Comparative evaluation of three methods for soil organic carbon quantification in Morocco’s Middle Atlas forests

This study compares three common methods for measuring Soil Organic Carbon (SOC) in Morocco’s Middle Atlas forests: the Walkley-Black (WB) method, the Loss on Ignition (LOI) method, and the Total Organic Carbon analysis by catalytic oxidation coupled with Non-Dispersive Infrared (NDIR) detection (TOC-CO-NDIR). Soil samples (n = 90) were taken from the upper organo-mineral horizon at depths of less than 30 cm of three major forests located in the Middle Atlas of Morocco, under different forest species such as Atlas cedar, green oak, Zeen oak, and maritime pine). The results show that SOC values ranged from 4.30 to 6.33%. Indeed, the TOC-CO-NDIR method showed the highest variability (standard deviation 3.22%), demonstrating its sensitivity to diverse SOC concentrations. In contrast, the LOI method showed a higher mean value of 6.33% and moderate variability, with a standard deviation of 2.35%, suggesting that it measures not only organic carbon but also other volatile compounds. The WB method displayed a mean of 4.55% with a standard deviation of 2.01%, indicating lower variability and greater stability of measurements. Correlation analysis showed a moderate and significant correlation between the TOC-CO-NDIR and LOI methods (r = 0.46, p = 0.031), while the correlation between TOC-CO-NDIR and WB was negligible (r= − 0.0015, p = 0.946), and a weak correlation between WB and LOI (r = 0.32, p = 0.149). The analysis of variance (ANOVA) revealed significant differences between the methods (p = 0.0227). The Root Mean Square Error (RMSE) values show that the LOI method is the most similar to the WB method, followed by LOI and TOC-CO-NDIR, and then WB and TOC-CO-NDIR, indicating LOI and WB provide the most consistent results for practical soil carbon analysis. A comparative cost analysis highlights the practicality of the LOI and WB methods due to their low equipment and operational costs, in contrast to the TOC-CO-NDIR method, which requires expensive instrumentation and specialized personnel.