3.2 Effects of grassland conversion on soil microbes
Conversion of grassland to cropland significantly decreased the soil total microbial biomass by 14%, Gram+ bacteria biomass by 19%, Gram- bacteria biomass by 22%, fungal biomass by 24%, actinomycete biomass by 17% and AMF biomass by 34%. Grassland conversion also significantly reduced the relative abundance of bacterial and fungal FLFAs and F:B ratio at surface 0-10cm soil (Figure 2A, B, C). In terms of soil functional composition, Gram+ and Gram- bacteria are the most abundant groups (Figure 2D).
The difference in soil microbial biomass between grassland and cropland varied with soil depth (Table 2); in the 0-10-cm and 10–20-cm soil layers, the cropland had lower soil microbial biomass at the various sites (P <0.01, Table 2) but had higher soil microbial biomass than grassland in the 20-30-cm soil layer (P <0.05, Table 2). For each soil layer, the differences in Gram+, Gram-, Fungi, Act and AMF between the cropland and grassland showed patterns similar to that of total microbial biomass (Table 2). The site (S), land use (LU) and soil depth (SD) significantly affected the soil microbial biomass (Table 3). There was no statistically significant interactions between S and the others factors (LU and SD) (P > 0.05), while LU and SD exist significant effect on soil microbial biomass (Table 3, P <0.05). Moreover, the correlation analysis showed that soil C and N were positively correlated with biomass of most lipid categories associated with bacteria, fungi and actinomycetes, while other soil parameters including pH, SM, NH4+, NO3- and aggregations had no effects on those variables (Table 4).