Carbon Sequestration in Fine Aroma Cocoa Agroforestry Systems in Amazonas, Peru

Abstract

One way to mitigate climate change is by reducing atmospheric CO₂ levels with the establishment of agroforestry systems (AFSs) that can capture and store atmospheric CO₂. This study therefore estimated the carbon sequestration in two components, aboveground (cocoa trees, other tree species, and leaf litter) and soil, in 15 fine aroma cocoa AFSs in Amazonas, Peru. These cocoa AFSs had a minimum area of 1.5 ha and were distributed into three age groups (each group consisted of five systems or farms): young cocoa trees between 8 and 15 years old, middle-aged cocoa trees between 16 and 29 years old, and adult cocoa trees between 30 and more than 40 years old. Generalized linear mixed model (GLMM) analysis followed by Fisher’s LSD mean comparison test (p > 0.05) determined the significant level of total aboveground biomass and total carbon content in the AFSs’ components. The present findings confirm that Theobroma cacao, Mussa sp., Cordia sp., and Persea sp. were the most common species in all AFSs. Clearly, biomass and carbon content in Theobroma cacao and Cordia sp. increased slightly with age, while fruit species Mussa sp. and Persea sp. decreased with age. The total aboveground carbon stock in young cocoa tree systems (13.64 Mg ha⁻¹) was lower than in middle-aged cocoa systems (20.50 Mg ha⁻¹) and adult cocoa systems (24.86 Mg ha⁻¹); nevertheless, no significant differences were found for any of the age ranges. On the other hand, carbon stocks in soil (up to 30 cm depth) in the AFSs ranged from 119.96 Mg ha⁻¹ to 131.96 Mg ha⁻¹. Meanwhile, the total carbon stored by aboveground and soil components in adults cocoa systems (156.81 Mg ha⁻¹) was higher compared to middle-aged cocoa systems (140.60 Mg ha⁻¹) and young cocoa systems (133.59 Mg ha⁻¹), although no statistically significant differences were found. Eventually, the CO₂ sequestration for young cocoa systems was 490.28 Mg ha⁻¹, and middle-aged and adult cocoa system recorded more than 500 Mg ha⁻¹ of CO₂. Furthermore, these data can further be used by national governments, local governments, and organisations of producers, particularly in accessing payments for environmental services, which may improve economic incomes and contribute to climate change mitigation by reserving biomass and sequestering C from these agroforestry cocoa systems.

1. Introduction

Global warming is considered a major concern worldwide due to massive emissions of greenhouse gases into the atmosphere [1]. In 2003, it was already warned that the concentration of greenhouse gases, such as carbon dioxide (CO₂), methane, and nitrous oxide, has increased considerably in the atmosphere, causing global warming, which leads to climate change [2]. Of the total global emissions, agriculture and land-use-related emissions contribute 17%, which means 9.3 billion tonnes of carbon dioxide equivalent (Gt CO₂eq), and agricultural activities associated with crops and livestock release methane and nitrous oxide at a total of 5.3 Gt CO₂eq [3]. In Peru, agriculture is the third activity reporting the highest amount of emissions: 26,044 Gg CO₂eq [4]. Therefore, the conservation and regeneration of forests within countries has been an outstanding action to mitigate climate change [5], as it holds considerable relevance for environmental services owing to its capacity to store and capture CO₂ [6]_. In fact, agroforestry is also important as a carbon sequestration strategy due to the storage potential of its diverse species of plants as well as soil [7], although their storage capacity may vary according to species and geography [8].

Given that climate change has direct consequences on agriculture, food production, and availability, putting food and nutritional security of the most vulnerable populations at risk [9], some previous studies have linked agri-food production and environmental concerns. Consequently, in recent years, private actors involved in the agri-food industry have been seeking to comply with environmental standards so as to promote sustainable supply in terms of their environment [10]. Social needs, concerns about environmental protection, and the growing demand for natural resources drive decision makers in this sector to focus on projects and programmes that support sustainability [11].

