*3.3. Organic Carbon Concentrations in Particle Size Fractions*

The average total fraction masses ranged between 969.33 and 991.84 mg·frac·g −1 soil indicating an average recovery rate varying between 96.9 and 99.1% (Table 4). Regardless of the cropping system and the soil layer, the non-particulate organic fractions (NOM) showed the highest carbon concentration 3.40–7.99 mg·kg−<sup>1</sup> . The fine particulate organic fractions (fPOM) and the coarse particulate fractions (cPOM) depicted the lowest carbon content between 0.56 and 2.3 mg/kg. In the layer 0–10 cm, carbon concentration in the cropping systems Ab10YF and 5YF were significantly higher in the three fractions NOM (*p* = 0.003), **(g·kg<sup>−</sup>1)**

**C stock (Mg C·ha<sup>−</sup>1)**

**Soil** 

fPOM (*p* = 0.01) and cPOM (*p* = 0.02). The cropping systems YMI and MCRC recorded the lowest carbon concentrations. For the layer 10–20 cm, 5YF exhibited the highest carbon content in NOM, while MCRC and YMI showed the lowest (1.05 and 1.26 mg·kg−<sup>1</sup> , respectively). For this layer, SOC concentration in the NOM fraction varied significantly between the cropping systems (*p* = 0.04). No significant difference was observed for the carbon concentration in the fPOM fraction. However, the carbon content in the cPOM fraction varied significantly (*p* = 0.04) between the cropping systems, with Ab10YF and 5YF and TP recording the highest carbon concentrations 1.24, 1.12 and 0.84 g/kg, respectively. 0–10 3.25 ± 0.52 c 7.34 ± 3.64 bc 4.72 ± 1.9 bc 5.73 ± 2.35 bc 18.1 ± 6.35 a <0.001 \*\*\* 10–20 3.0 ± 0.7 ab 2.98 ± 0.63 ab 3.31 ± 2.05 ab 6.29 ± 5.0 a 1.39 ± 0.5 b 0.001 \*\* 20–30 2.90 ± 1.07 a 2.90 ± 1.07 a 3.08 ± 1.13 a 2.47 ± 0.91 ab 1.35 ± 0.1 b 0.006 \*\* 30–40 2.24 ± 0.49 ab 1.81 ± 0.57 ab 1.97 ± 0.96 ab 3.26 ± 2.26 a 1.35 ± 0.57 b 0.012 \* MCRC: maize–cotton relay cropping; 5YF: 5-year fallow; Ab10YF: above 10-year fallow; YMI: yam– maize intercropping system; TP: teak plantation; SOC: soil organic carbon; C stock: carbon organic stock. Means that do not share a letter are significantly different at α = 0.05.; \* *p* value significant at 5%; \*\* *p* value significant at 1%; \*\*\* *p* value significant at 0.1%.

*C* **2022**, *8*, x FOR PEER REVIEW 4 of 5

**Properties Depth (cm) Cropping Systems** *p***-Value MCRC YMI TP 5YF Ab10YF** 

**Table 3.** Soil organic carbon content and carbon stock per soil depth layer*.* 

0–10 3.14 ± 0.98 b 7.37 ± 4.24 b 5.64 ± 2.62 b 4.94 ± 2.3 b 24.1 ± 11.6 a <0.001 \*\*\* 10–20 3.03 ± 1.33 b 2.21 ± 0.47 b 3.16 ± 1.04 ab 5.22 ± 3.78 a 2.43 ± 0.23 b 0.011 \* 20–30 2.55 ± 0.79 a 2.06 ± 0.55 a 2.84 ± 1.0 a 2.83 ± 1.04 a 2.10 ± 0.43 a 0.1 ns 30–40 2.24 ± 0.61 ab 1.36 ± 0.29 b 2.64 ± 0.85 ab 4.43 ± 4.37 a 2.27 ± 2.47 ab 0.021 \*

**Figure 2.** Total carbon stock for soil layers 0–30 cm (**A**) and 0–40 cm (**B**) MCRC: maize–cotton relay cropping; 5YF: 5-year fallow; Ab10YF: above 10-year fallow; YMI: yam–maize intercropping system; TP: teak plantation. Means that do not share a letter are significantly different at α = 0.05. **Figure 2.** Total carbon stock for soil layers 0–30 cm (**A**) and 0–40 cm (**B**) MCRC: maize–cotton relay cropping; 5YF: 5-year fallow; Ab10YF: above 10-year fallow; YMI: yam–maize intercropping system; TP: teak plantation. Means that do not share a letter are significantly different at α = 0.05.

