Inoculation with nitrous oxide (N2O)-reducing denitrifier strains simultaneously mitigates N2O emission from pasture soil and promotes growth of pasture plants

• Seven bacterial strains were screened from 40 denitrifier strains affiliated with Azospirillum and Herbaspirillum.
• These seven strains generate N2 only as a denitrification end product and produce indole-3-acetic acid.
• Inoculation with most of these strains mitigated N2O emissions from pasture soil.
• Inoculation with most of these strains promoted the growth of red clover and timothy.

The aim of this study was to screen nitrous oxide (N2O)-reducing denitrifier strains showing both N2O mitigation and plant growth-promoting (PGP) effects in soil systems, and the effects of selected strains were monitored in soil and plant, analyzed, and comparatively evaluated. Forty denitrifier strains affiliated with Azospirillum and Herbaspirillum, previously isolated from three different paddy soils, were evaluated. Of these, 11 produced indole-3-acetic acid (>5 μg mL−1), 9 promoted the growth of red clover (Trifolium pratense L. var. Medium) or timothy (Phleum pratense L. var. Horizon) on agar plates, and 7 were inoculated into two different soils for cultivating red clover and timothy in a greenhouse. Compared with non-inoculated control, N2O flux from red clover soil and from timothy soil were significantly lower 8 and 14 days and 9 days onwards, respectively, after inoculation with these seven strains. Cumulative N2O emissions from red clover soil were significantly lowered through inoculation with these seven strains. The growth parameters, including plant height, leaf area, fresh weight or dry weight, of the two pasture plants were significantly greater in soils inoculated with most of these seven strains than in non-inoculated soils. The uptake of C and N by the two pasture plants was significantly greater in soils inoculated with most of these seven strains than in non-inoculated soils. In conclusion, inoculating N2O-reducing denitrifiers to pasture soil could mitigate N2O emissions and simultaneously promote the growth of pasture plants in a greenhouse. These strains will be invaluable microbiological resources for developing novel biofertilizers.