Soil biochemical and chemical changes in relation to mature spruce (Picea abies) forest conversion and regeneration

To investigate soil changes from forest conversion and regeneration, soil net N mineralization, potential nitrification, microbial biomass N, L‐asparaginase, L‐glutaminase, and other chemical and biological properties were examined in three adjacent stands: mature pure and dense Norway spruce (Picea abies (L.) Karst) (110 yr) (stand I), mature Norway spruce mixed with young beech (Fagus sylvatica) (5 yr) (stand II), and young Norway spruce (16 yr) (stand III). The latter two stands were converted or regenerated from the mature Norway spruce stand as former. The studied soils were characterized as having a very low pH value (2.9 – 3.5 in 0.01 M CaCl₂), a high total N content (1.06 – 1.94 %), a high metabolic quotient (qCO₂) (6.7 – 16.9 g CO₂ kg^–1 h^–1), a low microbial biomass N (1.1 – 3.3 % of total N, except LOf₁ at stand III), and a relatively high net N mineralization (175 – 1213 mg N kg^–1 in LOf₁ and Of₂, 4 weeks incubation). In the converted forest (stand II), C : N ratio and qCO₂ values in the LOf₁ layer decreased significantly, and base saturation and exchangeable Ca showed a somewhat increment in mineral soil. In the regenerated forest (stand III), the total N storage in the surface layers decreased by 30 %. The surface organic layers (LOf₁, Of₂) possessed a very high net N mineralization (1.5 – 3 times higher than those in other two stands), high microbial biomass (C, N), and high basal respiration and qCO₂ values. Meanwhile, in the Oh layer, the base saturation and the exchangeable Ca decreased. All studied substrates showed little net nitrification after the first period of incubation (2 weeks). In the later period of incubation (7 – 11 weeks), a considerable amount of NO₃‐N accumulated (20 – 100 % of total cumulative mineral N) in the soils from the two pure spruce stands (I, III). In contrast, there was almost no net NO₃‐N accumulation in the soils from the converted mixed stand (II) indicating that there was a difference in microorganisms in the two types of forest ecosystems. Soil microbial biomass N, mineral N, net N mineralization, L‐asparaginase, and L‐glutaminase were correlated and associated with forest management.