The growing demand for cocoa and decreasing yields due to climate change have led stakeholders in the cocoa production chain to seek multiple strategies regarding cocoa farming [12]. One such strategy is the cultivation of cocoa under agroforestry systems, providing a variety of foodstuffs for local farming families [13] and improving soil fertility by controlling and preventing soil erosion [14]. However, agroforestry practices require intensive training for producers and collaborative work to guarantee the profitability of the crop [15].

Well-designed and well-managed agroforestry practices can turn these systems into effective carbon sinks [7]. Apart from sequestering atmospheric carbon in trees and soils, these agroforestry systems are also potentially sustainable [16]. As in other land-use systems, the amount of carbon sequestered depends on the levels of carbon in the standing biomass, the recalcitrant carbon remaining in the soil, and the carbon sequestered in wood products [7]. For cocoa agroecosystems, carbon sequestration is significant when planted at high densities [17].

To quantify the carbon biomass stock, we use regression models, which convert inventory data into an estimated aboveground biomass [18]. Consequently, measuring the biomass produced in cocoa agroecosystems is an important step to quantify potential carbon sequestration of cocoa production systems [19].

Even though there is substantial potential for carbon sequestration in cocoa AFSs, it is not properly recognized and still largely unexploited [7]. In the Amazonas region, studies related to cocoa AFSs are limited. Accordingly, this study aimed to evaluate and quantify the carbon sequestration of cocoa AFSs as an alternative to mitigating the effects of climate change and potentially contribute to household income from environmental services. To this end, carbon sequestration was calculated by the indirect method using allometric equations for the components: cocoa trees, other tree species, leaf litter, and soil of 15 fine aroma cocoa AFSs in the Amazonas region, one of the main cocoa growing regions of Peru.

2. Materials and Methods

2.1. Study Area

The study was carried out in cocoa agroforestry systems of the Cooperative de Servicio Múltiples APROCAM, which has 235 small cocoa farmers distributed in 4 districts of the province of Bagua (Aramango, Copallín, La Peca and Imaza), 2 districts of the province of Utcubamba (Cajaruro and El Parco), and 1 district of the province of Santa María de Nieva (Nieva) in the Amazonas region (Figure 1).

2.2. Estimation of Carbon Pools in Fine Aroma Cocoa Agroforestry System

2.2.1. Selection of Cocoa Farms and Data collection for the Calculation of Aboveground Biomass (Cocoa Trees, Other Tree Species, and Leaf Litter), and Carbon in the Soil

The selection of the farms used to calculate carbon was carried out following the methodology of Trinidad et al., 2016 [20] with some modifications made to meet the conditions of the cocoa farms in the study area. For this, 15 cocoa farms were selected, which were divided into three age ranges of cocoa cultivation: young cocoa trees that are 8–15 years old; middle-aged cocoa trees that are 16–29 years old, and adult cocoa trees that are 30–40 years old; furthermore, each farm had to have a minimum extension of 1.5 ha, making a total of 22.5 ha sampled for the implementation of this research. On these 15 selected farms, we collected the required data following the methodology described below.

• Only for adult and juvenile trees: In the centre of each plantation system, a rectangle of 1000 m² with a length of 50.00 m and a width of 20.00 m was delimited, which was divided into 2 smaller rectangles, each measuring a length of 25.00 m and a width of 20.00 m (500 m²) (Figure 2) [21]. Trees with a diameter at breast height (DBH) greater than 2.5 cm within the larger rectangle were measured and the DBH was recorded [20].
• Leaf litter: Within the small rectangles (500 m²), we delimited two rectangles of 5.00 m × 20.00 m (100 m²); within these quadrants, sub-quadrants of 0.5 m × 0.5 m (0.25 m²) were defined at 8 points, where all the plant material (leaf litter) found on the soil surface was weighed at each point. Then, a sub-sample of 500 g was taken in an airtight bag and transported to the laboratory to determine the humidity [22].
• Carbon in the soil: A homogeneous sample of 500 g of soil was collected from 10 random points per farm, sampling was done at a depth of 0–30 cm, and this sample was used to calculate the organic matter of the soil at the laboratory [21]. These data were used to calculate the amount of carbon that can be accumulated by the studied AFS.

