Abstract: Gap disturbance plays an important role in the processes of regeneration and succession of secondary forests, and influences the capability of soil nutrient supply. To indicate the effects of gap size on the processes of nutrient cycling in soil, we measured soil microbial biomass carbon, nitrogen and phosphorus during the peak growing season at the 0-20 cm depth in large, medium and small gaps (90-670 m²) formed 6 years ago in the  secondary forest in a montane region of eastern Liaoning Province, China. The results showed that compared with the control soil (i.e., the soil in the understory), there were no significant differences in microbial biomass carbon, microbial biomass phosphorus, the ratios of microbial biomass carbon to total carbon and microbial biomass phosphorus to total phosphorus in the three sizes of gaps at the soil depths of 0-10 and 10-20 cm. In addition, there were also no significant differences in microbial biomass carbon, nitrogen, phosphorus, the ratios of microbial biomass carbon to total carbon, microbial biomass nitrogen to total nitrogen and microbial biomass phosphorus to total phosphorus among the three gap sizes. In comparison with the control soil, microbial biomass nitrogen and the ratio of microbial biomass nitrogen to total nitrogen were significantly higher in the medium gaps with the size of 290 m² in the 10-20 cm soil layer, indicating the increase of cycling and availability of soil nitrogen in the medium gaps.

Key words: water quality, dissolved organic matter, liquid chromatography-organic carbon detection-organic nitrogen detection, molecular weight fraction