Soil net nitrogen (N) mineralization (Nmin) is a key process in the forest N cycle regulating the N availability of plant growth. However, it is unclear how N transformation responds to soil hydraulic properties changes. The soil inorganic N pools and N transformation in the early growing season in karst forestlands were investigated by using an intact soil core in situ incubation method. Three different typical vegetation types were selected. The results showed that the mean values of NH4+-N, NO3−-N, and inorganic N were 1.05–1.36, 1.55–3.85, and 1.05–2.34 times greater for ferns than for shrubs. NO3−-N and NH4+-N mainly occur at soil depths of 0–5 cm and 5–15 cm, respectively. The soil Nmin was 2.21–232.03 times higher at 0–5 cm than at the 10–15 cm. Net N immobilization was found for the juvenile ferns and shrubs at 5–15 cm. The Nmin of juvenile and mature ferns was 1.90–11.78 times and 1.17–16.20 times higher than shrubs, respectively, and shrubs had the highest Ks (69.91 mm h−1) but the lowest water-holding capacity. Both ferns and shrubs were able to hold more water and available water was richest in mature fern soil, which provided an extra water source for fern growth. Principal component analysis (PCA) was used to test whether the measured variables affected Nmin, and the results showed that soil organic matter (SOM), pH, and saturated volumetric water content (θs) were the main soil factors affecting Nmin. In addition, the NH4+-N, NO3−-N, and inorganic N stocks were reduced by 3.98 %–59.04 %, 48.07 %–63.30 % and 8.18 %–57.37 % after rainwater input, respectively. Our findings suggest that soil inorganic N and Nmin in the karst forest were regulated by soil hydraulic properties. Changes in the soil hydraulic properties might therefore influence the functioning of soil N transformation.