Effect of land utilization patterns on total and easy-to-use components of soil carbon, nitrogen and phosphorus in the karst area of Pingguo, Guangxi.

Soil carbon, nitrogen and phosphorus are important factors of soil quality, which play important roles in increasing soil nutrient storage, improving soil fertility and promoting plant growth. A large number of studies have shown that returning cultivated land back to forest and grassland and converting land use patterns will lead to changes in soil nutrient status and stoichiometry. However, systematic studies on total and easy-to-use components of soil carbon, nitrogen, and phosphorus are relatively few in Pingguo karst area of Youjiang River Valley in central Guangxi. In this study, plots of three types of land use (the restored forest and grassland from cultivated land and cultivated land for sugarcane and maize) inside and near Pingguo National Field Observation and Research Station of Karst Ecosystem, Guangxi were taken as research objects, and 0-15 cm surface soil was sampled in March, 2021. The physical and chemical properties of soil, total and easy-to-use components of carbon, nitrogen and phosphorus and their relationships were analysed in order to better understand the effects of returning cultivated land back to forest and grassland and converting land utilization patterns on total and easy-to-use components of carbon, nitrogen and phosphorus of soil in karst areas. The results showed as follows: 1) Compared to cultivated land, the soil pH values of the restored forest and grassland from cultivated land increased significantly. The macro-aggregates of restored forest and grassland (42.82%, 57.11% respectively) were significantly higher than those of cultivated land for sugarcane (16.94%) and maize (5.49%), and their micro-aggregates (4.58%, 1.76% respectively) were significantly lower than those of cultivated land for sugarcane (12.42%) and maize (16.34%). Meanwhile, compared to cultivated land for sugarcane (103.39 mg⋅kg^-1, 3.22 mg⋅kg^-1) and that for maize (105.02 mg⋅kg^-1, 3.07 mg⋅kg^-1), available nitrogen of restored forest and grassland significantly increased to 156.55 and 166.49 mg⋅kg^-1, respectively, while available phosphorus significantly decreased to 0.41 and 0.30 mg⋅kg^-1, respectively. 2) Compared to cultivated land, the organic carbon, microbial biomass carbon and total nitrogen of soil in restored forest and grassland increased significantly. The values of soil organic carbon of restored forest and grassland (34.12 g⋅kg^-1, 32.45 g⋅kg^-1 respectively) were 1.98 and 1.88 times of cultivated land for sugarcane (17.26 g⋅kg^-1), and 2.15 and 2.04 times of cultivated land for maize (15.89 g⋅kg^-1), respectively. The values of microbial biomass carbon of soil in restored forest and grassland (985.35 mg⋅kg^-1 1,110.04 mg⋅kg^-1 respectively) were 2.71 and 3.05 times of cultivated land for sugarcane (364.07 mg⋅kg^-1), and 3.14 and 3.54 times of cultivated land for maize (313.92 mg⋅kg^-1), respectively. The values of total nitrogen of soil in restored forest and grassland (4.14 g⋅kg^-1, 4.10 g⋅kg^-1 Respectively) were 2.48 and 2.46 times of cultivated land for sugarcane (1.67 g⋅kg^-1), and 2.19 and 2.16 times of cultivated land for maize (1.89 g⋅kg^-1), respectively. The total value of phosphorus in restored grassland was the highest (1.04 g⋅kg^-1), followed by that of cultivated land for maize (0.81 g⋅kg^-1), and the values of dissolved phosphorus of cultivated land for sugarcane and maize were significantly higher than those of restored forest and grassland. The values of stoichiometry of carbon, nitrogen, and phosphorus, OC/TP, TN/TP of soil in restored forest were significantly higher than those of restored grassland and cultivated land, and the values of restored grassland and cultivated land did not show significant difference. 3) Soil bulk density, aggregate structure, pH value, available nitrogen and available phosphorus were significantly correlated with total and easy-to-use components of carbon, nitrogen and phosphorus of soil. Properly speaking, soil pH value, large macro-aggregates, and available nitrogen showed a significantly positive correlation with organic carbon, microbial biomass carbon, total nitrogen and microbial biomass nitrogen of soil, and a significantly negative correlation with dissolved phosphorus. Meanwhile, bulk density, micro-aggregates, and available phosphorus of soil showed a significantly negative correlation with organic carbon, microbial biomass carbon, total nitrogen and microbial biomass nitrogen of soil, and a significantly positive correlation with dissolved phosphorus. The results indicated that the measures of returning cultivated land back to forest and grassland and converting land use patterns significantly improved soil carbon, nitrogen and soil quality in karst areas. These two measures are important and effective ways to control rocky desertification and protect ecological environment in karst areas. [ABSTRACT FROM AUTHOR]