Agricultural soils originating from carbonate rocks within karst regions exhibit inherent high concentrations of potentially toxic elements (PTEs) due to geogenic processes. However, the influencing factors of the elevated levels of PTEs in the naturally contaminated karst regions remain inadequately comprehended. This research investigates the chemical compositions of 278 soils derived from carbonate rocks. Descriptive statistics, stepwise multiple regression, and the random forest (RF) method were applied to screen the significant factors that affect the distribution, migration, and enrichment of the PTEs in soils. Cadmium (Cd) and arsenic (As) are the most highly contaminated PTEs in the soils of the study area, and the average contents of Cd and As in soils are 11.5 and 2.92 times the national soil background value, respectively. The pollution risk ranking of PTEs is Cd > As > Cr > Zn > Pb > Cu > Ni > Hg, using the proportion exceeding the risk screening thresholds of agricultural land as the standard. Soil Fe₂O₃ and Mn contents, soil pH and total organic carbon (TOC) values, and weathering intensity (characterized using the chemical index of alternation, CIA) are the most important factors influencing the PTE levels in agricultural soils.

The mineral composition of Fe–Mn nodules (FMNs) is closely related to the environment in which FMNs are formed. The formation mechanism of FMNs has consistently been one of the major topics in related research. In this study, the mineral composition of FMNs in soils derived from carbonate rocks in typical karst areas with high geochemical background in Guangxi, Southwest China, was investigated. The results showed that Fe₂O₃ (30.06%), SiO₂ (19.72%), Al₂O₃ (17.93%), TiO₂ (0.96%), P₂O₅ (0.78%), and MnO₂ (0.64%) were the main elemental composition, while four alkaline oxides K₂O, Na₂O, CaO, and MgO were less than 0.5% in soil FMNs. In addition, 10 mineral types were identified in soil FMNs in the study area, namely quartz, goethite, clinochlore, illite, kaolinite, boehmite, albite, microcline, lithiophorite, and hematite. There were no obvious differences in the mineral composition of soil FMNs in the study area compared with those observed in non-karst areas worldwide. The formation process of FMNs can be determined based on the surrounding environment that affects the soil mineral composition. The results suggested a relatively complex formation mechanism of soil FMNs. Moreover, both primary and secondary minerals were found in soil FMNs. However, some minerals can exist stably under normal redox conditions, while the other part of minerals can be easily weathered and dissolved, indicating a relatively high formation rate of soil FMNs and a relatively stable internal environment.