Proper Biochar Increases Maize Fine Roots and Yield via Altering Rhizosphere Bacterial Communities under Plastic Film Mulching

Biochar amendment is considered a sustainable agricultural strategy to improve crop yields. However, information on grain yield, fine roots and in relation to rhizosphere microbial communities in maize under plastic film mulching is very limited. Herein, biochar applied every 2 years (8.4 t ha−1, B1) and biochar applied every 5 years (21 t ha−1, B2) combined with nitrogen (225 kg ha−1), or nitrogen alone, were tested in a field experiment. The results showed that a biochar–fertilizer application significantly decreased the root length at the V9 stage, but biochar applied every 5 years significantly maintained the root length at the R6 stage. Biochar–fertilizer application increased grain yield under the B1 treatment while slightly decreasing under the B2 treatment. The rhizosphere of maize was preferentially colonized by Proteobacteria, Firmicutes, Sphingomonas, and Bradyrhizobium. Dominant phyla including Proteobacteria were enriched in bulk soils, while Bacteroidetes and Firmicutes were depleted in rhizosphere and bulk soils under the biochar–fertilizer application. Changes in root morphology and soil properties were responsible for bacterial community structure in response to different biochar applications. Thus, we concluded that the differential responses of maize yield and root attributes might be related to the specific biochar dose-specific effects on soil microbiome diversity.