Seasonal variability of sediment controls of nitrogen cycling in an agricultural stream

Agricultural streams receive large inputs of nutrients, such as nitrate (NO3-) and ammonium (NH4+), which impact water quality and stream health. Streambed sediments are hotspots of biogeochemical reactivity, characterised by high rates of nutrient attenuation and denitrification. High concentrations of nitrous oxide (N2O) previously observed in stream sediments point to incomplete denitrification, with sediments acting as a potentially significant source of global N2O. We investigated the effect of sediment type and seasonal variation on denitrification and N2O production in the streambed of an agricultural UK stream. Denitrification was strongly controlled by sediment type, with sand-dominated sediments exhibiting potential rates of denitrification almost 10 times higher than those observed in gravel-dominated sediments (0.026 +/- 0.004 N2O-N mu g g(-1) h(-1) for sand-dominated and 0.003 +/- 0.003 N2O-N mu g g(-1) h(-1) for gravel-dominated). In-situ measurements supported this finding, with higher concentrations of NO3-, nitrite (NO2-) and N2O observed in the porewaters of gravel-dominated sediments. Denitrification varied substantially between seasons, with denitrification increasing from winter to autumn. Our results indicate highest NO3- reduction occurred in sand-dominated sediments whilst highest N2O concentrations occurred in gravel-dominated sediments. This suggests that finer-grained streambeds could play an important role in removing excess nitrogen from agricultural catchments without producing excess N2O.