Comparison of soil nitrogen mineralization and nitrification in a mixed grassland and forested ecosystem in central Taiwan

We used the buried bag incubation method to study temporal patterns of net N mineralization and net nitrification in soils at Ta-Ta-Chia forest in central Taiwan. The site included a grassland zone, (dominant vegetation consists of Yushania niitakayamensis and Miscanthus transmorrisonensis Hayata) and a forest zone (Tsuga chinensis var. formosana and Yushania niitakamensis). In the grassland, soil concentration NH₄⁺ in the organic horizon (0.1–0.2 m) ranged from 1.0 to 12.4 mg N kg^–1 soil and that of NO₃^– varied from 0.2 to 2.1 mg N kg^–1 soil. In the forest zone, NH₄⁺ concentration was between 2.8 and 25.0 mg N kg^–1 soil and NO₃^–varied from 0.2 to 1.3 mg N kg^–1 soil. There were lower soil NH₄⁺ concentrations during the summer than other seasons. Net N mineralization was higher during the summer while net nitrification rates did not show a distinct seasonal pattern. In the grassland, net N mineralization and net nitrification rates were between –0.1 and 0.24 and from –0.04 to 0.04 mg N kg^–1 soil day^–1, respectively. In the forest zone, net N mineralization rates were between –0.03 and 0.45 mg N kg^–1 soil day^–1 and net nitrification rates were between –0.01 and 0.03 mg N kg^–1 soil day^–1. These differences likely result from differing vegetation communities (C₃ versus C₄ plant type) and soil characteristics.