2.2.2. Measurement of Total Aboveground Biomass (TAB) and Calculation of the Total Carbon of the Cocoa Agroforestry Systems

To measure aboveground biomass, the methodology for estimating and monitoring carbon sequestration used equations that were established by previous studies, which are described in

Table 1. Equations for estimating aboveground biomass (kg/tree).

Tree Species	Allometric Equations	Author
Timber trees	BA = 0.1184DBH2.53	[23]
Cacao trees	Y = 0.1208 DBH1.98	[24]
Musaceae	Y = 0.0303DBH2.13	[25]
Fruit trees	$\mathrm{B} ={10}^{\left(-1.11+2.64\ast \mathrm{Log}\left(\mathrm{D}\mathrm{B}\mathrm{H}\right)\right)}$	[26]

Y = biomass (kg/tree), DBH = diameter at breast heigh (cm), BA = biomass of forest trees; Log: Logarithm base 10.

.

In addition, the total aboveground biomass (TAB) of the studied AFSs included the total biomass of adult and juvenile trees (TTAB), plus the biomass of litter leaf (Bh) (Equation (1)). The TTAB per hectare was calculated by adding the biomasses of all adult and juvenile trees (TTAB_total) that were recorded in the transects by tree species multiplied by a conversion factor of 0.01; this conversion factor means that the transect was 1000 m² (Equation (2)) [6]. Bh was obtained from the division of the sampled fresh weight of the collected leaf sample (g) (SFW) by the final dry weight of the sample (g) (FDW); this result was multiplied by the total fresh weight per square meter (g/m²) (TFW) and a conversion factor of 0.004 (Equation (3)) [27].

$$\mathrm{TAB} \left(\mathrm{t}/\mathrm{ha}\right)=\left(\mathrm{TTAB}+\mathrm{Bh}\right)$$

(1)

$$\mathrm{TTAB}\left(\mathrm{t}/\mathrm{ha}\right)={{\displaystyle \sum }}^{}\mathrm{TTABtotal}\ast 0.01$$

(2)

$$\mathrm{Bh} \left(\mathrm{t}/\mathrm{ha}\right)=\left(\left(\mathrm{SFW}/\mathrm{FDW}\right)\ast \mathrm{TFW}\right)\ast 0.004$$

(3)

The total carbon (TC) of the agroforestry systems was composed of the carbon-bond in the aboveground biomass (CAB) and the total soil carbon (SC) (Equation (4)). For this purpose, CAB was obtained by multiplying the TAB by the conversion factor 0.5 (Equation (5)) [28]. Soil carbon was determined by multiplying the soil volume weight (SVW) by the percentage of carbon (%C), which was determined in the laboratory soil analysis and divided by the conversion factor 100 (Equation (6)) [6].

$$\mathrm{TC} \mathrm{t}/\mathrm{ha}=\mathrm{CAB}+\mathrm{SC}$$

(4)

$$\mathrm{CAB} \left(\mathrm{t}/\mathrm{h}\mathrm{a}\right)=\mathrm{TAB}\ast 0.5$$

(5)

$$\mathrm{SC} \left(\mathrm{t}/\mathrm{ha}\right)=\left(\mathrm{SVW}\ast \%\mathrm{C}\right)/100$$

(6)

Considering that the bulk density is associated with and varies according to the porosity and textural class of the soil [29], the calculation of the SVW was performed by multiplying the standard bulk density (BD), according to the proposed soil texture (

Table 2. Bulk density based on soil texture.

