Research on the soil organic carbon (SOC) distribution using different afforestation systems on limestone mountains is of great significance because it provides guidance for selecting afforestation systems and produces quantitative evaluations of soil conservation. By comparative analysis of the SOC content and SOC storage, the SOC distribution characteristics in the soils of four 7-year-old examples of afforestation systems and unused grassland (UNG) were studied in northern China’s Limestone Mountains. The results indicate that:
1) the four afforestation systems showed significantly improved soil properties in comparison with that of the UNG, including a decrease in soil bulk density and an increase in soil porosity, soil organic matter content, total nitrogen, and total phosphorus. The effective improvements in the mixed afforestation systems were greater than that in the pure afforestation systems, and the most effective improvement was a Platycladus orientalis-Robina pseudoacacia mixed plantation (PRM), followed by a Platycladus orientalis-Cotinus coggygria mixed plantation (PCM), a Platycladus orientalis-Prunus armeniaca mixed plantation (PPM), and a pure Platycladus orientalis plantation (POL);
2) the SOC content and SOC storage of the four afforestation systems were significantly higher than that of the UNG, and those same parameters in the 0-10 cm soil were significantly higher than those from the 10- 20 cm soil; the SOC content was ordered PRM > PCM > PPM > POL > UNG;
3) in the four afforestation systems, the SOC content showed a significant positive correlation with the silt and clay particle content and a non-significant negative correlation with the sand content. In addition, the SOC content showed a significantly positive correlation with soil total nitrogen, total phosphorous, and total porosity, and a significant negative correlation with soil bulk density.