 ha−1, resulting in a net profit of CNY 20,417 ha−1.

3.1.3. Soil Available Nutrients, Root Distribution, and Light Intensity

As shown in Figure 4, soil nutrient levels were significantly higher in the DR-I area than in both SR and DR-NI areas, primarily due to the greater fertilizer input in the former. Specifically, the NO3−, NH4+, and available P concentrations in the DR-I area ranged from 9.48 to 30.70 mg·kg−1, 18.72 to 57.39 mg·kg−1, and 20.98 to 33.09 mg·kg−1, respectively. Except for the NH4+ concentration three months after planting, nutrient levels in DR-I consistently exceeded those of the DR-NI and SR treatments at all the measured time points (p < 0.05). In terms of available K, differences among the three treatments were generally small, with the exception of the 12-month time point, where the DR-I treatment exhibited significantly higher K content (108.7 g·kg−1) than the DR-NI and SR groups (p < 0.05). The improved nutrient availability observed in the DR-F treatment can largely be attributed to the DR-I area.
The root distribution data in Table 3 reveals no evidence of root overlap between rubber trees and Ficus hirta Vahl. in the DR-F system. At a distance of 3 m from the rubber trees, the root biomass density of rubber trees was 0.16 g·dm−3, and Ficus hirta Vahl. roots were absent. Consequently, at a distance of 4 m, only Ficus hirta Vahl. roots were present, with a biomass density of 0.89 g·dm−3. These findings indicate effective spatial separation between the root systems of the two species.
Measurements of light intensity showed clear differences between the treatments. In the DR-F treatment, light intensity ranged from 15,004 to 139,457 lux, following a single peak diurnal pattern (Figure 5). In contrast, the SR treatment, characterized by a continuous canopy and heavy shade, exhibited a narrower range of 1858 and 23,518 lux, with smaller fluctuations throughout the day. The average light intensity was 84,037 lux in the DR-F system and only 13,282 lux in the SR system, a statistically significant difference (p < 0.05).

3.2. Resource Utilization Efficiency and Carbon Emissions

3.2.1. Resource Utilization Efficiency

Figure 6 illustrates the performance of different treatments with respect to SUE, PFPN, and CE. The DR-F treatment demonstrated notably higher resource utilization compared to the SR treatment. Specifically, the SUE reached 0.64% (Figure 6A), and the PFPN was 51.40 kg·kg−1 N (Figure 6B) in the DR-F system, both significantly higher than the corresponding values in the SR system (p < 0.05). While the CE of the DR-F treatment (6.93 kg·kg−1 C) exceeded that of the SR treatment (6.01 kg·kg−1 C), the difference was not statistically significant (p > 0.05; Figure 6C).

3.2.2. NECB and CF of Complex Systems

As shown in Figure 7, the DR-F treatment did not significantly alter the NECB compared to the SR treatment. However, it significantly reduced the CF (p < 0.05). The total carbon sequestration in the DR-F system reached 34.23 t CO2·ha−1, substantially higher than the 12.46 t CO2·ha−1 observed in the SR system. The gain was offset by a higher carbon output (26.70 CO2·ha−1) resulting from the harvest of Ficus hirta Vahl., which also led to a nonsignificant difference in NECB between treatments: 6.47 t CO2·ha−1 for SR and 7.53 t CO2·ha−1 for DR-F. Due to the higher productivity, the CF of the DR-F system was significantly lower at 0.33 kg CO2·kg−1, compared to 0.75 kg CO2·kg−1 in the SR system (p < 0.05).

