Responses of C-, N- and P-acquiring hydrolases to P and N fertilizers in a subtropical Chinese fir plantation depend on soil depth

Large amounts of carbon (C), nitrogen (N) and phosphorus (P) in soil are stored in forms that are not bioavailable unless they are modified by hydrolytic enzymes. The activity of hydrolases is affected directly and indirectly by fertilizers treated, but the effects depend on soil depth. We collected soil core (0–80 cm depth) from fertilized and unfertilized (control) plots in a subtropical Chinese fir plantation. The soil with fertilizers had been treated for five years with either N (50 kg N ha−1 yr−1), P (50 kg P ha−1 yr−1) or N and P together. The kinetics of three hydrolases (β-1,4-glucosidase (βGluc), β-1,4-N-acetylglucosaminidase (NAG) and acid phosphatase (Phos)) were measured, and their potential activities (Vmax), half-saturation constants (Km), and enzyme efficiencies (Vmax/Km) were calculated for 0–10 cm, 10–20 cm, 20–40 cm, 40–60 cm, and 60–80 cm soil depth increments. The effect of depth on enzyme kinetics was greater than fertilizers treated. Smaller soil organic carbon (SOC) contents were related to the stable efficiencies of NAG and Phos from 0 to 60 cm in the control plots. In general, decreases in SOC content with depth triggered the production of enzymes with low Km (i.e. high substrate affinity). The minimum and maximum decreases in the Km of βGluc, NAG and Phos were 0%, 22% and 64% in the top 20 cm and 30%, 54% and 79% at 80 cm depth, respectively, compared to 0–10 cm soil depth. Nitrogen fertilizer downregulated NAG production without changing Km with soil depth, leading to decreased Vmax and Vmax/Km compared to the control. Responses of Phos to P and NP fertilizers depended on soil organic matter (SOM) and available P contents. Relative to the control, the Phos Vmax decreased in the top 10 cm, increased between 10 and 40 cm, and lack of difference between 40 and 80 cm in the P- and NP-treated soils. The Vmax and Vmax/Km of βGluc increased by P fertilizers between 60 and 80 cm compared to the control, indicating microbial mobilization of P from SOM to meet energy and C requirements in nutrient-poor subsoil. We conclude that limitations in microbial activity caused by decreases in SOM with depth with N and P fertilizers Chinese fir plantations are offset by the production of more efficient enzymes. However, longer term management to build SOM in tree plantations is required to prevent further soil degradation and loss of productivity.