Nutrient characteristics and fertility evaluation of forest soil in hilly red soil region of South China: A case study of Zhaoqing City

This study focused on the soil systems of four forest types (coniferous forest, coniferous-broadleaved mixed forest, bamboo forest, and broadleaved forest) in Zhaoqing City. Adopting a combination of stratified random sampling and geostatistical point layout methodologies, we systematically established 447 sampling sites across the study area. Through quantitative determination of soil organic carbon (SOC), total nitrogen (TN) and total phosphorus (TP) concentrations, spatial distribution patterns were mapped using the Kriging interpolation method. Concurrently, a comprehensive soil fertility assessment was conducted to investigate the fundamental characteristics of forest soil fertility in Zhaoqing City, with particular emphasis on identifying the dominant nutrient elements governing soil quality in this subtropical red soil region. The results showed that the contents of SOC, TN and TP in the study area were the highest in the surface soil layer (0～20 cm), decreasing with soil depth and exhibiting significant surface aggregation. The SOC, TN and TP in each soil layer belong to the moderate to high variability, showing strong spatial heterogeneity. The SOC storage within the 0～100 cm profile was the highest in coniferous forests and the lowest in broadleaved forests. while soil TN and TP storage were highest in bamboo forests and the lowest in coniferous forests. Soil fertility evaluation revealed that the four forest types exhibited moderate fertility levels, with soil quality indices ranked as follows: bamboo forest (0.462) > coniferous and broadleaved mixed forest (0.434) > coniferous forest (0.432) > broadleaved forest (0.291). TN and SOC were identified as the primary limiting factors for soil fertility in Zhaoqing City's forest ecosystems. This study provides critical insights into the fertility dynamics of forest soils in the low-hill red soil region of South China, offering practical guidance for sustainable forest management and soil conservation strategies in subtropical ecosystems.