Conversion of Land Use from Upland to Paddy Field Changes Soil Bacterial Community Structure in Mollisols of Northeast China

Mollisols are extremely important soil resource for crop and forage production. In northeast China, it is a major land use management practice from dry land crops to irrigated rice. However, there is few data regarding soil quality and microbial composition in Mollisols during land use transition. Here, we analyzed the upper 30 cm of soil from land with more than 30 years of paddy use and from adjacent areas with upland crops. Our results showed that land use and soil depth had a significant effect on soil properties and enzyme activities. Soil moisture (SM) and soil organic carbon (SOC) contents were substantially higher in paddy fields than in upland crop lands, while nitrogen-related enzyme activities were lower. Following the land use change, bacterial diversity was increased and bacterial community composition changed. Taxonomic analyses showed that Proteobacteria, Chloroflexi, Firmicutes, and Bacteroidetes were the dominant phyla present. At family level, Gemmatimonadaceae decreased with land use change, while Syntrophorhabdaceae and Syntrophacea that play a part in methane cycling and nitrifying bacteria such as Nitrospiraceae increased, indicating that the structure and composition of the bacterial community might be a promising indicator of Mollisol health. Redundancy analysis indicated that land use type had a stronger effect on the soil bacterial community composition than soil depth. Additionally, bacterial community composition was closely associated with soil parameters such as soil moisture, pH, SOC, NO₃^--N, and NH₄⁺-N. Overall, land use change affects the physical and chemical properties of the soil, resulting in changes in the composition of the soil bacterial community and flora. These changes could provide a view of the bacterial community assembly and functional shifts following land use change.