Abstract: The seasonal variations of soil microbial biomass phosphorus (MBP), available phosphorus (AP) and total phosphorus (TP) were observed in seven size gaps (G1: 100 m², G2: 225 m², G3: 400 m², G4: 625 m², G5: 900 m², G6: 1225 m², G7: 1600 m²) of a masson pine plantation in a hilly area of the upper reaches of Yangtze River. The results showed that the soil MBP in the forest gaps was most sensitive to cutting disturbance, followed by soil AP, while TP was relatively stable. The effects of gap size were significant on soil MBP and AP, but not on soil TP. The center of forest gap with a size of 625-900 m² was more conducive to increase soil MBP, the center of larger gap (900-1600 m²) had a higher level of soil AP as well as the edge of medium-sized gap (225-625 m²), and the center of forest gap (625 m²) had a high level of soil TP. MBP and AP had significant changes in different periods of the growing season, but TP had no significant changes. Soil MBP of the centers of all gaps and the edges of gaps less than 625 m² was lower in the early period, but that of other areas was higher in early and lower in middle of growing season. Soil AP in the gap center was higher in the middle and lower in the early of the growing season, while lower in the early stage of gap edge. In addition, the interaction of gap size and seasonal variation had significant impacts on soil MBP. The location within forest gap had no significant influence on MBP, AP and TP, but the soil AP and TP of all gaps in the middle and late growing season were higher than in the understory soil, while the soil MBP of large-sized gap was only lower in the middle growing season. It is suggested that soil phosphorus was improved in a certain degree after the formation of forest gaps. Correlation analyses showed that soil MBP was significantly positively correlated with TP, but significantly negatively correlated with soil temperature, and soil MBP and TP were significantly positively correlated with soil water content. Therefore, soil temperature and moisture were major environmental factors affecting soil phosphorus in early formation of forest gaps.

Key words: Jiaozhou Bay, stable isotope, Rhinogobius pflaumi, trophic level, feeding habits.