Nitrogen enrichment stimulates rhizosphere multi-element cycling genes via mediating plant biomass and root exudates

Global nitrogen (N) deposition has a significant impact on the structure and function of above-ground plant communities; however, the impact of N enrichment on below-ground microbial communities remains insufficiently elucidated, especially regarding the functional gene structure. An 18-month pot experiment was conducted to examine the effects of N addition rate (0, 5, 10, and 15 g N m−2 yr−1) on the functional gene composition of rhizosphere microorganisms of Bothriochloa ischaemum, a native grass species of the semiarid area, and to evaluate the soil and plant variables connected with the observed variation. Using microarray GeoChip analysis, 2676, 744, 308, and 515 gene probes involved in carbon (C), N, phosphorus (P) and sulfur (S) cycling, respectively, were detected. Compared with the weak response of genes in the bulk soil, N addition significantly increased the abundance of genes involved in C fixation (tkta, rubisco, TIM), C degradation (amyA, ara, chitinase), methanogenesis (mcra), N2-fixation (nifH), denitrification (narG, nirK/S, nosZ), polyphosphate degradation (ppx), and sulfite reduction (dsrA, dsrB, cysJ) in the rhizosphere. The abundance of genes responsible for C-, N-, P-, and S-cycling increased with the N addition rate and was mainly regulated by variations in plant biomass and root exudates. The quantity of soil dissolved organic C played a crucial role in determining the abundance of genes related to the cycling of C, N, P, and S. Plant biomass indirectly affected gene abundance by increasing organic acids in root exudates and dissolved organic C. These findings facilitate our understanding of microbial function to elevated N levels and highlight the key role of root exudates and plant biomass in regulating microbial functions under future N-enrichment scenarios.