Abstract: We investigated the effects of long-term planting of the leguminous green manure-hairy vetch (Vicia villosa Roth L.) on the abundance of functional genes of nitrogen-fixing bacteria in red soils. We employed realtime quantitative PCR to determine the abundances of nifH and 16S rRNA genes. There were four treatments in the field experiment, including peanut, peanut + hairy vetch, sweet potato, and sweet potato + hairy vetch. Soil samples were collected from 0-15 and 15-30 cm layers from each treatment. The results showed that long-term planting of hairy vetch significantly reduced the available potassium content in the surface soils and lowered pH of surface soil under peanut. Planting hairy vetch significantly increased soil organic carbon, dissolved organic carbon and total nitrogen contents in sweet potato surface soils, but had little effect on peanut soils. Planting hairy vetch and soil layer had significant effect on the abundance of nifH gene. The abundance of nifH gene abundance in the surface soils was significantly higher than that in the deep soils across all treatments. Planting hairy vetch significantly reduced the nifH gene abundance in the surface soils but did not affect that in the deep soils. This might be due to the reduction in soil pH and available potassium content, which inhibited the growth of nitrogen-fixing microorganisms. Planting hairy vetch did not affect 16S rRNA gene abundance in either surface or deep soil, indicating that nitrogen-fixing bacteria were more sensitive to planting hairy vetch than whole bacteria. There was a significant positive relationship between nifH gene abundance and available potassium content in surface soils, suggesting that the decrease in available potassium content is a key factor contributing to the reduction in nifH gene abundance in surface soil under hairy vetch treatments. In conclusion, planting hairy vetch can promote soil carbon sequestration in sweet potato, but reduce available potassium content and nifH gene abundance in the surface soils of peanut and sweet potato, which warrants attention in agricultural practices.

Key words: hairy vetch, nitrogen-fixing microorganism, nifH gene, red soil