Historical surveys and current soil maps can identify soil differences and establish carbon monitoring baselines. We compared 330 samples from 1980 to 1983 with 376 independent samples from 2025 in Pingle County, China. Random forest models used 11 common covariates and nested spatial cross-validation. Land-cover maps supported comparisons of predicted soil organic matter (SOM). Cropland and forest sample means were 4.70 and 4.97 g/kg lower in 2025. Within the common valid area, predicted means were 26.43 and 23.54 g/kg. The mean predicted difference was −2.89 g/kg, with a conditional block-resampling range of −5.61 to −0.13 g/kg. Mean differences were −2.31, −3.47, and −4.81 g/kg for forest–forest, cropland–cropland, and forest–cropland combinations. With water bodies and built-up land excluded, a 20 cm depth and 0.58 conversion factor gave a 2025 soil organic carbon stock of 6.180 Tg C. Mean density was 3.35 kg/m², and the joint-model resampling range was 5.805–6.497 Tg C. Forest and cropland contributed 74.01% and 25.97%. Unpaired sampling and unverified analytical comparability prevent direct inference of temporal change or conversion effects. Conditional ranges exclude potential analytical bias between periods. These estimates provide a spatial basis for identifying priorities for soil carbon protection and soil quality resampling at the county scale. They can support sustainable land management and future soil carbon monitoring in karst landscapes.

Citric acid (CA) and nitrogen (N) fertilizers have been widely applied to enhance phytoremediation of various contaminated soils, yet their combined effects and underlying mechanisms in cadmium (Cd)–polycyclic aromatic hydrocarbon (PAH) co-contaminated systems remain poorly understood. Here, we investigated the combined effects of CA and ammonium nitrate on the phytoremediation of Cd–phenanthrene (Phe) co-contaminated soil using marigold (Tagetes patula L.), with random forest, mantel tests, and structural equation modeling employed to explore hypothesized pathways and associations. The results demonstrated that combined CA and N application significantly promoted plant growth, Cd uptake, and Phe dissipation. Among the tested treatments, the low-dose combination (1 g CA + 0.1 g N pot⁻¹) increased plant biomass by 88%, enhanced shoot Cd uptake by 121%, and achieved a Phe dissipation rate of 76.56%, representing the optimal remediation strategy. Mechanistically, the synergy between CA and N extends beyond simple growth promotion—N drives biomass production and, independently, facilitates Cd mobilization through rhizosphere acidification, while CA enhances contaminant bioavailability and buffers N-induced salt stress, together maintaining rhizosphere ionic homeostasis for efficient Cd translocation and Phe dissipation. This study provides a mechanistic framework for designing combined amendment strategies to enhance phytoremediation of co-contaminated soils.