I am amazed. Nitrogen-fixing plants are at worst neutral on the issue. At least for annuals, the peak of nitrogen fixation occurs at flowering or just before and reduces as the plants develop fruit. At the death of the plant, the remaining nodules of nitrogen-fixing bacteria and symbiotic nitrogen-fixing algae and other organisms become food for other organisms.

There is no realistic comparison between the contributions of biological nitrogen-fixing and those of agricultural and horticultural loading and loss of fertilizers.

There is no algae living in the soil fixing nitrogen, algae needs light for that. N2 fixing plants are unlikely to let nitrites and nitrates slip by, N2 from air isn't free, a plant will do better if it pulls up all it can from the soil. Fertilizers of any kind on the garden will lead to a bigger problem. Fixed nitrogen is only a problem with run off or if the water has very little ways to go under ground, there are bacteria that survive far from O2 on unfixing nitrogen, and every set of roots between the rain garden and the river gets a shot at it.