# Modeling the effect of land use and manure management on soil carbon sequestration in tropical mixed crop-livestock systems: A case study in Guadeloupe (Caribbean)

Jorge Sierra, Fabien Stark, Audrey Fanchone

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Jorge Sierra, Fabien Stark, Audrey Fanchone. Modeling the effect of land use and manure management on soil carbon sequestration in tropical mixed crop-livestock systems: A case study in Guadeloupe (Caribbean). Geoderma Régional, 2025, 41, pp.e00968. 10.1016/j.geodrs.2025.e00968. hal-05119122

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# Modeling the effect of land use and manure management on soil carbon sequestration in tropical mixed crop-livestock systems: A case study in Guadeloupe (Caribbean)

Jorge Sierra<sup>a,1</sup>, Fabien Stark<sup>b</sup>, Audrey Fanchone<sup>c,*</sup>

## ARTICLE INFO

**Keywords:**

Caribbean
Crop-livestock integration
Crop residues
Farm scale
Manure storage
SOC stock

## ABSTRACT

In tropical regions, mixed crop-livestock systems (MCLS) afford opportunities to achieve sustainable agriculture by supplying several ecosystem services, including soil C sequestration. The aim of this study was to assess soil C sequestration in representative tropical (Caribbean) MCLS in the Guadeloupe Archipelago. We designed a modeling approach at the scale of the farm and cropping system, based on a data-driven model of soil organic C (SOC) balance coupled to three databases. The variables included soil properties, manure properties (fresh and stored-composted), and farm characteristics of the eight MCLS. The results indicated that land use (i.e., relative area occupied by perennial crops) was the main factor controlling soil C sequestration at the farm scale and that pasture and sugarcane systems were crucial for maintaining or increasing SOC stocks. Under the current MCLS management, only 16 % of manure production is used to fertilize croplands. This mode of fertilization was found to be ineffective at the farm scale, given that it resulted in low (average + 0.03 % yr⁻<fnref n="1" />) or negative (up to -0.3 % yr⁻<fnref n="1" />) soil C sequestration; manure C inputs did not compensate for SOC outputs by mineralization in annual systems. It is estimated that increasing manure use to near 100 % would increase the level of soil C sequestration 3.5-fold (average + 0.11 % yr⁻<fnref n="1" />) and could compensate for up to 20 % of the reduction in C inputs from crop residues used as animal feed. These effects were 50 % higher for fresh manure than for stored-composted manure because of mass and C losses during storage, which reduced the level of manure C inputs to the soil. Overall, the results indicate that increasing manure use and shortening the storage period through, for example, more frequent split applications would be suitable to boost soil C sequestration and sustainability in tropical MCLS by decreasing soil C losses and increasing C retention in soil. However, trade-offs must be made between managing the frequency of applications, the cost and degree of manual labor required, and optimizing the overall health of the MCLS.

## 1. Introduction

The intensification of food systems during the past century has been characterized by a considerable use of non-renewable resources and the simplification of agricultural systems, leading to increasingly negative environmental impacts (FAO, 2017). Notwithstanding those modern practices—in particular, the decoupling of crop and livestock management—have contributed to increased food production, the loss of agricultural diversity at both farm and regional scales has negative consequences (Garrett et al., 2017). It has been suggested that

reintegrating crop and livestock systems spatially and over time could help overcome several environmental and economic challenges faced by intensified food systems (Rufino et al., 2009). Mixed crop-livestock systems (MCLS), which exploit the synergies between cropping and livestock systems, afford opportunities to achieve sustainable agriculture by increasing productivity and resource-use efficiency, especially in tropical regions (Ramakhanna et al., 2022). Crop-livestock integration is the hallmark of MCLS, as it reflects the extent of nutrient and energy exchanges between the two activities (Rufino et al., 2009). Such integration encompasses several practices such as the use of crop products

<footnote><sup>a</sup> INRAE, UR ASTRO, 97170 Petit-Bourg, Guadeloupe, France</footnote>

<footnote><sup>b</sup> SELMET, Univ Montpellier, CIRAD, INRAE, Institut Agro-Montpellier, 34060 Montpellier, France</footnote>

<footnote><sup>c</sup> INRAE, UR ASSET, 97170 Petit-Bourg, Guadeloupe, France</footnote>

<footnote>* Corresponding author at: UR ASSET, Domaine Duclos, Prise d'eau, 97170 Petit-Bourg, Guadeloupe, France.
E-mail address: audrey.fanchone@inrae.fr (A. Fanchone).</footnote>

<footnote n="1">¹ Deceased</footnote>
and byproducts to feed animals, the use of animal excreta to fertilize crops and amend soils, and using animals for draught power.