Soil Texture	Total Pore Space or Total Porosity (%)	Bulk Density BD (g/cm3)
Sandy	38 (32–42)	1.65 (1.55–1.80)
Sandy loam	43 (40–47)	1.50 (1.40–1.60)
loam	47 (43–49)	1.40 (1.35–1.50)
Clay loam	49 (47–51)	1.35 (1.30–1.40)
Sandy clay	51 (49–53)	1.30 (1.25–1.35)
Clay	53 (51–55)	1.25 (1.20–1.30)

Source: Adapted from Vasquez, A. et al., 2017 [30].

) [30], by the soil depth (SD) and the area (10,000 m²) (Equation (7)). For soils with a sandy clay loam texture, a bulk density of 1.40 g/cm³ was considered [31].

$$\mathrm{SVW} \left(\mathrm{t}/\mathrm{h}\mathrm{a}\right)=\mathrm{BD}\ast \mathrm{SD}\ast \mathrm{10,000}$$

(7)

Finally, we calculated the total CO₂ sequestrated in the cocoa AFSs by multiplying the amount of total carbon by 3.67 (conversion factor) [21].

2.3. Statistical Analysis

The data were analysed with the statistical software InfoStat/P version 2018 [32]. The results of aboveground biomass and total carbon of the agroforestry systems were reported in summary tables, with values of mean and standard deviation. Moreover, generalized linear mixed model (GLMM) analysis followed by Fisher’s LSD mean comparison test (p ≤ 0.05) determined the significance level of total aboveground biomass and total carbon content. GLMM relaxes the assumptions of normality, constant error variance, and a linear relationship between the effects of covariates and the mean to incorporate a wide variety of random effects [33]; effectively, this technique provided a more flexible approach by which to analyse data [34].

3. Results

3.1. Aboveground Biomass Capture and Carbon Stored in Cocoa Agroforestry Systems

Theobroma cacao, Mussa sp., Cordia sp., and Persea sp. were the most common species in cocoa AFSs. The aboveground biomass of the AFSs showed a slight increase with the age of trees in the system, although no statistically significant differences were found for any of the tree species that formed the AFSs (

Table 3. Aboveground biomass of agroforestry systems (mean ± standard deviation).

AFS	Theobroma cacao Mg ha−1	Mussa sp. Mg ha−1	Cordia sp. Mg ha−1	Persea sp. Mg ha−1	Other Species * Mg ha−1	Leaf Litter Mg ha−1	Total Aboveground Biomass Mg ha−1
1	9.35 ± 0.05	0.71 ± 0.03	13.65.04 ± 4.65	0.63 ± 0.04	27.64 ± 7.01	1.11 ± 0.10	53.09 ± 10.68
2	13.24 ± 0.10	0.85 ± 0.09	00 ± 00	00 ± 00	00 ± 00	0.83 ± 0.41	14.92 ± 05.28
3	14.15 ± 0.22	00 ± 00	1.58 ± 0.00	00 ± 00	4.72 ± 0.34	1.14 ± 0.22	21.59 ± 05.45
4	9.35 ± 0.04	1.81 ± 0.14	27.17 ± 4.96	00 ± 00	10.99 ± 4.87	0.98 ± 0.14	50.30 ± 02.66
5	14.47 ± 0.07	00 ± 00	13.96 ± 1.11	00 ± 00	5.08 ± 1.69	1.05 ± 0.15	34.56 ± 06.81
6	13.11 ± 0.04	1.14 ± 0.07	00 ± 00	00 ± 00	2.92 ± 0.38	1.03 ± 0.14	18.20 ± 05.05
7	11.59 ± 0.04	4.77 ± 0.05	11.24 ± 1.08	00 ± 00	00 ± 00	0.87 ± 0.21	28.47 ± 05.46
8	30.64 ± 0.05	3.61 ± 0.05	11.24 ± 1.08	3.68 ± 0.41	1.42 ± 0.27	1.06 ± 0.07	51.65 ± 11.40
9	24.02 ± 0.13	00 ± 00	00 ± 00	00 ± 00	18.79 ± 5.27	0.91 ± 0.10	43.72 ± 11.07
10	24.16 ± 0.11	00 ± 00	00 ± 00	00 ± 00	0.02 ± 0.00	1.07 ± 0.27	25.25 ± 09.78
11	17.76 ± 0.04	1.54 ± 0.03	16.34 ± 11.28	00 ± 00	0.61 ± 0.00	1.23 ± 0.26	37.48 ± 08.40
12	12.91 ± 0.09	0.06 ± 0.00	20.22 ± 1.06	00 ± 00	5.51 ± 0.59	0.87 ± 0.20	39.57 ± 08.57
13	07.35 ± 0.13	4.75 ± 0.04	4.03 ± 1.38	3.04 ± 1.32	14.25 ± 3.25	0.78 ± 0.14	34.20 ± 04.71
14	9.33 ± 0.06	0.19 ± 0.02	95.70 ± 22.52	0.25 ± 0.00	9.90 ± 3.63	0.61 ± 0.31	115.98 ± 37.69
15	10.01 ± 0.05	3.92 ± 0.04	00 ± 00	2.55 ± 0.00	5.63 ± 0.16	0.73 ± 0.37	22.84 ± 01.79
8–15 years	12.13 ± 2.90 a	2.13 ± 2.44 a	6.16 ± 6.13 a	0.61 ± 1.36 a	5.39 ± 5.34 a	0.97 ± 0.15 ab	27.40 ± 7.37 a
16–29 years	14.71 ± 6.31 a	0.82 ± 8.83 a	15.48 ± 10.04 a	0.13 ± 0.28 a	8.95 ± 11.34 a	1.05 ± 0.14 a	41.14 ± 11.12 a
30–40 years	17.45 ± 9.44 a	1.71 ± 1.90 a	21.39 ± 41.83 a	1.30 ± 1.71 a	7.15 ± 7.57 a	0.83 ± 0.17 b	49.82 ± 39.89 a

