*3.2. Total Soil Organic Carbon Content (SOC) and Soil Organic Carbon Stock (C Stock) across the Cropping Systems*

The variations in SOC and C stock from the four layers, 0–10, 10–20, 20–30 and 30–40 cm are presented in Table 3. For the soil layer 0–10 cm, the SOC content significantly varied (*<sup>p</sup>* < 0.001) between the cropping systems and ranged from 3.14–24.1 g C·kg−<sup>1</sup> . The highest SOC was recorded with Ab10YF while the lowest was recorded with MCRC. In the layer 10–20 cm, the SOC content showed significant (*p* = 0.011) differences between the cropping systems with 5YF and YMI recording the highest and the lowest SOC, respectively (5.22 and 2.21 g C·kg−<sup>1</sup> ). At the soil layer of 20–30 cm, the SOC content between the cropping systems was not significantly different (*p* = 0.1) with values ranging between 2.10–2.84 mg C·kg−<sup>1</sup> . In the soil layer of 30–40 cm, the SOC showed significant variation (*<sup>p</sup>* = 0.021) with values ranging from 1.36–4.43 g C·kg−<sup>1</sup> . The cropping systems YMI and 5YF exhibited the lowest and the highest concentrations of SOC, 1.36 and 4.43 g C·kg−<sup>1</sup> , respectively.

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


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%. ns: non-significant at 5%

The C stock showed significant variation between the cropping systems and soil depth layers (*p* < 0.001, *p* = 0.001, *p* = 0.006, *p* = 0.012). Regardless of the cropping systems, C stocks were higher for the surface layer 0–10 cm and showed a decreasing trend towards sub-surface layers. For the soil layer 0–10 cm, Ab10YF recorded the highest C stock (18.1 Mg C·ha−<sup>1</sup> ), while MCRC recorded the lowest (3.25 Mg C·ha−<sup>1</sup> ). In the layer of 10–20 cm, 5YF showed the highest C stock (6.29 Mg C·ha−<sup>1</sup> ), while Ab10YF recorded the lowest C stock 1.39 Mg C·ha−<sup>1</sup> and the cropping systems ranged in the order 5YF > TP > MCRC > YMI > Ab10YF. In the layer of 20–30 cm, C stock in the soil ranged from 1.35 to 3.26 Mg C·ha−<sup>1</sup> and Ab10YF and 5YF exhibited the lowest and the highest C stocks, respectively. In the layer of 30–40 cm, the C stock ranged between 3.26 and 1.35 Mg C·ha−<sup>1</sup> with Ab10YF and 5YF recording the lowest C stock values. In addition, there was a significant (*p* = 0.012 and *p* = 0.02) difference between the cropping systems for the C stock at the layer of 0–30 cm and at the layer 0–40 cm (Figure 2). Considering the total C stock of the layer 0–40 cm, the cropping systems Ab10YF exhibited the highest C stock (22.20 Mg C·ha−<sup>1</sup> ) while MCRC recorded the lowest (10.31 Mg C·ha−<sup>1</sup> ). Considering the total C stock for the surface layer of 0–30 cm (Figure 2), the highest C stock was recorded at Ab10YF and the lowest at MCRC 20.84 and 9.23 Mg C·ha−<sup>1</sup> , respectively.
