Impact of Agricultural Activities on Soil Particle Size, pH, and Organic Matter in the Dimonika Biosphere Reserve, Mayombe (Republic of Congo)

Land-use change disrupts several soil physico-chemical parameters. This study aimed to analyze the influence of agricultural activities, topography, and localities on soil texture, pH, and organic matter in the Dimonika Biosphere Reserve. To achieve this, 90 soil samples were collected using an auger based on land use types, topography, and localities. Analyses of soil texture, pH, total organic carbon (TOC), and total nitrogen (TN) were conducted at the IRSEN laboratory in Pointe-Noire. The results showed that clay texture predominates in the studied area, which is related to the nature of the soils. The Kruskal-Wallis ANOVA test highlighted a significant effect of agricultural activities on soil acidity, with an acidic pH ranging from 4.2 to 5 in cultivated areas, compared to a very acidic pH (3.5 to 4.2) in mature forests. However, neither topography nor localities affected pH. Total organic carbon significantly decreased in old plantations (1.3±0.3%) and fallows (1.5±0.5%) compared to mature forests (1.9±0.5%) and savannahs (2.3±0.6%), while total nitrogen showed no notable variations. Topography also had no influence on organic status (TOC, TN). However, at the local level, the Makaba area stood out with significantly higher TOC (2±0.5%) and TN (0.2±0.03%) compared to Les Saras (0.14±0.03%) and Kayes (0.16±0.2%). The C/N ratio, influenced only by agricultural activities, was below 15, indicating rapid organic matter mineralization and a low TOC content. Several potential solutions were proposed for sustainable soil management.