Means with letters in common are not significantly different, according to Fisher’s LSD (p > 0.05). * Other forest and fruit species found within the cocoa AFSs.

). Adult AFSs (30–40 years old) accumulate 0.83 Mg ha⁻¹ of biomass in the leaf litter, which is numerically lower than young AFSs (8–15 years old) accumulating 0.97 Mg ha⁻¹, and statistically lower than middle-aged AFSs (16–29 years old) accumulating 1.05 Mg ha⁻¹. On the other hand, the accumulation of total aboveground biomass in adult AFSs (49.82 Mg ha⁻¹) is higher; however, there are no significant differences with the accumulation in young and middle-aged AFSs (41.14 Mg ha⁻¹ and 27.40 Mg ha⁻¹, respectively).

The total carbon stored in the aboveground component of Theobroma cacao, Mussa sp., Cordia sp., and Persea sp. had no significant statistical differences in the evaluated cocoa AFSs. However, as the plantation gets old, there is a slightly higher amount of C stored. On the other hand, C stored in leaf litter showed significant difference, with adult cocoa systems storing less C than middle-aged and young cocoa systems (0.47 Mg ha⁻¹ and 0.44 Mg ha⁻¹, respectively). Finally, in the cocoa systems, carbon stored in trees and leaf litter had no significance between them, but the adult cocoa systems store a little bit more C than the other systems (

Table 4. Carbon in aboveground biomass of agroforestry systems (mean ± standard deviation).