*3.3. Organic Carbon Concentrations in Particle Size Fractions*  The average total fraction masses ranged between 969.33 and 991.84 mg·frac·g−1 soil **Table 4.** Carbon concentrations in particle size fractions across the cropping systems for 0–10 and 10–20 cm.


nificantly between the cropping systems (*p* = 0.04). No significant difference was observed for the carbon concentration in the fPOM fraction. However, the carbon content in the cPOM fraction varied significantly (*p* = 0.04) between the cropping systems, with Ab10YF and 5YF and TP recording the highest carbon concentrations 1.24, 1.12 and 0.84 g/kg, re-MCRC: maize–cotton relay cropping; 5YF: 5-year fallow; Ab10YF: above 10-year fallow; YMI: yam–maize intercropping system; TP: teak plantation; fPOM: fine particulate organic matter; cPOM: coarse particulate organic fraction; NOM: non-particulate organic matter. Means that do not share a letter are significantly different at α = 0.05; \* *p* value significant at 5%; \*\* *p* value significant at 1%.

### spectively. *3.4. Carbon Enrichment Factor (EF) in Particle-Size Fractionation*

The contribution of each particle-size organic matter fraction to the total organic carbon content expressed using the enrichment factor for the two layers revealed that regardless of the cropping systems and the layer, the NOM fraction exhibited the greatest contribution to the total SOC, while cPOM exhibited the lowest contribution (Figure 3). In the two layers, MCRC recorded the highest contribution of the NOM fraction 71.9% and 75.03%. The cropping systems Ab10YF, YMI and TP recorded the highest carbon contribution from the cPOM between 7.1% and 22% for the two layers and between 28.89% and 44.41% for fPOM. In addition, EF values showed that the cPOM and fPOM were the most influenced by the cropping systems.

**0–10**

**10–20**

**Total fraction mass**

**Total fraction mass** 

**Table 4.** Carbon concentrations in particle size fractions across the cropping systems for 0–10 and

MCRC: maize–cotton relay cropping; 5YF: 5-year fallow; Ab10YF: above 10-year fallow; YMI: yam– maize intercropping system; TP: teak plantation; fPOM: fine particulate organic matter; cPOM: coarse particulate organic fraction; NOM: non-particulate organic matter. Means that do not share a letter are significantly different at α = 0.05.; \* *p* value significant at 5%; \*\* *p* value significant at 1%.

The contribution of each particle-size organic matter fraction to the total organic carbon content expressed using the enrichment factor for the two layers revealed that regardless of the cropping systems and the layer, the NOM fraction exhibited the greatest contribution to the total SOC, while cPOM exhibited the lowest contribution (Figure 3). In the two layers, MCRC recorded the highest contribution of the NOM fraction 71.9% and 75.03%. The cropping systems Ab10YF, YMI and TP recorded the highest carbon contribution from the cPOM between 7.1% and 22% for the two layers and between 28.89% and 44.41% for fPOM. In addition, EF values showed that the cPOM and fPOM were the most

**Depth (cm**) **Fractions MCRC (g·kg<sup>−</sup>1) YMI (g·kg−1) TP (g·kg<sup>−</sup>1) 5YF (g·kg<sup>−</sup>1) Ab10YF (g·kg<sup>−</sup>1)** *p* **Value** 

**(g·kg<sup>−</sup>1 soil)** 986.62 978.67 971.22 981.45 969.33

**(g/kg soil)** 991.84 988.33 975.52 978.56 989.42

influenced by the cropping systems.

NOM 3.40 ± 0.4 d 4.46 ± 0.38 c 3.73 ± 0.33 d 5.93 ± 0.39 b 7.99 ± 0.21 a 0.003 \*\* fPOM 1.19 ± 0.21 b 1.23 ± 0.32 b 1.28 ± 0.3 b 1.89 ± 0.2 ab 2.24 ± 0.12 a 0.01 \* cPOM 0.70 ± 0.4 b 0.71 ± 0.42 b 1.08 ± 0.9 ab 2.09 ± 0.8 a 2.3 ± 5.33 a 0.02 \*