In general, MCLS remain understudied (Garrett et al., 2017), although there have been a number of studies over the past 15 years in which researchers have modeled MCLS as a network of mass flows. The goal of these modeling approaches was to investigate the impact of integration practices on the environmental performance of the systems, mainly in tropical regions, for example, Ethiopia (Rufino et al., 2009), Guadeloupe and the Caribbean (Stark et al., 2016, 2018), Brazil (Szymczak et al., 2020), and Madagascar (Fanjaniaina et al., 2022). In most of these studies, the soil was not considered as a specific compartment of the system but was included implicitly in the crop compartment. Szymczak et al. (2020) included soil as a system compartment, but viewed it as a storage box for nutrients without considering the biological processes involved. Thus, processes such as soil C sequestration were not modeled despite their potential importance as a major ecosystem service offered by the MCLS (Brewer and Gaudin, 2020; Terefe et al., 2020). Moreover, the theoretical benefits of MCLS warrant further investigation, considering that the integration of crop and livestock activities may have contradictory effects on soil organic C (SOC) stocks. For example, the use of crop residues to feed animals rather than returning these residues to the soil can negatively affect SOC stocks by reducing C inputs (Sierra et al., 2015). In contrast, using manure to fertilize crops can increase C inputs and, hence, SOC stocks (Xia et al., 2017). In grazing-based systems, it has been reported that SOC stocks under improved pastures might increase because of C inputs from animal excreta and root turnover, as well as the low SOC outputs linked to the lack of soil disturbance by tillage (e.g., Lorenz and Lal, 2018). As the intensity of soil tillage is greater with annual crops, which reduces SOC stocks (Anindita et al., 2023), it can be hypothesized that land use changes (e.g., the grassland-to-cropland area ratio) would affect the potential for soil C sequestration in the MCLS by changing the relative impact of soil tillage at the farm scale. Indeed, a focus on the multidimensional assessment of MCLS—an important and challenging area of research—will clarify the trade-offs between farm production and ecosystem services, such as soil C sequestration.

Few studies on the functioning of the MCLS have been performed in the tropics (Garrett et al., 2017); in particular, long-term field experiments are lacking. Such studies are costly and require additional information to select farm management scenarios to be tested at the field level (Puech and Stark, 2023). In this context, modeling the SOC balance in MCLS offers an opportunity to test several farm management scenarios under different pedoclimatic conditions to calibrate field experiments and identify how soil can best be used in such systems as a C sink (Kane, 2015). This is particularly important in the tropics where climate warming markedly affects SOC stocks and soil quality, both of which are key factors underpinning food production and security in these regions (FAO, 2017). Although some models have been proposed to assess soil C sequestration at the regional scale in tropical MCLS (e.g., Ramakhanna et al., 2022), to our knowledge, there are as yet no modeling studies at the farm scale, which would reflect decisions made regarding land, crop, and livestock allocation. This type of analysis is crucial for MCLS in the tropics, which exhibit a large diversity of structures and functions (Stark et al., 2016; Bisson et al., 2019) that may exert different effects on soil C sequestration.

Here we present a data-driven modeling approach designed to assess soil C sequestration at the cropping system and farm scales in a representative tropical MCLS area in Guadeloupe. Previous studies conducted in Guadeloupe generated data on SOC stocks, soil properties (Sierra et al., 2015), manure properties (Faverial, 2016), and MCLS structure and practices (Stark et al., 2016), which are useful for modeling purposes. More specifically, we aimed to model the impact of land use (farm area occupied by perennial and annual cropping systems) and manure management (level of use and storage) on soil C sequestration on eight MCLS farms.

# 2. Materials and methods

## 2.1. Study area

The study was carried out in the Guadeloupe Archipelago, which forms part of the Lesser Antilles in the eastern Caribbean Sea (16° 15' N, 61° 45' W). The archipelago consists of two main islands (Basse-Terre, 848 km² and Grande-Terre, 586 km²) and several smaller islands. Only the main islands were included in this study. Grande Terre is characterized by a gently undulating land surface, the local relief rarely exceeding 40 m. In contrast, the central region of Basse-Terre is dominated by a mountain chain oriented from northwest to southeast, reaching 1500 m at the La Soufrière volcano in the south. The northern, eastern, and southern parts of Basse Terre have extensive areas of inclined flat surfaces. The land west of the crest slopes steeply towards the Caribbean Sea. Unlike the Basse-Terre soils, which developed on old (north) or young (south) volcanic ash deposits, soils in Grande-Terre developed on coral reef limestone. Sierra et al. (2015) divided the Guadeloupe Archipelago into five agroecological regions (AER) characterized by very different pedoclimatic conditions (Table 1). We used these AER to set the parameter values of the SOC balance model, as proposed by Sierra et al. (2015, 2017). (Details are given in Section 2.4). The eight farms included in our analysis are distributed among four of the five AERs (Table 1). No farms were selected from AER 3.

## 2.2. Characteristics of MCLS

Information concerning the farm structure and functioning of the MCLS was obtained from a database developed by Stark et al. (2016). Data on farm structure are presented in Table 2. Quantitative data concerning the plant material used as livestock feed are given in Table A1 in Appendix A. Industrial supplementary feed was also used but was not included in the model because it does not affect the SOC balance (Stark et al., 2016). There were nine crop and four livestock compartments (Table 2). Semi-confined poultry and pigs were placed in paddocks subdivided into open and covered areas for feeding and rest. Manure was a mixture of excreta and bedding material (straw and sawdust). The rabbits were reared in individual cages using sawdust as bedding material. In the rearing of cattle, two farmers applied a traditional grazing system on natural (farm A) or improved pastures (farm D), and animal excreta were recycled directly in the grassland. Three

<caption>**Table 1**</caption>

Pedoclimatic characteristics of the agroecological regions (AER) of the Guadeloupe Archipelago as described by Sierra et al. (2015).

<table><thead><tr><th>AER</th><th>Island<sup>a</sup></th><th>Temperature<sup>b</sup><br/>°C</th><th>Rainfall<sup>b</sup><br/>Mm*yr<sup>-1</sup></th><th>Soil type</th><th>Soil<br/>pH<sup>c</sup></th><th>SOC<br/>stock<sup>c</sup><br/>mg<br/>C*ha<sup>-1</sup></th></tr></thead><tbody><tr><td>1</td><td>GT</td><td>26.5</td><td>1120</td><td>Calcic<br/>Vertisol</td><td>7.0-8.2</td><td>60-75</td></tr><tr><td>2</td><td>BT<br/>(north<br/>and<br/>north-<br/>east)</td><td>25.4</td><td>2350</td><td>Ferralisol</td><td>4.5-5.5</td><td>50-60</td></tr><tr><td>4</td><td>BT<br/>(south<br/>and<br/>south-<br/>east)</td><td>25.0</td><td>2210</td><td>Nitisol</td><td>5.0-6.5</td><td>40-55</td></tr><tr><td>5</td><td>BT<br/>(west)</td><td>26.6</td><td>920</td><td>Calcisol</td><td>7.0-8.0</td><td>45-55</td></tr></tbody></table>

No farms were selected from AER 3.