4. Discussion

4.1. The Significance of Rubber Agroforestry in Enhancing Comprehensive Output

Economic viability is one of the primary constraints influencing the sustainable development of rubber plantations. Over the past decade, the market price of rubber has remained relatively stable at CNY 12–14 kg−1, resulting in a total output value of approximately CNY 8000–12,000 ha−1. Given that this value is close to the cost of cultivation, profit margins are minimal. Due to the strategic importance of natural rubber in China, there is a national consensus to maintain planting areas in order to preserve the rubber supply potential. Thus, policy does not support converting rubber plantations into other high-value crops [30,31,32]. To address this economic challenge, enhancing comprehensive output through composite planting has become a key strategy. However, in conventional SR systems, intercropping is typically limited to the first 3–4 years after rubber planting due to constraints posed by reduced light availability and root competition. Beyond this period, intercropping is either discontinued or limited to shade-tolerant crops. In contrast, the DR planting pattern helps mitigate these limitations, allowing for long-term intercropping without compromising rubber yield [20]. The DR system thus presents a promising framework for establishing rubber-based agroforestry systems. In the present study, Ficus hirta Vahl. was introduced as an intercrop during the fourth to fifth year after rubber planting. Its yield reached 17.55 t·ha−1, which is comparable to the 18.22 t·ha−1 yield intercropped for four years and notably higher than the 10.80 t·ha−1 yield achieved for two years in a Masson pine-based agroforestry system [23].
Light availability and nutrient competition are known to strongly influence productivity in agroforestry systems [33,34]. Rubber trees are typically dominant in such systems and can suppress the performance of intercrops [35]. However, in this study, no rubber tree roots were detected 4 m from the trees in the DR-F treatment, and only a small number of Ficus hirta Vahl. roots were observed at that distance, indicating minimal underground competition (Table 2). Previous studies have also shown that even at 12 years of age, rubber trees exhibit very limited root growth at a 4 m distance [36]. Light measurements further support the advantage of the DR-F system. Higher light penetration was observed (Figure 4), which not only supported the healthy growth of Ficus hirta Vahl. but also benefited the rubber trees (Table 1). Notably, even at 17 years old, the intercropped areas in DR systems still receive 2–4 h of direct sunlight daily [20], unlike SR systems, where continuous heavy canopy shade prevails. This increased light availability makes DR systems more conducive to long-term, diversified intercropping.
Therefore, the DR system offers a structurally and ecologically favorable basis for establishing rubber-based agroforestry systems that enhance overall productivity per unit area. The rubber-Ficus hirta Vahl. system developed in this study represents a promising model for such integrated land-use strategies.

4.2. Resource Utilization Efficiency of Rubber Agroforestry and Its Contribution to Carbon Sequestration and Emission Reduction

Efficient utilization of resources such as light, water, nitrogen, and carbon is essential for improving crop productivity and ensuring the stable supply of agricultural products [37,38,39]. Benefiting from optimized spatial configuration, the DR-F system allows 50% of the land to be used for intercropping Ficus hirta Vahl. while maintaining a rubber tree density of 420 trees·ha−1. The SUE of DR-F was 0.64%, which, while lower than that of maize (1.14–2.13%), was higher than that of wheat (0.43–0.52%) and comparable to rice (0.63–0.67%) [26,40]. It was also significantly higher than the SUE of the SR system (0.23%). Similarly, DR-F demonstrated higher nitrogen use efficiency, and its PFPN was 1.8 times that of SR. This finding is consistent with prior studies [41,42]. Differences in agricultural inputs and productivity among crop systems often result in large variations in CE. For example, the CE values of wheat and maize have been reported as 10.99 and 9.68 kg·kg−1 CO2, respectively, while that of cotton is only 2.96 kg·kg−1 CO2 [5]. Crop diversification tends to increase carbon input [4]. In the DR-F system, the introduction of Ficus hirta Vahl. led to additional carbon inputs from fertilizers, pesticides, and machinery. However, this was offset by the increased biomass. As a result, the CE in DR-F was slightly higher than in SR, though the difference was not significant (Figure 6C). This was attributed to the comparable biomass and input levels of Ficus hirta Vahl. and rubber trees, which prevented significant changes in CE.
Land use systems can serve as net sources or sinks of GHG emissions depending on their carbon sequestration capacity and emission levels [43]. In rubber-based composite systems, the addition of intercrops at the same planting density generally increases biomass and NECB [44]. In this study, although the DR-F system employed a lower rubber planting density, its rubber biomass remained equivalent to that of SR. The additional biomass from Ficus hirta Vahl. made the total biomass in DR-F approximately three times higher than that in SR. However, since the biomass of Ficus hirta Vahl. was removed from the system after harvest, the NECB in DR-F remained similar to that in SR. When calculating NECB, changes in biomass exert a stronger influence than factors such as Rh, soil N2O emissions, and agricultural inputs. Planting woody crops to increase carbon retention and returning crop residues to the field are recognized strategies for improving NECB [44,45]. As Ficus hirta Vahl. is a small shrub, the aboveground biomass is removed during root harvest and not returned to the soil. It is therefore recommended that future studies explore the potential for crushing and incorporating this biomass back into the field to enhance the carbon sequestration capacity of the system.
Diversified agroforestry systems offer a promising strategy for reducing CF [46], though the specific impact depends on the crop combination. For instance, “rubber–peanut” systems have not shown reductions in CF compared to SR, whereas “rubber–yam bean” systems have demonstrated significant CF reductions [4]. In this study, while NECB did not significantly differ between SR and DR-F, the higher productivity of DR-F resulted in a significantly lower CF (Figure 7B). This suggests that DR-F is capable of generating more output without increasing net emissions, representing a climate-resilient and sustainable model for rubber production.

