The core soil bacterial genera and enzyme activities in incubated soils from century-old wheat rotations

Abstract

Long-term field trials are ideal for characterising soil microbial communities because lasting communities in a given cropping system evolve over a long time. However, the soil microbiome has not been characterized in most long-term trials. We used a field trial established in 1911 to identify the core soil bacteria and their functioning in century-old wheat rotations, and how soil moisture deficit affected them. We collected soil samples from three wheat rotations: continuous wheat (W), fallow-wheat (FW) and fallow-wheat-wheat (FWW), all with or without N + P fertilizer, and incubated them with or without soil moisture deficit. The crop rotation effects on microbial biomass C (MBC) were in the order: W > FWW = FW. Fertilizer increased soil MBC by 21%. The activities of β-glucosidase (C cycling), N-acetyl-β-glucosaminidase (C and N cycling), and acid phosphomonoesterase (P cycling) followed similar trends, as did soil N supply measured in the field. The α-diversity of the soil bacteria was lower in continuous wheat than in the other rotations, and decreased with fertilizer application. Actinobacteria and Bacteroidetes were more abundant in continuous wheat than other rotations, and the relative abundance of Bacteroidetes increased with fertilizer application. By contrast, Acidobacteria were less abundant in continuous wheat than in other rotations. Soil moisture deficit during incubation had no effects on the soil microbiome. The core bacterial genera present in 90% of the soil samples were Bradyrhizobium, Rubrobacter, Friedmanniela and Marmoricola. β-diversity analysis revealed that the bacterial community structure in continuous wheat with fertilizer application was different from the structures in the other treatments. These and previously-published soil organic C, N and wheat yield results suggest that microbe-mediated nutrient cycling enabled sustainable continuous wheat cropping when fertilizer was applied.