Biological nitrogen fixation in paddy soils is driven by multiple edaphic factors and available phosphorus is the greatest contributor

Biological nitrogen (N) fixation (BNF) is important for sustainable rice cultivation. Various edaphic factors have been individually evaluated for their effects on BNF in paddy soils. However, no single factor could fully explain the different soil outcomes. Paddy BNF is more likely to be simultaneously influenced to various degrees by combinations of several edaphic factors; however, the relative importance of the interaction of multiple edaphic factors on the regulation of BNF in paddy soils is still unclear. Twenty-seven paddy soil samples with different soil properties were collected from the major rice cropping areas in Southwest and Northeast China to determine the edaphic factors affecting paddy BNF amount. Rice was transplanted into pots filled with paddy soils and grown in a ¹⁵N₂-enriched airtight chamber. Estimation of BNF was based on the measurements of ¹⁵N enrichment in the paddy soils and rice plants at the end of a 77-d incubation period. The BNF amounts ranged from 0.66 to 12.3 kg ha^-1, with a significant positive relationship with available phosphorus (AP) and significant quadratic relationships with available molybdenum (AMo) and total N (TN). Available P explained 42% of the observed variation in BNF, TN explained 17%, and AMo explained 13%. The specific interaction between soil cation exchange capacity and available soil N (as determined by rice N uptake) accounted for 28% of the variation in BNF. The BNF amount was decreased when AP was < 14 mg kg^-1, AMo < 0.09 mg kg^-1, or TN was > 3.2 g kg^-1. These results provide valuable benchmarks that could be used to guide farmers in managing paddy soils to improve the potential contribution of paddy BNF to soil fertility.