Nitrogen-fixation activity and the abundance and taxonomy of nifH genes in agricultural, pristine, and urban prairie stream sediments chronically exposed to different levels of nitrogen loading
Caton, I.R., Caton, T.M. & Schneegurt, M.A. Arch Microbiol (2018) 200: 623
Small streams exert great influences on the retention and attenuation of nitrogen (N) within stream networks. Human land use can lead to increased transport of dissolved inorganic N compounds and downstream eutrophication. Microbial activity in streams is important for maintaining an actively functioning N cycle. Chronically high N loading in streams affects the rates of the central processes of the N cycle by increasing rates of nitrification and denitrification, with biota exhibiting decreased efficiency of N use. The LINXII project measured N-cycle parameters in small streams using (NO3)-N-15 (-) tracer release experiments. We concurrently measured N-2 fixation rates in six streams of three types (agricultural, pristine, and urban prairie streams) as part of this broader study of major N-cycle processes. Nitrogen fixation in streams was significantly negatively correlated with nitrate levels, dissolved inorganic N levels, and denitrification rates. Algal mat and leaf litter samples generally exhibited the highest rates of N-2 fixation. The abundance of nifH genes, as measured by real-time PCR, was marginally correlated with N-2-fixation rates, but not to other N-cycle processes or stream characteristics. The nifH sequences observed were assigned to cyanobacteria, Deltaproteobacteria, Methylococcus, and Rhizobia. Seasonal changes, disturbances, and varying inputs may encourage a diverse, flexible, stable N-2-fixing guild. Patchiness in the streams should be considered when assessing the overall impact of N-2 fixation, since algal biomass exhibited high rates of N-2 fixation.