Abstract: The seasonal variations of ammonium nitrogen (NH₄⁺-N), nitrate nitrogen (NO₃^--N), activities of urease and nitrite reductase in differentsized forest gaps (G1: 100 m², G2: 400 m², G3: 900 m², G4: 1600 m²) of Pinus massoniana plantations in hilly area of the upper reaches of the Yangtze River were analyzed. Our results showed that the ammonium nitrogen content gradually increased, and then decreased with the increasing size of forest gap, while the nitrate nitrogen content showed a decrease trend. The NH₄⁺-N, NO₃^--N contents, and urease activity of soil were higher in the forest gaps of 400 m², 100-400 m² and 900-1600 m² than in the other gaps. However, the nitrite reductase activity in the forest gap of 400 m² was lower than in the other gaps. These findings indicated that the NH₄⁺-N and NO₃^--N contents were significantly affected by forest gap size, and smaller size of forest gaps contributed to soil nitrogen mineralization. The NH₄⁺-N content in spring was significantly higher than in other seasons, and the content of NO₃^--N and urease activity were higher in autumn and winter. However, the nitrite reductase activity was highest in autumn. Compared to under tree canopy, the available nitrogen content in smaller forest gaps significantly increased in spring and summer; the enzyme activity was higher but no significant differences were observed except that the nitrite reductase activity in the gap edge was significantly higher than in the center of forest gap. Moreover, the available nitrogen content and enzyme activity were closely related to soil temperature and moisture. Our findings suggested that after the formation of gaps, the improvement of soil moisture may enhance the enzyme activity and further contribute to the transformation of nitrogen in soil.

Key words: Acanthogobius ommaturus, monthly change, Yangtze River estuary, diet composition