Effects of Typical Forest Management Measures (Thinning and Fertilization) on Soil Nitrogen Mineralization in Chinese Fir Plantations

The influence of forest tending management measures on soil N turnover has attracted significantly more concern in recent decades. Nevertheless, uncertainties remain surrounding the effects and mechanisms of different tending management measures on soil nitrogen mineralization. In the process of forest management, thinning and fertilization are two common tending measures that are of great significance to the process of the forest nitrogen cycle. This study was conducted in a typical subtropical Chinese fir plantation, where we established a two-factor experimental design involving thinning intensity (light, moderate and heavy thinning) and fertilization treatment (fertilized and unfertilized). The objective was to evaluate the effects of these treatments on soil nitrogen mineralization rates. The results show that the fertilization treatment significantly increased soil ammonium nitrogen, nitrate nitrogen content, microbial biomass carbon and nitrogen, and net nitrogen mineralization rate by stimulating enzyme activity through exogenous nutrient input (p < 0.05). Additionally, light thinning and moderate thinning significantly promoted nitrogen mineralization, while heavy thinning showed an inhibitory effect. A reasonable forest management model can effectively improve the forest structure and soil microenvironment, increasing the soil nitrogen mineralization rate and nitrogen availability. In summary, systematically elucidating the synergistic regulatory effects and underlying mechanisms of different thinning intensities and fertilization treatments on nitrogen mineralization is crucial for gaining deeper insights into forest nitrogen cycling processes and the restoration of ecological functions.