5. Conclusions

In this study, the SR system was used as a control to assess the feasibility of the DR-F system in terms of productivity, resource utilization efficiency, and GHG emissions. Over a two-year period, the DR-F system produced significantly higher biomass and additional yield of Ficus hirta Vahl., resulting in an economic gain of CNY 20,417 ha−1 (p < 0.05). Although the DR-F system did not significantly enhance CE or NECB, it substantially improved SUE and PFPN and significantly reduced the CF (p < 0.05). These findings indicate that the DR-F system demonstrates more efficient resource utilization and reduced climate impact. Overall, the DR-F model offers a more productive, resource-efficient, and climate-resilient approach to sustainable rubber agroforestry.

Author Contributions

Conceptualization, J.H.; methodology, J.P.; validation, X.Z., Z.T. and H.T.; investigation, Y.X. and Y.Z.; resources, X.W.; data curation, J.H.; writing—original draft, J.P. and X.Z.; writing—review and editing, J.H.; supervision, J.H.; project administration, X.W.; funding acquisition, X.W. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Opening Project Fund of Rubber Research Institute, CATAS (RRI-KLOF202401), Hainan Provincial Natural Science Foundation of China (325MS136), CATAS Program Advancement of High-efficiency Tropical Specialty Agriculture in Hainan to Support Hainan Free Trade Port Development (JSJCSF2025001), Central Financial Forestry Science and Technology Promotion and Demonstration Project (Qiong [2022]-TG04), and the Earmarked Fund for China Agricultural Research System (CARS-33-ZP3).