AFS	Theobroma cacao Mg ha−1	Mussa sp. Mg ha−1	Cordia sp. Mg ha−1	Persea sp. Mg ha−1	Other Species * Mg ha−1	Litter Leaf Mg ha−1	Total Aboveground Biomass Mg ha−1
1	4.69 ± 0.03	0.35 ± 0.01	6.83 ± 2.33	0.32 ± 0.02	13.82 ± 3.50	0.50 ± 0.05	26.51 ± 5.35
2	6.62 ± 0.05	0.43 ± 0.05	00 ± 00	00 ± 00	00 ± 00	0.37 ± 0.19	7.42 ± 2.64
3	7.08 ± 0.11	00 ± 00	0.79 ± 0.00	00 ± 00	2.36 ± 0.17	0.51 ± 0.10	10.74 ± 2.73
4	4,69 ± 0.02	0.9 ± 0.07	13.58 ± 2.48	00 ± 00	5.5 ± 2.43	0.44 ± 0.06	25.11 ± 5.15
5	7.24 ± 0.03	00 ± 00	6.98 ± 0.56	00 ± 00	2.55 ± 0.80	0.47 ± 0.06	17.24 ± 3.34
6	6,52 ± 0.02	0.56 ± 0.04	00 ± 00	00 ± 00	1.46 ± 0.20	0.47 ± 0.07	9.01 ± 2.52
7	5.81 ± 0.02	2.36 ± 0.02	5.62 ± 0.54	00 ± 00	00 ± 00	0.39 ± 0.09	14.18 ± 2.74
8	15.30 ± 0.06	1.79 ± 0.02	5.62 ± 0.54	1.84 ± 0.21	0.7 ± 0.13	0.48 ± 0.03	25.73 ± 5.70
9	11.98 ± 0.06	00 ± 00	00 ± 00	00 ± 00	9.39 ± 2.63	0.41 ± 0.05	21.78 ± 5.53
10	11.92 ± 0.06	00 ± 00	00 ± 00	00 ± 00	0.01 ± 0.00	0.48 ± 0.12	12.41 ± 4.83
11	8.86 ± 0.02	0.77 ± 0.02	8.17 ± 0.64	00 ± 00	0.31 ± 0.00	0.55 ± 0.12	18.66 ± 4.27
12	6.50 ± 0.05	0.03 ± 0.00	10.12 ± 0.53	00 ± 00	2.75 ± 0.29	0.39 ± 0.09	19.79 ± 4.30
13	3.64 ± 0.07	2.38 ± 0.02	2.01 ± 0.69	1.52 ± 0.66	7.11 ± 1.62	0.35 ± 0.06	17.01 ± 2.35
14	4.62 ± 0.03	0.09 ± 0.02	47.86 ± 11.26	0.12 ± 0.00	4.93 ± 1.82	0.27 ± 0.14	57.89 ± 18.860
15	5.04 ± 0.02	2.00 ± 0.02	00 ± 00	1.28 ± 0.00	2.81 ± 0.08	0.33 ± 0.17	11.46 ± 1.79
8–15 years	6.06 ± 1.46 a	1.06 ± 1.22 a	3.08 ± 3.06 a	0.30 ± 0.68 a	2.70 ± 2.67 a	0.44 ± 0.07 ab	13.64 ± 3.69 a
16–29 years	7.33 ± 3.08 a	0.41 ± 0.41 a	7.74 ± 5.02 a	0.06 ± 0.14 a	4.48 ± 5.67 a	0.47 ± 0.06 a	20.50 ± 5.63 a
30–40 years	8.71 ± 4.71 a	0.86 ± 0.96 a	10.70 ± 20.92 a	0.65 ± 0.86 a	3.57 ± 7.78 a	0.37 ± 0.08 b	24.86 ± 19.91 a

Means with letters in common are not significantly different, according to Fisher’s LSD (p > 0.05). * Other forest and fruit species found within the cocoa AFSs.

).

Soil carbon stock within the cocoa AFSs has had a similar behaviour to the carbon stored in the aboveground component; adult systems (131.96 Mg ha⁻¹) stored more carbon than middle-aged and young cocoa AFSs. Importantly, more than 50% of the evaluated systems presented clay and sandy clay loam soil textures (

Table 5. Soil organic carbon at 30 cm depth.

