Abstract: The decline of the productivity and soil fertility of larch (Larix gmelinii) plantation is an urgently addressed issue in forestry production. Especially for the second generation larch plantation, whether its closetonature transformation can improve the soil fertility, soil microbial diversity, and soil nutrient contents is lacked of indepth study. In this paper, an investigation was made on the effects of closetonature transformation of the second generation larch plantation on the soil microbial diversity and structure as well as the soil nutrient contents within two years of the transformation. The results showed that there was a close correlation between gap size and soil microbial diversity. After the first year of transformation, the soil microbial diversity in small gaps (25 m²) had a significant increase, with the Shannon index, richness index, and evenness index being higher than those in large gaps (100 m²) and the control (no-thinning). In the second year of transformation, less difference was observed in the three indices among the three treatments. The Shannon index and richness index were higher in the first year than in the second year, while the evenness index was in adverse, suggesting that in the first year of transformation, soil microbial diversity was higher but in uneven distribution. The soil microbial genetic similarity in small gaps was similar to that in the control; while in large gaps, this similarity was significantly different from the other two treatments, because of the greater changes of microclimate conditions in large gaps resulting in an intense change in soil microbial diversity. In the first year of transformation, soil nutrient contents had a significant increase. The soil available P content in large gaps was increased by 80%, and that in small gaps was increased by 52%; soil available N content was also increased; while soil available K content had less increase. In the second year, soil nutrient contents were decreased, but the decrements differed between large gaps and small gaps. The soil available P content had a significant decrease in large gaps, but the soil nutrient contents had less decrease in small gaps. This study provided evidences for the feasibility and scientific implementation of closetonature forest management.

Key words: saline water irrigation, subsurface drainage, soil watersalt movement, high water table, winter wheat yield.