Topsoil organic matter and its effect on the soil nutrients contents of Cunninghamia lanceolata plantations

Soil organic matter plays an important role in forest ecosystems and is an important index for estimating soil carbon storage and soil fertility and quality. In this study, the distribution of soil organic matter and its influence on soil nutrients were analyzed on a provincial scale and the impact of environmental factors on changes in the soil organic matter content of Chinese fir plantations (CFPs) was assessed to provide a theoretical basis for the sustainable management of CFPs. Data from 1092 forest farms in six provinces of China (Guangdong, Guangxi, Hunan, Jiangxi, Zhejiang, and Fujian) were used to characterize the distribution of soil organic matter and its effect on the soil nutrients in the topsoil (0–20 cm) in CFPs. The results showed that the average soil organic matter content of the CFPs was 31.02±13.44 g·kg⁻¹, the nutrient grade was at a medium level, and the coefficient of variation was 43.33%, which represented a moderate variation level. The available phosphorus (AP) content was 5.41±8.01 mg·kg⁻¹, the nutrient grade was rich, and the coefficient of variation was as high as 148.06%. The total phosphorus (TP) content was 0.49±0.38 g·kg⁻¹, which was extremely poor, and the coefficient of variation was 77.55%, being moderately variable. The soil organic matter content of different soil types varied greatly. The content in mountain yellow soil was the highest (46.63±16.88 g·kg⁻¹), and that in dark red soil was the lowest (15.81±4.38 g·kg⁻¹). Stand density, elevation, slope, and soil pH were the main factors that affected the soil organic matter content in the topsoil of CFPs, with relative contributions of −0.35, 0.28, 0.11, and 0.11, respectively. The contributions of soil organic matter to total nitrogen (TN), available nitrogen (AN), total potassium (TK), and available potassium (AK) were positive, with relative contributions of 0.17, 0.47, 0.16, and 0.21, respectively. However, soil organic matter contributes less to soil TP, with a relative contribution of −0.09. In general, the nutrient grade of soil organic matter in the surface layer of CFPs was medium. The main grading performance was mainly characterized by moderate to inferior levels, and few rich levels and no extremely poor levels were observed. Altitude, slope, and soil pH had positive effects on soil organic matter, whereas stand density had a negative effect. Soil organic matter had a positive effect on soil TN, AN, TK, and AK and a weak negative effect on TP. The contribution of soil organic matter to TP was low; therefore, the lack of phosphorus in the soil may be the main factor limiting Chinese fir growth.