AFS	SVW Mg ha−1	Texture of Soil	Bulk Density (g/cm3)	% Carbono	Total Carbon in Soil Mg ha−1
1	3750	Clay	1.25	1.60	60.00
2	3900	Sandy clay	1.30	2.13	83.07
3	4200	Loam	1.40	2.13	89.46
4	3750	Clay	1.25	3.15	118.13
5	3750	Clay	1.25	2.75	103.13
6	4200	Sandy clay loam	1.40	3.50	147.00
7	3900	Sandy loam	1.30	3.57	139.23
8	4200	Sandy clay loam	1.40	3.65	153.30
9	4200	Sandy clay loam	1.40	3.31	139.02
10	3750	Clay	1.25	4.50	168.75
11	3750	Clay	1.25	4.10	153.75
12	3750	Clay	1.25	3.73	139.88
13	4200	Sandy clay loam	1.40	2.88	120.96
14	4500	Sandy loam	1.50	3.55	159.75
15	4500	Sandy clay	1.50	2.77	124.65
8–15 years		—	—	2.97 ± 0.59	119.96 ± 24.07
16–29 years		—	—	2.42 ± 1.13	128.10 ± 42.39
30–40 years		—	—	3.08 ± 0.63	131.96 ± 30.50

).

Although, the total C stored in aboveground (trees and leaf litter) and soil components in adult cocoa AFSs were higher (156.81 Mg ha⁻¹) than middle-aged and young cocoa AFSs (148.60 and 133.59 Mg ha⁻¹, respectively), there were no significant statistical differences between the three age ranges evaluated (Figure 3).

3.2. CO₂ Sequestration of Cocoa Agroforestry Systems

Young systems had the lowest CO₂ retention with 490.28 Mg ha⁻¹, followed by middle-aged and adult systems with more than 500 Mg ha⁻¹ of CO₂, even though no significant statistical differences in CO₂ retention were found between them (Figure 4).

4. Discussion

AFSs provide ecosystem goods and services that mitigate climate change by sequestering biomass and carbon [35]. In the fine aroma cocoa AFSs in Amazonas; Theobroma cacao, Mussa sp., Cordia sp., and Persea sp. were the most common species; having said that, the aboveground component in these cocoa AFSs can store from 14 to 115 Mg ha⁻¹ of plant biomass and from 7 to 54 Mg ha⁻¹ of carbon, and soils can store from 60 to 168 Mg ha⁻¹ of C. On the other hand, previous studies have shown that the cocoa AFSs composed of fruit trees such as Inga edulis, Citrus sinensis, and Averrhoa carambola can reserve 76.6 Mg ha⁻¹ of carbon in aboveground biomass, 8.9 Mg ha⁻¹ of carbon in dead biomass, and 25.6 Mg ha⁻¹ of carbon in soil [36]. Systems that store an average of 122 ± 24 Mg ha⁻¹ of total carbon, where 43% of the stored carbon comes from aboveground biomass and 41% from soil, are considered systems with an intermediate to high level of storage [21]; therefore, the cocoa systems in our study are above these results showing high-level systems for carbon storage.

The amount of aboveground carbon in cocoa AFSs can be positively influenced by the proportion of timber trees and negatively influenced by the proportion of cocoa plants [37]. Indeed, cocoa AFSs that are well-associated with timber, fruit, and industrial forest species have efficiently the highest carbon sequestration [38]. Cocoa AFSs between 12 and 20 years old can reserve carbon above 40 Mg ha⁻¹, while 5-year-old systems capture C below 30 Mg ha⁻¹ [38]; accordingly, the amount of carbon stored can also be influenced by the age of crops, i.e., as the age of the cocoa increases, the amount of carbon stored is higher [24]. This same behaviour was observed in the cocoa AFSs studied in this research, showing that as the age of the systems increased, the amount of stored C increased.