<footnote><sup>a</sup> GT: Grande-Terre, BT: Basse-Terre.</footnote>

<footnote><sup>b</sup> Mean annual values.</footnote>

<footnote><sup>c</sup> For the 0-0.25 m soil layer.</footnote>
<table><caption>**Table 2**

Characteristics of crop-livestock activities in eight farms in Guadeloupe.</caption><thead><tr><th rowspan="2">Production factor</th><th colspan="8">Farm</th></tr><tr><th>A</th><th>B</th><th>C</th><th>D</th><th>E</th><th>F</th><th>G</th><th>H</th></tr></thead><tbody><tr><td>Agroecological region</td><td>1</td><td>1</td><td>1</td><td>2</td><td>2</td><td>2</td><td>4</td><td>5</td></tr><tr><td>Total area (ha)</td><td>2.9</td><td>12.9</td><td>18.3</td><td>10.0</td><td>13.0</td><td>15.1</td><td>9.0</td><td>2.7</td></tr><tr><td>Grassland (ha)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Natural pasture</td><td>1.7</td><td>5.0</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Improved pasture</td><td>-</td><td>1.0</td><td>6.0</td><td>3.0</td><td>-</td><td>3.5</td><td>-</td><td>-</td></tr><tr><td>Cropland (ha)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Bananas: export</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>9.0</td><td>-</td></tr><tr><td>Bananas: animal feed</td><td>-</td><td>-</td><td>0.1</td><td>-</td><td>1.0</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Sugarcane</td><td>-</td><td>6.8</td><td>8.8</td><td>6.0</td><td>6.0</td><td>10.5</td><td>-</td><td>-</td></tr><tr><td>Vegetables</td><td>-</td><td>-</td><td>1.0</td><td>1.0</td><td>1.0</td><td>0.5</td><td>-</td><td>2.0</td></tr><tr><td>Tubers</td><td>0.2</td><td>0.1</td><td>2.4</td><td>-</td><td>1.0</td><td>0.5</td><td>-</td><td>-</td></tr><tr><td>Pineapples</td><td>-</td><td>-</td><td>-</td><td>-</td><td>4.0</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Orchard crops</td><td>1.0</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.1</td><td>-</td><td>0.7</td></tr><tr><td>Livestock (TLU)<sup>a</sup></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Cattle</td><td>2.4</td><td>17.6</td><td>19.2</td><td>4.8</td><td>-</td><td>32.8</td><td>-</td><td>-</td></tr><tr><td>Pigs</td><td>2.0</td><td>-</td><td>6.8</td><td>4.4</td><td>21.0</td><td>-</td><td>2.4</td><td>2.2</td></tr><tr><td>Poultry</td><td>2.0</td><td>-</td><td>-</td><td>-</td><td>3.0</td><td>-</td><td>-</td><td>1.0</td></tr><tr><td>Rabbits</td><td>0.2</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Total</td><td>6.6</td><td>17.6</td><td>26.0</td><td>9.2</td><td>24.0</td><td>32.8</td><td>2.4</td><td>3.2</td></tr></tbody></table>

<sup>a</sup> Tropical Livestock Unit as estimated by Fanchone et al. (2022). Data are taken from Stark et al. (2016). Farms are ordered by agroecological region and then by total area.

farmers applied a semi-confined system using natural (farm B) or improved pastures (farms B, C, and F) for grazing, and a covered area for supplementary feeding and resting. The manure used in these systems was a mixture of excreta and bedding material (straw). All bedding materials were sourced from the farm. Manure from animal confinement facilities was either used as a direct organic fertilizer, stored prior to application, or not used to fertilize the crops. Unused manure is frequently transferred to neighboring farms where vegetables and tubers are grown. However, in the case of farms with few animals (and thus not subject to effluent management regulations) the manure can be stored on the ground in the more remote areas of the farm. In Table 2, natural pastures correspond to unfertilized native grasslands dominated by *Dichanthium aristatum* (Poir) C.E. Hubbard, and improved pastures correspond to fertilized artificial grasslands dominated by *Digitaria decumbens* Stent.

## 2.3. Manure management scenarios

In calculating soil C sequestration, we considered two different scenarios concerning the amount of manure used at the cropping system scale and two scenarios concerning the type of manure used. In terms of the amount of manure used, we defined the first scenario (Current) as corresponding to the current rate applied by farmers to each crop (Table 3). In the second scenario (Manure +), we simulated the distribution of total manure production. In Manure+, the distribution of unused manure was based on farming practices observed in Guadeloupe, according to the following order of priority (Sierra et al., 2015): vegetables, tubers, bananas for export, sugarcane, and orchards. On some farms, the amount of unused manure was sufficient to fertilize only one or two prioritized crops (Table 3).

