Nitrogen mineralization and potential nitrification at different depths in acid forest soils

Soil samples were collected from the litter, humus and five mineral soil layers to a depth of 50 cm in acid, but highly productive, Norway spruce forests in S Sweden and E Denmark for determination of net N mineralization and potential nitrification. The samples were sieved while still fresh and incubated at a constant temperature (15°C) and soil moisture for 74-117 days with periodic subsamplings. Net N mineralization rates, expressed per g organic matter or per g of total N, decreased with increasing depth. Net N mineralization in the ten soils studied ranged from 35 to 105 kg N ha⁻¹ yr⁻¹, of which the organic horizons contributed 32-74%. Nitrate formation patterns were variable. Almost no nitrification could be detected at pH (H₂O) values lower than 4.0. Nitrate was formed in humus layers with pH values of 4.0-4.5, but the nitrification was never complete. By contrast, the nitrification was almost complete at a depth of 10-50 cm, where the pH (H₂O) was 4.1-4.5. Addition of CaCO₃ stimulated nitrification in the humus layer, indicating the presence of acid-sensitive nitrifiers, while nitrification in the mineral soil was sometimes stimulated and sometimes inhibited by the addition of CaCO₃. Tests with the acetylene block method showed that nitrification, when it occurred, was autotrophic in the humus layer but less easy to characterize in the mineral soil. Thus, further studies are needed to characterize the nitrifier organisms and identify the factors regulating the activity of these organisms. Actual nitrification rates in the field could not be determined, but our findings showed that the B horizon in acid forest soils has a great potential for nitrification. Because N deposition does not appear to decrease in Scandinavian forests, we conclude that an ammonium surplus in the forest floor followed by a downward transport of ammonium to the nitrifying subsoil can increase the risk of nitrate leaching in the future.