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

Agricultural streams receive large inputs of nutrients, such as nitrate (NO 3 − ) and ammonium (NH 4 + ), 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 (N 2 O) previously observed in stream sediments point to incomplete denitrification, with sediments acting as a potentially significant source of global N 2 O. We investigated the effect of sediment type and seasonal variation on denitrification and N 2 O 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 N 2 O–N μg g −1 h −1 for sand-dominated and 0.003 ± 0.003 N 2 O–N μg g −1 h −1 for gravel-dominated). In-situ measurements supported this finding, with higher concentrations of NO 3 − , nitrite (NO 2 − ) and N 2 O observed in the porewaters of gravel-dominated sediments. Denitrification varied substantially between seasons, with denitrification increasing from winter to autumn. Our results indicate highest NO 3 − reduction occurred in sand-dominated sediments whilst highest N 2 O 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 N 2 O.