NOM 1.05 ± 0.38 c 1.26 ± 0.25 c 1.66 ± 0.30 b 2.53 ± 0.27 a 1.81 ± 0.99 b 0.04 \* fPOM 1.14 ± 0.47 a 0.99 ± 0.29 a 0.78 ± 0.21 a 0.92 ± 0.29 a 0.98 ± 0.33 b 0.044 \* cPOM 0.56 ± 0.11 b 0.41 ± 0.21 b 0.84 ± 0.32 ab 1.12 ± 0.17 a 1.24 ± 0.13 a 0.04 \*

*3.4. Carbon Enrichment Factor (EF) in Particle-Size Fractionation* 

**Figure 3.** Carbon enrichment factor EF of the organic matter fractions (**A**) 0–10 cm layer, (**B**) 10–20 cm layer. MCRC: maize–cotton relay cropping; 5YF: 5-year fallow; Ab10YF: above 10-year fallow; YMI: yam–maize intercropping system; TP: teak plantation. **Figure 3.** Carbon enrichment factor EF of the organic matter fractions (**A**) 0–10 cm layer, (**B**) 10–20 cm layer. MCRC: maize–cotton relay cropping; 5YF: 5-year fallow; Ab10YF: above 10-year fallow; YMI: yam–maize intercropping system; TP: teak plantation.

### **4. Discussion 4. Discussion**

10–20 cm.

The present study assessed the soil organic carbon stock and its distribution in three particle-size fractions considering five cropping systems in the Kiti sub-watershed in the Zou watershed in central Benin. This paper contributes to a growing understanding of the dynamics of soil organic carbon storage in coarse structure tropical soils in sub-Saharan Africa (SSA). The C stock in this study was estimated using the method considering the sum of the stocks of the different layers of the soil profile (0–10, 10–20, 20–30 and 30–40 The present study assessed the soil organic carbon stock and its distribution in three particle-size fractions considering five cropping systems in the Kiti sub-watershed in the Zou watershed in central Benin. This paper contributes to a growing understanding of the dynamics of soil organic carbon storage in coarse structure tropical soils in sub-Saharan Africa (SSA). The C stock in this study was estimated using the method considering the sum of the stocks of the different layers of the soil profile (0–10, 10–20, 20–30 and 30–40 cm), known as the classical method for C stock calculation. The limitation of this method is that it does not consider the variations in soil mass. To curb this, Ellert et al. [72] and Arrouays et al. [73] introduced soil equivalent masses and suggested C stocks estimation by the equivalent masses rather than the estimation by soil depth. This approach allows to reliably assess the changes in organic matter quantities linked to time or soil management practices. This method of calculation was used by Barthès et al. [14]; Aholoukpè [33] and Houssoukpèvi et al. [45].

The C stock recorded in this study ranged between 9.23 and 20.84 Mg C·ha−<sup>1</sup> for the layer 0–30 cm and between 11.48 and 22.20 Mg C·ha−<sup>1</sup> for the layer 0–40 cm. The carbon stock recorded in this study was slightly smaller than those recorded by previous studies in Benin [32,33,38,45,74]. However, the stocks recorded were higher than those recorded by Saidou et al. [42]. The low stock recorded in this study compared to previous studies could be attributed to various factors including tillage, the absence of crop residue restitution and seasonal vegetation fires which have been proven to negatively influence soil organic carbon stocks [74]. Moreover, the low stock observed in the study could also be attributed to the high proportion of concretion in the soil which can lead to a low proportion of fine particles and consequently affect soil organic carbon stocks. This is in line with research by Hairiah et al. [75] and Reichenbach et al. [76] who illustrated that the geochemical properties of the soil parent material leave a footprint that affects SOC stocks and mineral-related C stabilization mechanisms.

The soil under fallow (5YF and Ab10YF) had the highest C stock compared to teak plantation (TP) and the croplands (MCRC and YMI). The results are consistent with other work [32,77,78], highlighting that soils under fallow are enriched in organic matter from decaying litter, leaves and branches with lignified materials, which decompose progressively and replenish the soil organic carbon pool. The C stocks recorded in this study were low compared to those recorded by [75] (13.68; 12. 73 and 24.40 Mg C. ha−<sup>1</sup> for the layer 0–20 cm) on vegetable farmland receiving organic amendment (poultry manure and sheep dung) and a 5-year fallow. These differences could be attributed to differences in farm management practices implemented in regard to vegetable farming versus staple and cash crop farming. Previous studies assessing the effect of farm management practices revealed

