Abstract
Background: Soil microbiota exert fundamental functions in maintaining ecosystem functioning and services, including pedogenesis, biogeochemical processes and plant productivity, especially for agriculture system. Despite their ubiquitousness from the epipedon to deep soil, the vertical characteristics of microbiomes (especially for functional microorganisms) and their contribution to soil element cycling when considering soil developmental features are poorly understood. Here, nine profiles (0~135 cm) of two canonical paddy soil types (Fe-accumuli- and Hapli-stagnic anthrosols; 111 samples in total) at a local scale were collected, which represented relative long- and short-term water flooding history, respectively. The vertical variations in edaphic characteristics and assemblies of soil bacterial and diazotrophic communities, and microbial contribution to element cycling were explored. Results: Across soil profiles, Hapli-stagnic anthrosol was characteristic of higher concentrations in free iron oxides and total iron in the epipedon, and contained higher amounts of ammonia along the subsurface layers, as compared with acidic Fe-accumuli-anthrosol. Community assemblies of bacteria and diazotrophs, as well as edaphic properties, were mainly shaped by soil depths, followed by soil types. Furthermore, random forest analysis revealed that, for Fe-accumuli-anthrosol, available Fe could best predict nitrogen cycling index and nitrogen status was significantly related to iron cycling index; while in Hapli-stagnic anthrosol, available sulfur was the most important variable in predicting nitrogen and iron cycling indices. Among the dominant genera, some distinctive biomarkers that varied remarkably between the two soil types were noticeable for their contributions to both nitrogen and iron transformation, including iron-reducing diazotroph Geobacter and iron-oxidizing bacterium Rhodanobacter that characterized Fe-accumuli type, and sulfur reducing diazotroph Desulfobacca as main discriminant clades for Hapli-stagnic type.Conclusions: A novel perspective was proposed on the vertical characteristics of edaphic properties and bacterial and diazotrophic communities in the two paddy soil types. The findings indicated the nitrogen-iron cycling processes for Fe-accumuli-anthrosol and nitrogen-iron-sulfur coupling interaction for Hapli-stagnic anthrosol, advancing our understanding of the significant multiple role played by soil microorganisms, especially for diazotrophs, in element biogeochemical cycles.
Movements of free oxides in paddy soil profiles (Part 1)