Ammonium Transformations in Springwater within the Riparian Zone of a Small Woodland Stream

Field enrichments with ammonium were conducted for periods of 4–4.5 h on two springs in the riparian zone of a small woodland stream near Toronto, Ontario. Springwater lost much of the injected ammonium as it flowed over organic and sandy riparian substrates before entering the stream. Negligible nitrate release occurred during the ammonium enrichments. Laboratory experiments revealed that riparian substrates rapidly removed ammonium from enriched springwater. Sediments covered with aerated ammonium-enriched water showed a small nitrate-N increase of 9–12 μg∙L−1 after 5 h. An absence of ammonium depletion in sterilized organic substrates and very small losses in low-temperature (1 °C) incubations suggested that microbial activity rather than adsorption was responsible for ammonium uptake. The rapid loss of ammonium from springwater overlying acetylene-treated substrates, despite the complete inhibition of nitrification, indicated that microbial immobilization was mainly responsible for ammonium depletion. We suggest that ammonium transformations in spring-fed areas of overland flow within the riparian zone may contribute significantly to the regulation of nitrogen inputs to small woodland streams.