Nitrogen fixation by Pueraria lobata as a nitrogen source in the midstream sediment bar of a river

It was assumed that the symbiotic N fixation in a floodplain site is highly affected by flood disturbances. To test this hypothesis, we collected soil and plant samples from several quadrats of the midstream flood plain of a relatively steep urbanized river that was subject to a different magnitude of flooding. Soil carbon and nitrogen contents had a significant positive correlation with the elevation of sites; i.e. they negatively correlated with a frequency of flood inundation. Soil pH values ranged between 6.0 and 7.23 at low elevation sites and 5.16 and 6.81 for high elevation sites. There was a significant difference ( p  < 0.001) in N fixation among sampling points, which suggests that flooding disturbs the process. A pH value of 6.5 is the best for N fixation; N fixation decreases while the pH increases or decreases. Site elevation also directly affected soil characteristics, especially nitrogen and carbon content. The fractionation of 15 N value in leaf tissue (δ 15 N) suggests that N fixation is extensive when the total N (TN) is ≤ 0.3%. Below this TN, Pueraria lobata can derive up to 80% of their TN requirement from the atmosphere. Therefore, it can be concluded that site elevation, or flood pulses, affects the symbiotic N fixation in a floodplain by modifying the edaphic factors. Copyright © 2015 John Wiley & Sons, Ltd.