The compiled MCLS data revealed marked diversity between farms concerning the type of manure used on croplands (fresh or stored-composted), which may vary during the growing season as a function of cropland availability, manure production, and the labor force required to collect and spread the manure (Stark et al., 2016). As such diversity is difficult to model, we defined two extreme scenarios: manure applied within 24 h of excretion (Fresh manure) and manure applied after three or four months of storage (Composted manure). Thus, we assumed that the manure stored during this period displayed the

<table><caption>**Table 3**

Rates of manure application for each cropping system and farm. The pair of values indicates the rates used in the Current and in the Manure+ scenarios, respectively. All the values are expressed as fresh manure. For the simulations carried out for the Composted manure scenario, the actual rates of applied manure were estimated accounting for the mass loss during storage presented in Table A6. Natural and improved pastures were not amended with manure collected from animal confinement facilities. The rates of manure application in the Current scenario were calculated from the database elaborated by Stark et al. (2016).</caption><thead><tr><th rowspan="3">Crop</th><th colspan="8">Farm</th></tr><tr><th colspan="8">Mg fresh manure•ha<sup>-1</sup>•yr<sup>-1</sup></th></tr><tr><th>A</th><th>B</th><th>C</th><th>D</th><th>E</th><th>F</th><th>G</th><th>H</th></tr></thead><tbody><tr><td>Bananas: export</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0<sup>a</sup>/<br>1.0<sup>b</sup></td><td>-</td></tr><tr><td>Bananas: animal feed</td><td>-</td><td>-</td><td>0/0</td><td>-</td><td>0/0</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Sugarcane</td><td>-</td><td>0/<br>10.0<sup>b</sup></td><td>0/0</td><td>0/0</td><td>0/0</td><td>0/<br>10.0<sup>b</sup></td><td>-</td><td>-</td></tr><tr><td>Vegetables</td><td>-</td><td>-</td><td>1.0/<br>12.0</td><td>1.0/<br>6.0</td><td>4.5/<br>4.5</td><td>11.0/<br>11.0</td><td>-</td><td>1/<br>4.5</td></tr><tr><td>Tubers</td><td>5.0/<br>25.0</td><td>0/<br>15.0</td><td>0/<br>6.3</td><td>-</td><td>0/<br>3.0</td><td>0/<br>15.0</td><td>-</td><td>-</td></tr><tr><td>Pineapples</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0/0</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Orchard crops</td><td>0/<br>11.0</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0/0</td><td>-</td><td>0/<br>0</td></tr></tbody></table>

<sup>a</sup> Commercial compost was applied at a rate of 30 Mg/ha to each banana plantation every five years.

<sup>b</sup> Manure is applied at each sugarcane plantation every 5 years.

properties of a composted product (Faverial, 2016). We considered that the actual soil C sequestration from manure C was intermediate between the values calculated under these extreme scenarios. Lastly, the two scenarios were combined to obtain four scenarios for manure management: Current/Fresh, Current/Composted, Manure+/Fresh and Manure+/Composted.

We considered that the mineral fertilizer practices applied by farmers were not modified when using manure, as reported by Sierra et al. (2017). Fertilizer practices are summarized in Table A2. This farming strategy is discussed in Section 4.2. We also considered that the use of manure did not affect crop yields or the amount of crop residue, which is in line with the results obtained from several studies conducted in temperate regions (e.g., Maltas et al., 2012) as well as the tropics (e.g., Sileshi et al., 2019) in which cropping systems involving relatively high rates of mineral fertilizers were analyzed, as is the case in Guadeloupe (Sierra et al., 2015) (Table A2). Nevertheless, we performed additional simulations to test the impact of this assumption by accounting for a moderate increase in the amount of crop residues in response to manure application; e.g., +0.5 % per Mg/ha of manure application, regardless of manure type and animal species.

## 2.4. Modeling of SOC balance

In this section, we describe our approach to modeling SOC balance in the MCLS farms (Fig. 1). To simplify the presentation, most of the parameters used in the model and their values are presented in Tables A3 to A5 of Appendix A. The equations describing soil C inputs are presented in Text A1 of Appendix A.

The SOC balance for each cropping system and farm was calculated using the MorGwanik model coupled with three databases (Fig. 1). MorGwanik is a process-based model that was calibrated and evaluated using a diachronic approach on 253 cropping systems, including 827 measurements of SOC stocks recorded over 20 years in Guadeloupe, covering a wide range of soils, climates, and farming systems (Sierra et al., 2015). MorGwanik has been successfully applied to assess SOC balance in many cropping systems in Guadeloupe (Sierra et al., 2017)
<figure><img src="image_4.png" /><figcaption>Fig. 1. Overview of the data-driven modeling approach designed to assess soil C sequestration at the cropping system and farm scales in mixed crop-livestock systems. AER refers to the agroecological region.</figcaption></figure>

and La Reunion (Indian Ocean) (Acosta-Alba et al., 2022). The model simulates the SOC balance at the cropping system scale as a function of annual C inputs and outputs. Carbon inputs correspond to C originating from crop residues and organic amendments (e.g., manure and compost), and C outputs correspond to the C released from SOC mineralization.

