Effects of biofertilizers on nonsymbiotic nitrogen fixation in different paddy soils

Abstract

The excessive application of chemical nitrogen fertilizers to paddy fields has led to numerous environmental problems. Nitrogen-fixing biofertilizers are adequate substitutes for chemical nitrogen fertilizers. In this study, two representative paddy soils (HH and SH) in China with significant differences in their nitrogen fixation activities and physicochemical properties were selected. The effects of 11 kinds of commercial nitrogen-fixing biofertilizers on the properties, nonsymbiotic biological nitrogen fixation activities and nifH gene abundance of the soil were assessed. The results revealed that different biofertilizers exerted distinct effects on the biological nitrogen fixation (BNF) rate and nifH gene abundance of these soils. The BNF rates of HH soil and SH soil ranged from 10.35 to 21.95 μg kg⁻¹·d⁻¹ and from 2.53 to 35.27 μg kg⁻¹·d⁻¹, respectively. The highest increase in BNF was 80.87 % in HH soils and more than tenfold in SH soils. Mo is a component of nitrogen-fixing enzymes, and the results showed that Mo content was positively correlated with the rate of nitrogen fixation in SH soils, which was the most important factor affecting the rate of nitrogen fixation. Application of biofertilizers with high Mo content in Mo-deficient paddy soils may be an effective measure to increase soil N input. The size of the nitrogen-fixing strain population in the 11 biofertilizers ranged from 2.6 × 10³ to 1.2 × 10⁸ CFU g⁻¹, and 32 strains were identified by 16S rRNA gene sequencing. Strains B4 (Pantoea wallisii), B5 (Pantoea anthophila), and B8 (Bacillus paralicheniformis) had the highest BNF rates in nitrogen-free medium. This study elucidated the contributions of various commercial biofertilizers to nonsymbiotic nitrogen fixation in paddy soils and revealed the underlying mechanisms. These findings provide a theoretical foundation for the development and practical application of biofertilizers, and will inform strategies to increase the efficacy of biofertilizers in agricultural settings.