(This message also posted on Plants, Soils and Climate)
I have a query about the wisdom (or not) of planting nitrogen fixers in areas subject to high moisture levels such as swales, rain gardens, channels. I know the advantage of providing nitrogen to plants, but presumably the nitrogen generated (or part thereof) will be liable leaching, draining or run off with potentially adverse impacts on waterways downstream (eg algal blooms). I’d be interested in thoughts and commentary on this issue.
Excess nitrogen/algal blooms are the result of extremely high concentrations of nitrogen, generally from industrial scale agriculture. You’re only going to get those super-high concentrations of nitrogen from synthetic fertilizers, urine, feces, or MAYBE if you take a large monoculture of a N fixer and leave the dead plants in the field to decompose without planting anything else that would use the N. If you plant a few nitrogen fixers among non-fixers in a swale, I really doubt the plants will produce enough bio-available N to leach. If you’re worried about it, though, you can always remove N from the system by cutting back the N fixers or removing the dead plant matter and letting them decompose on a different area that has less leaching potential. I don’t think that is necessary, though.
Nitrogen fixation in plants is bacterial working in symbiosis with the “nitrogen fixing” plant. This is hardly the same thing as free nitrogen as found in fertilizers. The nitrogen is retained in the soil, assuming soil health is maintained. Unless the water also washes away the bacteria in the soil, I don’t see a problem.