that farm management practices, including a fallow period from five years and above, soil amendments, cover crops, mulching and crop residues restitution have a positive effect on C stocks in the soil [2,29,32,33]. The cultivated lands (MCRC and YMI) recorded the lowest C stocks. This could be attributed to the low crop residue restitution and the tillage system which can induce soil aggregate crumbling and therefore rapid carbon mineralization. The C stock observed in the soil under teak plantation was lower compared to the 30.5 and 31.4 Mg C·ha−<sup>1</sup> recorded by Houssoukpèvi et al. [45] for cropland and tree plantations in southern Benin. This could be attributed to the age of the plantation and the uneven exploitation scheme, making the plantation have a scattered structure and therefore a lower carbon input. Since existing studies reporting on C stocks in Benin were conducted at different layers making their comparison challenging, an extrapolation offers the possibility to compared different cropping systems on the basis of the surface layer of 0–30 cm. This extrapolation depicted that for the Acrisol in southern Benin, C stocks of 73 Mg C·ha−<sup>1</sup> under fallow, 41 Mg C·ha−<sup>1</sup> under vegetable farming systems with chicken manure, 38 Mg C·ha−<sup>1</sup> with sheep ruminant dung [32] and 32 Mg C·ha−<sup>1</sup> under the maize–mucuna cropping system [14]. This confirms that farm management practices have a substantial effect on soil organic carbon stocks. Since limited studies have focused on C stock evaluation across different farming systems, different agroecological zones, and different soil types as well as the long-term influence of these systems, more in-depth studies will help to identify and implement sustainable farming systems for better carbon sequestration and resilient food systems. In addition, as watersheds have a proven propensity to soil erosion [79], understanding the impact of landform on soil organic carbon storage could have a great contribution to developing sustainable farming systems at the watershed scale.

The particle-size distribution of organic carbon in the different particle-size fractions indicated that the non-particulate organic carbon fraction, associated with silt-clay (<53 µm), held the largest contribution to the total organic carbon. The contributions of the coarse particulate organic matter (cPOM) to the total soil organic carbon reserves were the lowest in all the cropping systems. The carbon associated with the organo-mineral fraction are localized in clay and silt bonds which protect the carbon from mineralization. Indeed, under conditions highly favorable to biological decomposition and humification, such as those in tropical regions, particulate organic matter is exposed to mineralization processes and therefore represents a small portion of the total organic carbon pool in the soil [80]. The two particulate fractions, cPOM and fPOM, have been proven to be the most affected by cropping systems [81]. These results are consistent with previous studies that emphasized the vulnerability of this fraction to mineralization processes [65,81]. These fractions, being free from soil mineral particles, are more accessible to microorganisms. The low cPOM and fPOM in the cultivated land, could be explained by tillage and soil erosion. Tillage induces soil aggregate breakdown and accelerates organic carbon mineralization. Furthermore, the cPOM and fPOM are lighter fraction and therefore susceptible to be streamed away during storm rain. For example, Akplo [79] pointed out in the Zou watershed that the particulate portion (cPOM + fPOM) of soil organic carbon was significantly affected by soil erosion.

Although the carbon content in the biomass and soil amendments applied to soil are generally accumulated more in the fPOM and cPOM fractions, the long-term accumulation of carbon in soil is predominantly determined by the carbon in the silt and clay fractions, NOM [82]. The higher concentration of the cPOM and fPOM in the two fallow lands can be attributed to the shoots and residues from the thick vegetation that accumulated during the fallow period. The fine compartment organic matter (NOM) is associated with micro-aggregates that protect organic carbon by adsorption and occlusion on mineral surfaces. The abundance of fine elements favors stabilization of soil organic carbon at a higher level of dynamic equilibrium, good structural stability of soil and makes the system more sustainable. According to our results, NOM is significantly higher in the fallow land. The biologically and chemically active fractions, fPOM and cPOM, belonging to the labile compartment is very sensitive to cultivation practices [83]. Ploughing is considered

as an unfavorable factor for the storage of organic matter in the soil [84]. It favors the destruction of soil micro-aggregates and accelerates soil organic carbon mineralization. This explains the low concentrations observed in the cultivated lands. Long-term fallows remain good sustainable land management practices. However, in the current context, where croplands are shrinking in favor of urbanization and development, yet food need is growing and cropland expansion is limited, there is a need for in-depth studies establishing more sustainable and resilient intensification of farming systems [85,86].
