Effects of Astragalus smicus growth together with nitrogen fertilization on N-cycling genes in Cd-contaminated soil

Abstract Heavy metal contamination in soil is a severe environmental problem that has caused negative effects on plant growth as well as element cycling, microbiome and diversity in cropland ecosystem. Nitrogen cycling is an extremely important element cycling process in the ecosystem, including three primary processes, nitrogen (N) fixation, nitrification and denitrification. The process of nitrification and denitrification can also cause nitrogen loss and emission of a greenhouse gas N2O. Nitrogen cycling is driven mainly by soil microorganisms. Plant growth and artificial nitrogen fertilization both can improve soil physiochemical properties and affect soil nitrogen cycling as well as nitrogen turnover to a certain degree. In the present study, Astragalus smicus growth and nitrogen fertilization are used to test their effects on soil physiochemical properties in Cd-contaminated soil by using pot experiments. qPCR and 16S-rDNA high throughput sequencing are employed to examine their effects on microbial community structure and abundance of N-cycling related genes. Results show that A.smicus growth can significantly reduce soil pH, but markedly increase the contents of soil organic matter and available N compared to the control. A. smicus growth and N fertilization markedly reduce soil pH, and significantly increase soil total N, organic matter, available N, NO3-N, contents of urease and alkaline proteinase as well as abundance of N-cycling related genes, nifH, AOA amoA, and AOB-amoA compare to the control. A. smicus growth and N fertilization profoundly increase soil total N, available N, NO3-N, contents of urease and alkaline proteinase and abundance of AOB-amoA compare to the control group and the A. smicus growth group. Hyphomicrobium, Nitrospira and Nitrosospira are the primary drivers for N fixation, nitrification and denitrification.