Assimilatory Uptake Rather than Nitrification and Denitrification Determines Nitrogen Removal Patterns in Streams of Varying Land Use

• Published in: Limnology and oceanography 2008-11, Vol.53 (6), p.2558-2572
• Main Authors: Arango, C. P., Tank, J. L., Johnson, L. T., Hamilton, S. K.
• Format: Article
• Language: English
• Subjects: Agriculture; Earth sciences; Earth, ocean, space; Exact sciences and technology; Forest canopy; Geochemistry; Hydrology; Hydrology. Hydrogeology; Land use; Mineralogy; Nitrification; Nitrogen; Nutrient uptake; Organic farming; Riparian forests; Sediments; Silicates; Streams; Water geochemistry
• ISSN: 0024-3590; 1939-5590
• DOI: 10.4319/lo.2008.53.6.2558

Agricultural and urban land use increase nitrogen (N) concentrations in streams, which can saturate biotic demand by plants, algae, and bacteria via assimilative uptake, and by nitrification and denitriflcation. We studied six streams per year in each of three land-use categories (agricultural, urban, and forested) for 3 yr (n = 18 streams), and we compared whole-stream N uptake and microbial N transformation rates during spring, summer, and autumn. We measured whole-stream removal of added ammonium ${\rm{NH}}_4^ +$ and nitrate ${\rm{NO}}_3^ -$ in the field and quantified sediment nitrification and denitriflcation rates in laboratory assays. Relative demand for ${\rm{NH}}_4^ +$ (as uptake velocity, $V_f$) was highest in spring and in streams with open canopies, implying a link with aquatic autotrophy. In agricultural and urban streams, whole-stream removal (as areal uptake, U) of ${\rm{NH}}_4^ +$ and ${\rm{NO}}_3^ -$ nitrification, and denitriflcation rates approached saturation at higher inorganic N concentrations. Nitrification and denitriflcation rates measured in redox-optimized laboratory assays were roughly equivalent, suggesting that in situ redox conditions will determine whether stream sediments are a net source or sink of ${\rm{NO}}_3^ -$. Though nitrification and denitriflcation rates were measured under ideal redox conditions, they were always more than an order of magnitude lower than whole-stream ${\rm{NO}}_3^ -$ uptake, demonstrating their limited influence on whole-stream ${\rm{NO}}_3^ -$ dynamics. Assimilatory processes, which temporarily store N removed from the water column, dominated whole-stream N demand and controlled downstream N flux. The ultimate fate of assimilated N remains unknown; in-channel storage cannot account for it, and thus a key question is what fraction may eventually be stored in downstream depositional zones or denitrified upon remineralization.