The basic equation of the MorGwanik model is:

$$d\text{SOC}_{\text{stock}}/dt = (C_{\text{res}} \times h_{\text{res}}) + (C_{\text{ame}} \times h_{\text{ame}}) - \text{SOC}_{\text{stock}} \times (k_{\text{AER}} \times k_{\text{crop}}) \quad (1)$$

where $d\text{SOC}_{\text{stock}}/dt$ (Mg C•ha⁻¹•yr⁻¹) is the annual change of SOC stock in the 0–0.25 m soil layer; $C_{\text{res}}$ (Mg C•ha⁻¹•yr⁻¹) is the annual C input from crop residues (aboveground and roots); $C_{\text{ame}}$ (Mg C•ha⁻¹•yr⁻¹) is the annual C input from organic amendments; $h_{\text{res}}$ and $h_{\text{ame}}$ (Mg C/Mg C) are the humification coefficients of crop residues and organic amendments, respectively; $k_{\text{AER}}$ (yr⁻¹) is the mineralization rate constant of SOC in each AER, and $k_{\text{crop}}$ (unitless) is a coefficient accounting for the effect of the cropping system on $k_{\text{AER}}$. The coefficient $k_{\text{AER}}$ is specific to each AER and reflects the impact of soil type and climate on SOC mineralization (Table A3), whereas $k_{\text{crop}}$ is specific to each cropping system and reflects the effect of soil tillage intensity on SOC mineralization (lower for perennial crops than for annual crops) (Table A4). The values of $C_{\text{res}}$ and $h_{\text{res}}$ are listed in Table A5. For each cropping system, the applied value of $C_{\text{res}}$ takes account of the fraction of residues used as animal feed (Table A1). The model parameter values for C inputs from manure (for example $C_{\text{ame}}$ and $h_{\text{ame}}$ in Eq. (1)) were obtained from the database developed by Faverial (2016) (Fig. 1) and are summarized in Table A6. These data were obtained from several laboratory studies conducted in Guadeloupe, using manure collected from farms under livestock management, as described above. Data on the rate of manure application in each cropping system were obtained from a database developed by Stark et al. (2016) (Table 3).

The original version of the MorGwanik model was validated for all cropping systems included in this study, except for the grazed pastures. Accordingly, the parameters were calibrated (Eq. (1)) for natural and improved pastures using the soil–crop database developed by Sierra et al. (2015). The calibration procedure and model parameter values for the pastures are presented in Text A2 of Appendix A.

## 2.5. Calculation of changes in SOC stocks at the cropping system and farm scales

The first step in determining changes in SOC was to perform calculations following the MorGwanik model at the cropping system scale for each system on each farm (Table 2). Because soil C sequestration is not a linear process, the change in SOC in each cropping system was calculated as the mean rate over a time series of 30 years, which is a reasonable period for studying SOC dynamics (IPCC, 2008). The 30-yr time series was used to take into account the exponential trend of SOC dynamics in the estimate of soil C sequestration but not to analyze the temporal dynamics in each cropping system. The rate of soil C sequestration were expressed as % initial SOC stock by year to compare cropping systems and farms of different sizes located in the AER, with soils presenting very different levels of SOC (Table 1). Soil C sequestration at the cropping system scale was then calculated as follows:

$$\text{Soil C sequestration} = \left\{ \left[ \left( \frac{\text{SOC}_{\text{stock 30}} - \text{SOC}_{\text{stock 0}}}{\text{SOC}_{\text{stock 0}}} \right) \times 100 \right] / 30 \right\} \quad (2)$$

where SOC stock 0 and SOC stock 30 are the SOC stocks at the beginning and the end of the time series, respectively. The model was initialized for each system in each farm with SOC stock 0 values taken from the soil–crop database developed by Sierra et al. (2015) (Fig. 1). Data on crop management, which can affect the SOC balance, were obtained from the
database developed by Stark et al. (2016). The rate of soil C sequestration at the farm level was calculated as the pooled mean of the values obtained at the cropping system level, by accounting for the area of each system relative to the total farm area (Table 2).

In Guadeloupe, only the yields of export crops (sugarcane and bananas) are readily available, as these data are obtained by farmers from the records of sugar factories or exporting companies. In the MorGwanik model, these yields are used to calculate C inputs from the residues of these crops (Eqs. (A2) to (A5)). In the present study, data on the crop yields of sugarcane and bananas were obtained from the database developed by Stark et al. (2016). In contrast, crops produced in the local market (e.g., vegetables, tubers, pineapples, and orchard fruit) are partially or fully distributed outside commercial networks, and thus yield data are unavailable (Sierra et al., 2017). For these crops, we used the crop residue values reported by Sierra et al. (2015, 2017), which corresponded to the mean values observed in Guadeloupe (Table A5).

Differences in soil C sequestration between the farm management scenarios were assessed by ANOVA using the XLSTAT™ 2018.5 program applying a factorial design (amount of manure × manure type) and considering the farms as replicates (n = 8). Differences between scenarios were considered statistically significant at *P* < 0.05.

# 3. Results

## 3.1. Overview of the current use of manure at the farm and cropping system scales

The total amount of fresh manure produced on farms averaged 15 Mg per year (expressed as fresh matter), with marked variability among the farms (ranging from 6 Mg/yr at farm D to 34 Mg/yr at farm F; Table A7). Higher values were observed on farms with high livestock activity dominated by cattle in semi-confined systems (e.g., farms C and F, Table 2). Under current farm management (i.e., the current scenario in Table A7), the total amount of manure used to fertilize crops averaged 2 Mg per farm and was zero at farms B and G. Therefore, the fraction of unused manure was relatively high, averaging 84 % of manure production, although the portion of unused manure was lower (40 %) at farm E, which is characterized by pig and poultry production. In contrast, higher values of unused manure were observed at farms specializing in cattle production in semi-confined systems, for example 96 % at farm C and 100 % at farm B. Commercial compost was only used at farm G, where, for example, 30 Mg/ha of fresh compost is applied at each banana plantation every five years (Table 2).

