Long-term organic farming may prevent nitrate leaching by enhancing the abundance and co-occurrence of nasA-type nitrate assimilatory bacteria

Organic farming is considered to have advantages in reducing soil nitrate (NO₃⁻) leaching. Previous studies have established that excessive NO₃⁻ accumulation serves as the primary driver of leaching, with NO₃⁻ assimilation process identified as one of the important reasons for underpinning such accumulation dynamics. However, the distribution and diversity of NO₃⁻ assimilating microorganisms participating in this process in the organic system is poorly understood. In the present study, the abundance and characteristics of nasA-type nitrate assimilatory bacteria (NAB) in organic (ORG), integrated (INT), and conventional (CON) systems set up in 2002 were investigated via quantitative real-time PCR (qPCR) and Pacbio high-throughput sequencing over two vegetable growing seasons. The abundance of nasA gene was significantly higher in ORG than the other two systems. Composition analysis revealed that nasA gene bacterial community structures shifted in ORG, and Bradyrhizobium (an important nitrogen fixing genus) was a dominant nasA-type NAB with the highest relative abundance and occurred in all samplings in ORG. Further Redundancy Analysis showed the community of nasA-type NAB in ORG was mainly affected by soil available phosphorus and available potassium. Our results demonstrate that higher abundance and different communities of nasA-type NAB in ORG may be one of the important reasons reducing soil nitrate accumulation and leaching, which could provide a theoretical basis for nitrate leaching in farmland.