Notably, Theobroma cacao and Cordia sp. (the former a main crop and the latter a timber shade species) report the highest carbon storage with 6.06 and 3.08 Mg ha⁻¹, respectively, in young cocoa AFSs, increasing with the age of the system based on the age of the cacao crop. In our study, in adult systems, Theobroma cacao and Cordia sp. can store 8.71 and 10.70 Mg ha⁻¹, respectively. It was observed that in timber species, there is a positive relationship between total height, DBH, and age of the species; therefore, there is a linear relationship between DBH and the accumulation of total biomass and, consequently, the accumulation of total carbon [39]. In a 25-year-old cordia-cocoa system, cordia trees can store 80–85% of the total biomass carbon [40], explaining why in the middle-aged and adult cocoa AFSs that we studied, cordia reported the highest amount of carbon stock.

On the other hand, fruit species Mussa sp. and Persea sp. reported low levels of car-bon storage with 0.86 Mg ha⁻¹ and 0.65 Mg ha⁻¹ of carbon in adult cocoa AFSs, although this storage may be higher when the cocoa AFSs are younger. These results are similar to other studies, which have reported that banana farms in production can store up to 0.69 Mg ha⁻¹ of C [41]. So, Mussaa sp. can report low levels of carbon storage especially as a monoculture [42]. Nonetheless, Mussa sp. in combination with woody tree crops potentially constitute an important component of agroforestry [43]. In general, agroforestry systems with perennial crops can be important carbon sinks, while intensively managed agroforestry systems with annual crops have more of a similarity to conventional agriculture [7].

On the other hand, soil carbon sequestration was relatively higher compared to the aboveground component in these cocoa AFSs, ranging from 119.96 Mg ha⁻¹ of carbon in young cocoa AFSs to 131.96 Mg ha⁻¹ in adult cocoa AFSs. Previous studies affirm that soil may hold larger stocks of organic carbon within a farm [1,6]. However, it may be influenced by the clay content that it has [44]. Moreover, agricultural soils with perennial vegetation may have higher soil organic carbon than soils with annual vegetation, as the concentration of organic matter may favour carbon sequestration in soils [45]. Likewise, belowground tree roots also contribute to belowground carbon storage, which is essential for maintaining a stable soil environment [46]. In systems with more frequent tree pruning, the total organic matter contribution is higher [16]. This analysis may explain why adult cocoa AFSs are able to sequester more carbon than younger cocoa AFSs. Conversely, soil degradation, misuse, and mismanagement can result in the depletion of carbon stocks in this component [47]; therefore, cocoa AFSs with perennial crops need minimal tillage for soil conservation. In effect, in adult cocoa AFSs, the accumulation of organic matter increases and may favour carbon sequestration.

The amount of carbon in any agroforestry system depends on the structure and function of the different components of the systems implemented [48]. Consequently, if we measure the efficiency of carbon sequestration in different systems, traditional land use systems are less efficient than cocoa agroforestry systems in association with timber forest species [38]. In cocoa AFSs older than 15 years, the amount of CO₂ sequestered exceeds 500 Mg ha⁻¹ and not only does it improve with age but also with good soil management, such as the implementation of mulching, minimum tillage, and organic amendments on the farm [49]. Given that agroforestry practices are identified as one of the main strategies to reduce CO₂ emissions from the agricultural sector [50], cocoa AFSs can become a good alternative to mitigate climate change and producers can use them as a way to access payments for environmental services, increasing their economic income.

5. Conclusions

The biomass and carbon sequestration in cocoa AFSs is higher with the age of the system. Timber tree species like Cordia sp. can capture more than 10 Mg ha⁻¹ of carbon in systems older than 29 years. On the contrary, in the fruit species Musa sp. and Persea sp., carbon sequestration is lower and it tends to decrease with age of the system. On the other hand, in cocoa agroforestry older than 29 years, soil can capture more than 130 Mg ha⁻¹ of carbon and this is higher than the carbon captured by the aerial component. Finally, cocoa AFSs can retain more than 490 Mg ha⁻¹ of CO₂.

Carbon sequestration and emissions trading offers an innovative approach to rural economic development and environmental conservation. Therefore, the results obtained can be an alternative for national, regional, and local governments and/or farmers’ organizations to implement and promote ecosystem service programs in order to contribute to the income derived from cocoa production and mitigate climate change.