Higher Glucose-Induced Biological Nitrogen Fixation Potential in Paddy Soils Than in Upland Soils

Paddy fields generally contain greater total soil nitrogen than adjacent upland fields. However, whether and how biological nitrogen fixation, the primary source of bioavailable soil nitrogen, contributes to the difference in nitrogen nutrition between these two land-use types remains uncertain. In this study, we focused on biological nitrogen fixation in paired adjacent upland and paddy soils, using ¹⁵N₂ tracing with high-throughput sequencing. Sufficient available carbon input (glucose) induced an 18.2-fold higher biological nitrogen fixation rate under nitrogen-limited conditions in paddy soil (14.9 mg N kg^-1 day^-1) relative to adjacent upland soil. Compared with upland soil, paddy soil exhibited higher abundance, different community composition and lower deterministic community assembly (variable selection) of diazotrophs (nitrogen-fixing microorganisms) upon glucose-induced biological nitrogen fixation. Glucose-induced nitrogen fixation in paddy soil enhanced interactions among prokaryotes under nitrogen-limited conditions and, in turn, supported faster prokaryotic growth compared to upland soil. Soil organic carbon and diazotrophic abundance, especially biomarker abundance, were the dominant abiotic and biotic factors influencing nitrogen fixation. Improved soil nitrogen in paddy fields, compared with adjacent upland fields, can therefore be substantially attributed to biological nitrogen fixation.