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Precipitation and air temperature during the experimental period.
Figure 1. Precipitation and air temperature during the experimental period.
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Figure 2. Planting design in the DR-F system.
Figure 2. Planting design in the DR-F system.
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Figure 3. Yield and economic benefit of intercropped Ficus hirta Vahl.
Figure 3. Yield and economic benefit of intercropped Ficus hirta Vahl.
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Figure 4. Soil nutrient content under different planting systems. Different letters indicate significant differences between treatments (p < 0.05). SR: Single-row rubber plantation; DR-NI: non-intercropping area in double-row rubber plantation; DR-I: intercropping area in double-row rubber plantation.
Figure 4. Soil nutrient content under different planting systems. Different letters indicate significant differences between treatments (p < 0.05). SR: Single-row rubber plantation; DR-NI: non-intercropping area in double-row rubber plantation; DR-I: intercropping area in double-row rubber plantation.
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Figure 5. Daily light intensity under different planting systems. SR: Single-row rubber plantation; DR-F: double-row rubber plantation intercropping with Ficus hirta Vahl.
Figure 5. Daily light intensity under different planting systems. SR: Single-row rubber plantation; DR-F: double-row rubber plantation intercropping with Ficus hirta Vahl.
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Figure 6. Resource utilization efficiency under different treatments ((A) solar utilization efficiency SUE; (B) partial factor productivity from applied nitrogen, PFPN; (C) carbon efficiency, CE). Different letters indicate significant differences between treatments (p < 0.05). SR: Single-row rubber plantation; DR-F: double-row rubber plantation intercropping with Ficus hirta Vahl.
Figure 6. Resource utilization efficiency under different treatments ((A) solar utilization efficiency SUE; (B) partial factor productivity from applied nitrogen, PFPN; (C) carbon efficiency, CE). Different letters indicate significant differences between treatments (p < 0.05). SR: Single-row rubber plantation; DR-F: double-row rubber plantation intercropping with Ficus hirta Vahl.
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Figure 7. NECB (A) and CF (B) under different treatments. Different letters indicate significant difference between treatments (p < 0.05). SR: Single-row rubber plantation; DR-F: double-row rubber plantation intercropping with Ficus hirta Vahl.
Figure 7. NECB (A) and CF (B) under different treatments. Different letters indicate significant difference between treatments (p < 0.05). SR: Single-row rubber plantation; DR-F: double-row rubber plantation intercropping with Ficus hirta Vahl.
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Table 1. Agricultural inputs and greenhouse gas emission factors.
Table 1. Agricultural inputs and greenhouse gas emission factors.
ItemUnitGHG CoefficientUnit
Nkg CO2·kg−17.76kg·ha−1
P2O52.33
K2O0.66
Diesel2.50
Herbicide18.00
Electricitykg CO2·kwh−10.95kwh·ha−1
Table 2. Biomass of different planting systems.
Table 2. Biomass of different planting systems.
Planting SystemHevea DBH (cm)Hevea Biomass Incrementation
(t·ha−1)		Ficus hirta Vahl.
(t·ha−1)		Total Biomass
(t·ha−1)
4-Year6-Year
SR4.27 a8.11 b8.49 a—8.49 b
DR-F4.32 a9.12 a10.49 a12.8423.34 a
Different letters indicate significant difference between treatments (p < 0.05). SR: Single-row rubber plantation. DR-F: Double-row rubber plantation intercropping with Ficus hirta Vahl.
Table 3. Root biomass density in DR-F system (g·dm−3).
Table 3. Root biomass density in DR-F system (g·dm−3).
Distance from HeveaHeveaFicus hirta Vahl.
2 m1.16 a—
3 m0.16 a—
4 m—0.89
“—” indicates no crop root was detected. Different letters indicate significant differences in root biomass density between different distances (p < 0.05).
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Pan, J.; Zeng, X.; Tian, Z.; Zhang, Y.; Xian, Y.; Tu, H.; Huang, J.; Wang, X. Rubber-Ficus hirta Vahl. Agroforestry System Enhances Productivity and Resource Utilization Efficiency and Reduces Carbon Footprint. Agriculture 2025, 15, 1750. https://doi.org/10.3390/agriculture15161750

AMA Style

Pan J, Zeng X, Tian Z, Zhang Y, Xian Y, Tu H, Huang J, Wang X. Rubber-Ficus hirta Vahl. Agroforestry System Enhances Productivity and Resource Utilization Efficiency and Reduces Carbon Footprint. Agriculture. 2025; 15(16):1750. https://doi.org/10.3390/agriculture15161750

Chicago/Turabian Style

Pan, Jian, Xiu Zeng, Zhengfan Tian, Yan Zhang, Yuanran Xian, Hanqi Tu, Jianxiong Huang, and Xiuquan Wang. 2025. "Rubber-Ficus hirta Vahl. Agroforestry System Enhances Productivity and Resource Utilization Efficiency and Reduces Carbon Footprint" Agriculture 15, no. 16: 1750. https://doi.org/10.3390/agriculture15161750

APA Style

Pan, J., Zeng, X., Tian, Z., Zhang, Y., Xian, Y., Tu, H., Huang, J., & Wang, X. (2025). Rubber-Ficus hirta Vahl. Agroforestry System Enhances Productivity and Resource Utilization Efficiency and Reduces Carbon Footprint. Agriculture, 15(16), 1750. https://doi.org/10.3390/agriculture15161750

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