[Objective] Red soil is one of the important soil types in China. Its low fertility is a problem for agricultural output. Exploring the effect of different improvement measures on soil fertility is integral for realizing sustainable use of middle and low yield fields.[Method] In this study, dryland red soils derived from Quaternary red earth were collected from a field experiment in Yueyang, Hunan Province and used to measure the changes in soil pH and nutrient contents across different soil layers and years of different treatments. These treatments included fallow (F), no fertilizer control (CK), a single application of inorganic fertilizer of nitrogen, phosphate, and kalium (NPK), inorganic fertilizer combined with straw-return (NPKS), inorganic fertilizer combined with lime (NPKL), inorganic fertilizer combined with amendments of organic crushed-bones (NPKA), and inorganic fertilizer combined with commercial bio-organic fertilizer (NPKC). We also compared the activity of enzymes related to carbon, nitrogen, and phosphorus cycles in this soil using the microplate fluorescence method.[Result] Some treatments significantly affected soil nutrients and enzyme activities. Soil organic matter (SOM), total nitrogen (TN), total phosphorus (TP), and available phosphorus (AP) in the 0-20 cm soil layer treated with NPKC in 2020 increased by 73%, 29%, 61%, and 1 847%, respectively, relative to the control. This treatment also significantly increased the enzyme activities of the enzymes that participate in the carbon cycle, including α-1, 4-glucosidase (αG), β-1, 4-glucosidase (βG), β-1, 4-xylosidase (βX), and cellobiohydrolase (CBH). Also, the activity of β-1, 4-N-acetylglucosamine (NAG), which is involved in the nitrogen cycle, was increased. Correlation analysis showed that SOM was significantly positively correlated with enzyme activities of αG, βG, βX, CBH, and NAG (P < 0.01). Additionally, the pH value was significantly negatively correlated with acid phosphatase (ACP) activity (P < 0.01). The effect of improvement measures on the enzyme activity of 0-20 cm soil layer was greater than on deeper layers. In 2019, compared with the control, NPKA treatment increased CBH enzyme activity in the 0-20 cm soil layer by 352%, but only by 2% in the 20-40 cm soil layer. Besides, ACP enzyme activity in the soil also showed a trend of increasing with years of treatment.[Conclusion] The combination of inorganic fertilizer and organic materials can significantly improve the nutrient status and soil enzyme activity in red soils. This can be used to efficiently improve the fertility of barren red soil.