Because of the large quantity of unused manure, current rates of application were very low, averaging 0.2 Mg/ha at the farm scale (Table 3). Higher values were observed at the smallest farms (<3 ha, Table 2): 0.7 Mg/ha at farm H and 0.4 Mg/ha at farm A. Although all

farmers cultivating vegetables applied manure on these crops, only one farmer among the five who cultivated tubers used manure as fertilizer (farm A, Table 3). No manure was applied on crops of bananas, sugarcane, pineapples, or in fruit orchards under the current scenario. It should be noted that under the simulated Manure+ scenario the rate of manure application at the farm scale increased 10-fold (i.e., average 2 Mg/ha); the greatest increases were observed at farms A (16-fold) and C (27-fold) (Table 3).

## 3.2. Soil C sequestration at the cropping system and farm scales

Soil C sequestration varied markedly among farms and scenarios (Fig. 2a). For example, the coefficients of variation among farms ranged from 170 % for Manure+/Fresh to 850 % for Current/Composted (data not shown). Soil C sequestration was negative at two farms under all scenarios (farms E and H) and positive values were observed at farms B, C, F, and G (Fig. 2a). Farm A presented slightly negative soil C sequestration under the Current scenario, and positive values under the Manure + scenario for both manure types. At farm D, soil C sequestration was very low, and SOC stocks were near the steady state under all scenarios. At the farm scale, only farm G attained a threshold of 0.4 % C/yr for the Manure+/Fresh scenario, as established by the ‘4 per 1000 Initiative’ proposed at COP21 as a measure to mitigate rising atmospheric CO₂ levels (Minasny et al., 2017). This result was mainly associated with the application of commercial compost to banana crops (Table 3). Considering the eight farms, the mean soil C sequestration was positive under all scenarios and increased significantly in the order Current/Composted < Current/Fresh < Manure+/Composted < Manure+/Fresh (*P* < 0.05) (Fig. 2b). The effect of the quantity of manure applied (i.e., Current vs. Manure+) on soil C sequestration was greater than that of the type of manure (i.e., Fresh vs. Composted) (*P* < 0.05). For example, although soil C sequestration was on average 3.5 times higher under the Manure+ scenario than under the Current scenario, it was 1.5 times higher under the Fresh scenario than under the Composted scenario (*P* < 0.05).

Based on the data on the C content and humification coefficient of the manures assessed in this study (Table A6), we estimated that the effectiveness in terms of C retained in the soil was, on average, 46 % higher for composted manure (79 kg C/Mg fresh matter) than for fresh manure (54 kg C/Mg fresh matter). The apparent contradiction between the effectiveness of each manure type expressed by unit of product, and its impact on soil C sequestration expressed at the farm level is analyzed in Section 4.2.

Additional simulations showed that the general trend observed for the effect of manure application at the farm scale did not vary when considering a moderate increase in crop residue as a function of manure application (Fig. A2). In this case, soil C sequestration at the farm scale

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Predicted soil C sequestration in the 0–0.25 m layer: (a) for each farm and scenario, (b) for each scenario expressed as the mean values of the eight farms. In (b), the values of bars with different letters are significantly different at *P* < 0.05.</figcaption></figure>
increased by an average of 10%; however, the relationship between the scenarios and farms was the same as described above. In the next sections we will focus on the obtained results, excluding any effects of manure application on crop residues (which is discussed in Section 4.2).

Despite considerable differences between the Manure+ and Current scenarios, no significant correlation was observed between soil C sequestration and the rate of manure application (P < 0.05) (Fig. 3a). Indeed, soil C sequestration at the farm scale was mainly affected by land use, and more specifically, by the proportion of perennial crops (P < 0.05) (Fig. 3b), which masked any effects of manure application. The effect of land use on soil C sequestration was approximately twice that of manure application (P < 0.05). Under both the Current and Manure+ scenarios, soil C sequestration was only positive on farms with more than 60% of their cultivated area occupied by perennial crops (i.e., sugarcane, bananas, and orchard crops) as well as pasture (Fig. 3b). Although Fig. 3 corresponds to the fresh scenario, the general pattern observed for the composted scenario was similar to that presented in Fig. 3 (Fig. A3).

The results indicate that the application of fresh cattle manure at a rate of 12 Mg fresh matter•ha⁻¹•yr⁻¹ could offset up to a 20% reduction in C input from the residues of annual crops used to feed animals (Fig. 4). It is interesting to note that without manure application, the rates of soil C sequestration were always negative: from -0.30 % yr⁻¹ to -0.64 % yr⁻¹ for 0 % and 20 % crop residue reductions, respectively.

# 4. Discussion

## 4.1. Impact of land use on soil C sequestration

Most Caribbean soils are relatively rich in organic matter because of the presence of clay minerals with a high retention of organic C (e.g.,

<figure><img src="image_6.png" /></figure>

**Fig. 3.** Predicted soil C sequestration in the 0-0.25 m layer at the farm scale as a function of (a) the rate of manure application, and (b) the farm area occupied by perennial crops, under the Current and Manure+ scenarios. In (a), the correlation between the two variables was not significant (P < 0.05). Data correspond to the Fresh manure scenario.

