Responses of bacterial communities and nitrogen-cycling microbiomes to the conversion from cereals to legumes in the rice-based system

Abstract

Legumes are gaining popularity in rice-centric agroecosystems to boost yield, improve sustainability, and enhance soil fertility. However, their effects on soil nitrogen(N)-transforming networks, and predominant drivers governing microbial communities shift remain largely opaque. Here, we investigated soil bacterial community composition and N-cycling genes after converting to legume rotations in a 7-year field experiment for both rice and winter cropping seasons. Two conventional rice-based systems, rice-wheat (RW) and rice-rape (RRa), and four rice-legume rotations, rice-milk vetch (RM), rice-fava bean (RF), rice-milk vetch-rice-fava bean after continuous rice-milk vetch (RMF), and rice-fava bean-rice-milk vetch following continuous rice-fava bean (RFM) were constructed. Results showed that legume integration imposed greater impacts on soil biogeochemistry properties and bacterial communities in winter versus rice seasons. Integration of legume into rice-based cropping systems elicited significant alterations in soil water content, CEC, pH, total N content (TN), microbial biomass N, and dissolved organic carbon. Taxa including Acidobacteria (RFM), Rokubacteria (RMF), and Chloroflexi (RM) related to decomposition potentials were altered. Shift in soil bacterial community were attributed to alterations in environmental factors including soil pH, CEC, and TN. Legume mulching during rice seasons significantly reduced potential for assimilatory nitrite reduction (RM). Legume cultivation, on the other hand, suppressed the processes of N fixation (RFM), nitrate and nitrite reduction (RF and RFM) during winter seasons, potentially decreasing N leaching and N2O emissions. This study demonstrates strategic legume integration can reshape soil N cycling to improve agricultural sustainability.