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The potential impact of oxygen from roots on the source of inorganic nitrogen for Ruppia maritima L. (Potamogetonales) was investigated in laboratory experiments. Roots released oxygen at an average rate of 2 to 3 μg O2 (mg dry wt)-1h-1. A distinctive oxygenated zone with a radius of 0.75 to 1.25 mm developed in the sediment around the roots. Although nitrate and nitrite could have been present in the oxygenated zone, these were unlikely to be significant sources of nitrogen for R. maritima. Root hairs extended up to 6 mm beyond this zone. In addition, roots of plants cultured in a flow-through system took up ammonia at a rate approximately 9 times greater than that for either nitrate or nitrite.