Print PageThe success of early farming in Sweden relied on natural nutrition processes sustaining annual crops; however, the source of this productivity remains poorly understood. Anthropogenic pollution and river regulations have drastically limited the number of opportunities to study key natural processes in large alluvial systems in many parts of the world. The relatively pristine conditions in northern Sweden allowed us to investigate the role of cyanobacteria in maintaining site productivity in alluvial meadows that sustained centuries of annual harvesting of native sedges and grasses (as winter fodder). Replicate samples of surface soils were collected at 72 alluvial meadow sites at 10 rivers in northern Sweden between latitudes 64-68 N and longitudes 16-24 E. Nitrogen fixation rates were estimated by use of a 15N calibrated acetylene reduction method at numerous points in time and space. Intensive sampling by season was conducted along four distinct river segments and intensive sampling by river elevation was conducted at numerous points along a single river (Tornälven). At each site, 12 replicate samples of surface detritus (0 – 2 cm) were collected with a 2.5 cm coring device. A replicated fertilization and harvesting trial was conducted at one river segment. Replicate samples from eight individual sites along three separate river systems were analysed for diazotrophic cyanobacterial community composition using specific primer sets for the nifH gene marker.
Results/Conclusions
Nitrogen fixation rates in alluvial meadows along large rivers in Sweden were temporally variable, but account for 12 – 35 kg N ha-1 yr-1. Nitrogen fixation was measurable from May through October and rates were highest in late summer. Nitrogen fertilizers significantly reduced nitrogenase activity yet the harvesting of fodder increased nitrogenase activity in soil/sediment samples. Nitrogen fixation rates declined with decrease in river elevation, apparently related to increasing inorganic N concentrations. The N fixing consortium at these sites was found to be highly diverse. At least six major genera of cyanobacteria were identified in the alluvial sediments. Nitrogen fixation by cyanobateria appears to account for roughly one third of the annual N historically removed as fodder and likely accounts for much of the sustained productivity in alluvial meadows of high latitude ecosystems. Nitrogen fixation in alluvial meadows of large river systems is likely a significant contributor to the river N spiral through out the boreal biome.
phone 202-833-8773