<figure><img src="image_7.png" /></figure>

**Fig. 4.** Effect of the fraction of crop residues used to feed animals on soil C sequestration at the system scale. Simulations were performed for the vegetable cropping system of farm C with (amended) and without (no amended) the application of 12 Mg fresh cattle manure ha⁻¹ yr⁻¹.

smectite in Calcic Vertisols and Calcisols, Fe and Al hydrous-oxides in Ferralsols; Sierra et al., 2015) (Table 1). These soil properties indicate that the SOC balance is more sensitive to changes in C output (i.e., SOC mineralization) than to C input (e.g., C derived from crop residues and organic amendments) (Chopin and Sierra, 2021). In other words, a relative change in C output affects the SOC balance more than the same change in C input. Variations in C inputs are low in the studied systems, as, the production strategy (type of production, crop-livestock integration practices) and the production factors (family labor, available land and capital) of such system does not vary significantly over time, whereas, variation of C output can be more pronounced. This was reflected in the present study by the higher soil C sequestration observed under perennial crops compared to annual crops, which was directly associated with lower C outputs due to the lower tillage intensity with the former (that is, a lower k_crop under our modeling approach, Eq. (1)). Balesdent et al. (2000) found that the disturbance of the soil structure by tillage is a key factor in enhancing SOC losses by exposing the SOC that is physically protected in microaggregates to biodegradation. Several studies performed on tropical soils have shown that no-till and reduced tillage may markedly reduce SOC loss and promote soil C sequestration (e.g., Powlson et al., 2016). This was the case for the perennial crops assessed in this study: soil tillage was performed every five years on sugarcane and banana plantations, whereas no-till was practiced in orchards and pastures.

Interestingly, both observed and simulated data indicate that improved pastures could be a sink for atmospheric C in a tropical climate (Text A2). Eze et al. (2018) reported that heavy grazing in the tropics may induce a decrease in SOC stocks in improved pastures because of the large quantity of biomass removed by the animals. However, this was not the case in our study because of the low grazing intensity (Table 2), which probably increased C returns from the aboveground biomass and roots (Lorenz and Lal, 2018). In contrast, the management of annual crops (mainly vegetables and tubers) involves the implementation of several tillage operations during the growing season, which promotes SOC loss (Sierra et al., 2017). Our results regarding annual crops were in line with those reported by Minasny et al. (2017), who observed strong impacts of conventional tillage on SOC losses in the tropics, mainly in soils with relatively high SOC content (e.g., >20 g C/kg).

The soil C sequestration results obtained in this study were within the broad range of values reported for other tropical MCLS, for example, the study by Zanatta and Salton (2010) in Brazil and Terefe et al. (2020) in Ethiopia. However, a direct comparison between the farming systems assessed by these authors and ours is very difficult because of the
considerable diversity in climate, farming practices, land use, and initial SOC stocks. For example, the soil C sequestration observed by Terefe et al. (2020) in some MCLS in the central highlands of Ethiopia was two times higher than that estimated in our study, which corresponded to soils with initial SOC stocks that were two or three times smaller than ours. As discussed above, the feasibility of achieving high soil C sequestration by applying a given farming practice strongly depends on the initial SOC stock, which determines the SOC output level (Chopin and Sierra, 2021). Therefore, the extrapolation of our results to other tropical regions should be performed with caution.

## 4.2. Impact of manure management on soil C sequestration

Despite the strong effect of land use on SOC stocks, the impact of manure management was not a minor factor: soil C sequestration at the farm scale under the Manure+ scenario was significantly enhanced compared to sequestration under the current scenario as a result of reduced negative SOC balance under annual cropping systems. Interestingly, farms displaying highly positive soil C sequestration under Manure+ (e.g., farms A and F, Fig. 2a) encompassed substantially different profiles in terms of farm size and functioning. For example, farm A is small and has a relatively large area of pasture and diversified livestock activities, whereas farm F is relatively large and includes diverse cropping systems and large-scale cattle production. This shows that MCLS can achieve relatively high soil C sequestration across a range of farm structures and management practices. Moreover, under the Manure+ scenario, manure application contributed to decreasing or counterbalancing the negative effects of reducing C inputs from crop residues used to feed animals. Indeed, increasing manure recycling and the use of crop residues to feed animals would boost the level of integration between crop and livestock activities, which is a crucial concern in terms of the ecological and economic sustainability of MCLS in the tropics (Brewer and Gaudin, 2020). This is particularly important for the eight farms analyzed in the present study, which currently exhibit a low level of integration, as reported by Stark et al. (2016) and Fanchone et al. (2022). The large quantity of unused manure observed on most farms under the Current scenario reflects this low integration. This may be linked to the lack of mechanical manure spreaders in Guadeloupe, which hinders the application of organic amendments at high rates (Fanchone et al., 2022).

Several authors have reported that greenhouse gas (GHG) emissions during manure storage can represent a relatively high loss of C and nutrients and strongly affect the GHG balance at the farm scale (e.g. Tittonell et al., 2010; Garrett et al., 2017). Indeed, GHG released during manure storage affect net soil C sequestration at the farm scale, which can be expressed as the difference between gross soil C sequestration (i.e., as estimated in the present study) and the equivalent C lost by GHG emissions. As GHG emissions are much higher during manure storage than after manure spreading (IPCC, 2008), it may be hypothesized that differences between the fresh and composting scenarios for net soil C sequestration would be higher than those observed in our estimates of gross soil C sequestration (Fig. 2). To our knowledge, there are as yet no published studies of GHG emissions during manure storage under tropical conditions, which prevents the direct application of the emission factors determined in temperate regions (e.g., IPCC, 2008) to estimate net soil C sequestration in tropical MCLS. Therefore, future research should consider not only the impact of different livestock activities on GHG emissions, mainly during manure storage, but also the impact of storage systems, which can vary markedly among smallholder MCLS in the tropics (Tittonell et al., 2010).

