Expression of N‐cycling genes of root microbiomes provides insights for sustaining oilseed crop production

Summary Agricultural production is dependent on inputs of nitrogen (N) whose cycle relies on soil and crop microbiomes. Crop diversification has increased productivity; however, its impact on the expression of microbial genes involved in N‐cycling pathways remains unknown. Here, we assessed N‐cycling gene expression patterns in the root and rhizosphere microbiomes of five oilseed crops as influenced by three 2‐year crop rotations. The first phase consisted of fallow, lentil or wheat, and the second phase consisted of one of five oilseed crops. Expression of bacterial amoA , nirK and nirS genes showed that the microbiome of Ethiopian mustard had the lowest and that of camelina the highest potential for N loss. A preceding rotation phase of lentil significantly increased the expression of nifH gene by 23% compared with wheat and improved nxrA gene expression by 51% with chemical fallow in the following oilseed crops respectively. Lentil substantially increased biological N 2 fixation and reduced denitrification in the following oilseed crops. Our results also revealed that most N‐cycling gene transcripts are more abundant in the microbiomes associated with roots than with the rhizosphere. The outcome of our investigation brings a new level of understanding on how crop diversification and rotation sequences are related to N‐cycling in annual cropping systems.