Soil Nutrients and Soil Microbial Community Diversity as Affected by Land Use in the Minjiang River Valley, Sichuan

The Minjiang River Valley in Sichuan is a region diversified in type of land use (shrub-grassland, slope farmland, planted forest and secondary forest). Soil microbial community diversity and distribution of different types of land use in the region was studied using the Biolog plate and phospholipid fatty acid methyl ester (FAMEs) methods, separately. Results show that the soils regardless of whatever type of land use were undertended to be slightly acidic, following an order of slope farmland >shrub-grassland >planted forest >secondary forest in soil pH, fluctuated somewhat in soil electric conductivity, bulk density and total porosity, and followed an order of secondary forest> planted forest>shrub-grassland >slope farmland in content of soil nutrients and available nutrients. The soils varied significantly in functional diversity of microbial community relative to land use. Average well color development (AWCD) is an index that directly reflects soil microbial activity and functional diversity. It increased with the usage going on time. The soils displayed an order of secondary forest >planted forest >shrub-grassland >slope farmland in soil microbial community activity. Soil microbes' use of carbon sources varied in intensity relative to type of the source. Carboxylic acids and carbohydrates were the major ones and followed by amino acids, phenolic acids, polymers, and amines. The soils also displayed the order of secondary forest >planted forest >shrub-grassland >slope farmland in Simpson index (H), Shannon-Wiener index (E), richness index (D_S) and McIntosh index (S), but the differences between the four types of land use in D_S were not big enough to be rated as significant. Principal component analysis (PCA) shows that principal components No. 1 and No. 2 out of 31 affecting factors related to carbon source utilization explained 63.89% and 18.55% of the variation, respectively. Among the principal components, carboxylic acids and carbohydrates were the two major contributors. Correlation analysis shows that soil microbial diversity index was in significantly or very significantly positive relationship with the content of soil organic matter and of total nitrogen, but in negative one with pH, that is to say, the content of soil organic matter and the content of total nitrogen are the two major factors affecting soil microbial diversity. All findings mentioned above suggest that the soil under secondary forest is the highest in nutrient content and more favorable for survival of microorganisms, which play a very important role in maintaining stability of the ecosystem in this area.