Residue Return Effects Outweigh Tillage Effects on Soil Microbial Communities and Functional Genes in Black Soil of Northeast China

Conservation tillage as an effective alternative to mitigate soil degradation has attracted worldwide attention, but the influences of conservation tillage on soil microbial community and especially function remain unclear. Shotgun metagenomics sequencing was performed to examine the taxonomic and functional community variations of black soils under three tillage regimes, namely no-tillage with residue (maize straw) return (NTS), moldboard plow with residue return (MPS), and moldboard plow without residue return (MPN) in Northeast China. The results revealed: 1) Soil bacterial and archaeal communities differed significantly under different tillage regimes in contrast to soil fungal community. 2) The overlay of less tillage and residues return under NTS led to unique soil microbial community composition and functional composition. Specifically, in contrast to other treatments, NTS increased the relative abundances of some taxa such as Bradyrhizobium, Candidatus Solibacter, and Reyranella, along with the relative abundances of some taxa such as Sphingomonas, Unclassified Chloroflexi and Nitrososphaera decreased; NTS had a unique advantage of increasing the relative abundances of genes involved in 'ATP-binding cassette (ABC) transporters' and 'quorum sensing (QS)' pathways, while MPN favored the genes involved in 'flagellar assembly' pathway and some metabolic pathways such as 'carbon' and 'glyoxylate and dicarboxylate' and 'selenocompound' metabolisms. 3) Significantly different soil bacterial phyla (Acidobacteria, Gemmatimonadetes, and Chloroflexi) and metabolic pathways existed between MPN and another two treatments (NTS and MPS), while did not exist between NTS and MPS. 4) Dissolved organic carbon (DOC) and soil bulk density were significantly affected (P < 0.05) by tillage and accounted for the variance both in microbial (bacterial) community structure and functional composition. These results indicated that a change in tillage regime from conventional to conservation tillage results in a shift of microbial community and functional genes, and we inferred that residue return played a more prominent role than less tillage in functional shifts in the microbial community of black soils.