As stated earlier, mineral fertilization practices are not affected by the use of manure in the cropping systems currently applied in Guadeloupe (Sierra et al., 2017). The results of our modeling showed that soil C sequestration should increase if manure has a positive effect on yield and the amount of crop residue, mainly under the Manure + scenario. However, we argue that this would probably not be the case in

view of the fact that the actual rates of mineral fertilizer use are high (e. g., Table A2: 70–250 kg N·ha⁻¹·yr⁻¹; Sierra et al., 2015) and that the application of manure would induce a nutrient excess relative to crop requirements. This agrees with the results reported by Sileshi et al. (2019) for many cropping systems in sub-Saharan Africa. These authors observed that the application of manure in systems involving the high use of mineral fertilizers did not significantly increase yields but resulted in a less efficient use of the nutrients applied, which favored nutrient leaching. Similar results were reported by Zhang et al. (2015) for an intensive maize-cropping system in China. In any case, the nutrient application rates from mineral fertilizers and manure should be calculated based on crop requirements to avoid pollution risks, particularly in humid tropics. Replacing a fraction of mineral fertilizers with manure could be promoted by improving access to information on the global impact of manure application (i.e., as an organic amendment as well as an organic fertilizer), chiefly among smallholder farmers who have little contact with peasant associations and extension services (Fanchone et al., 2022).

The differences found between the manure types when soil C sequestration was considered per unit of applied product were consistent with the higher soil C retention from composted manure, as reported by Xia et al. (2017) in their meta-analysis. Although fresh manure supplies more labile C, which can be mineralized rapidly after spreading, composted manure contains more recalcitrant compounds such as lignin, which have a longer half-life in the soil (Faverial et al., 2016; Biala et al., 2021). This was accounted for in our modeling approach by the different coefficients of humification and C content of each manure type (Table A6). However, in our approach, we also included mass loss during manure storage, which reduced the amount of composted product available for spreading. Thus, soil-C sequestration at the farm scale was lower under the composting scenarios. This result highlights the fact that a reliable assessment of the global impact of manure type on soil C sequestration needs to consider the effect of storage on the quantity and quality of available manure; it should thus be expressed at the cropping system and farm levels, and not by the unit of product.

## 4.3. General considerations

In the modeling approach used in this study, the focus was on biotechnological aspects, thus enabling the identification of practices that could improve C sequestration in tropical MCLS. The identification of promising practices is a prerequisite for studying their impact on a broader scale, including health and socioeconomic performance. The Manure+ scenario described in this study suggests the application of fresh manure, which is already the case in the Current scenario. The application of fresh manure followed by its rapid incorporation is a widespread practice in the study area (Fanchone et al., 2022; Stark et al., 2016). This practice offers the opportunity to reduce storage time, which is known to be a GHG-emitting phase (IPCC, 2008), but also raises other agronomic and health concerns (Bicudo and Goyal, 2003). Composting allowed manure to be sanitized during the thermophilic phase. However, on-farm composting is not a widespread practice in the study area, and on a regional scale, the use of compost is hampered by several factors, including the lack of large-scale local composting platforms (Faverial, 2016).

The results also suggest that increasing the amount and frequency of manure application increases C sequestration. The fractional and multiple applications of manure may increase labor costs and effort, thus decreasing economic benefits. Fanchone et al. (2022) reported that labor intensity and the cost of manual application were among the main constraints reported by non-adopter farmers as reasons for refusing to collect and use manure. Several levers need to be combined to increase manure recycling and soil C sequestration in MCLS. Adapted public policies may propose subsidies to assist in the purchase of adapted materials or compensation for lost earnings. Research plays an
important role in proposing solutions that respect the environment, consumer health, and farmers' expectations.

## 5. Conclusions

Overall, the results of this case study demonstrate that the level of soil C sequestration in tropical MCLS relies mainly on the presence of perennial crops and the impact of manure storage on SOC balance. Therefore, pasture and sugarcane systems are crucial for maintaining or increasing SOC stocks at the farm scale. These systems are important components of Caribbean agriculture and may play a major role in ensuring ecological sustainability by reducing the negative effects of climate change on soil quality. The results also indicate that current manure management is ineffective because it induces low or negative rates of soil C sequestration and does not compensate for the negative effects of annual crops on SOC stocks. Although manure storage represents a strategic node for nutrient redistribution between livestock and crops, shortening its duration through more frequent application and incorporation into the soil could contribute to increasing C and nutrient retention in these farming systems. This should include not only annual crops but also perennial crops, such as bananas, sugarcane, and orchard crops, which are currently little fertilized with manure. Moreover, such practices could replace C inputs from crop residues used to feed animals, which would increase the degree of integration and sustainability of tropical MCLS.

A corollary of this study is that relatively high soil C sequestration at the farm scale could be achieved in MCLS with very different farm structures and functioning (e.g., size, pedoclimatic conditions, crops, and animal species), provided there is an increase in manure use. To achieve this, a reliable assessment of the impact of manure type (i.e., fresh or composted) on soil C sequestration at the farm level should account for the overall effect of storage on both the quality and quantity of products available for application. Although the efficiency of soil C sequestration at the farm scale was greater for fresh manure than for composted manure, the use of fresh manure requires precautions to minimize chemical and biological risks. Further research is needed to determine the conditions under which the improved practices proposed here could be adopted and implemented by farmers, including the negative impact of GHG emissions, and how the rates of mineral fertilizers could be reduced when the application rates of manure are increasing.

### Availability of data

Data will be made available by the authors upon request.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

This study formed part of the AgroEcoDiv Project funded by the European Regional Development Fund (grant 410-00235) and the Regional Council of Guadeloupe (grant 410-00232). We thank the eight farmers who were interviewed for this study and Victoria Hawken for reviewing the English manuscript. We salute the memory of our esteemed colleague Jorge Sierra, who worked with us on this project but passed away during the publication process.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.geodrs.2025.e00968.

## Data availability

Data will be made available on request.

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