Stable-isotope Probing Highlights the Active Microbes Associated with Carbon Flow under Different Cultivation Conditions: Rhizosphere Soil Versus Bulk Soil and Upland Soil Versus Paddy Soil

Plant carbon (C) released into the rhizosphere will ultimately impact the bulk soil, varying with soil water status. However, the microbial communities involved, and how they vary between rhizosphere and bulk soil, and under varying water regimes, are little understood. Here, within a continuous 13CO2 labeling chamber, a rhizobox-like system was utilized to cultivate ryegrass (upland) and rice (paddy) to identify the root-derived C flow and associated active microbes. The 15N-labeled fertilizer was used to measure the N flow from the soils and plants. The 13C labeled soil organic C (13C-SOC) values of upland soil were greater than those of paddy soil. There was a reduction in 13C-SOC values from the rhizosphere to the bulk soils, but the total 15N labeled N (15N-TN) values exhibited an increase trend from this direction. The isotopic enrichment of PLFAs and DNA sequencing results indicated that the bacterial and fungal abundance and diversity exhibited augmentation in the rhizosphere soil under both cultures compared with the bulk soils. Bacteria played a much more important role than fungi in the C flow in the paddy soil. However, fungi dominantly contributed to this flow in the upland soils. Our findings revealed a vital role of Ascomycota fungi (especially Talaromyces) and Actinobacteria bacteria (Actinomycetales and Gaiellales) in contributing to the assimilation of root-derived C for the upland and paddy soils, respectively.