Is Endophyte-Plant Co-Denitrification a Source of Nitrous Oxides Emission?—An Experimental Investigation with Soybean

Abstract

The biological pattern of plants’ nitrous oxide (N2O) generation is not well understood because plant cells cannot form N2O from nitric oxide (NO) reduction. Hypothetically, we consider that plant-endophytic co-denitrification is potentially capable of making up for the functional loss of NO reduction in the plant cell and is thus the source of plant-derived N2O. In order to test the above hypothesis, field-cultured soybean seedlings and aseptic-cultured seedlings were used to establish plant holobionts with differentially established endophytes. The N2O fluxes, copies of the bacterial NO reductase encoding gene, and a diversity of endophytic denitrifying bacteria of these holobionts were observed by gas chromatography, real-time PCR, and 16s rDNA sequencing. The flux of N2O by the field seedlings was significantly higher than the fluxes of the aseptic seedlings and bulk soil. The N2O flux of the soybean seedlings was significantly correlated to the abundance of the bacterial NO reductase encoding gene. Eleven genera of denitrifying bacteria were observed in the soybean seedlings, and among them, two genera of aerobic denitrifying bacteria were specifically associated with colonizing plant samples. In this work we have also showed that plant N2O emission is affected by the abundance of total endophytic denitrifying bacteria. We conclude that plant sourced N2O is cooperatively generated by the plant-endophyte symbiotic system in which endophytic denitrifying bacteria help plants to emit N2O by taking over the NO–N2O reduction process.