Rhizosphere-induced shift in the composition of bacterial community favors mineralization of crop residue nitrogen

Aims: In agricultural systems, residue amendment is an important practice for nutrient management, but the role of microbes in mineralization of crop residue nitrogen (N) is not well known. Therefore, this study aimed to examine how the residue N mineralization was associated with changes of the microbial community composition in crop rhizosphere. Methods: A rhizobox system was deployed to separate the rhizosphere zone into the root-growth (central), and 2 mm (proximal) and 4 mm (transitional) zones away from the central zone, and the gradient change of the residue-N mineralization along the zones was assessed. Soybean plants were grown in a Mollisol without or with amendment of 15N-labelled soybean and maize residues. Furthermore, amplicon sequencing was performed to detect the shift of microbial community composition associated with the residue-N mineralization. Results: The residue-N was mineralized faster in the rhizosphere than the bulk soil, and from soybean residue than maize residue. Greater enrichment of taxa against the unit of residue-N mineralization in the soybean than maize residue treatment was correspondent with the enriched ammonification genes, likely contributing to the enhanced mineralization of soybean residue-N in the rhizosphere. A gradual increase in dissolved organic C and a decrease in available N concentration from the central root zone to the bulk soil, might shift bacterial community favoring the residue-N mineralization in the rhizosphere. Conclusions: The spatial changes in chemical properties across the rhizosphere lead to the recruitment of microbiome taxa to enhance the mineralization of N derived from crop residues.