Nitrate Transformations in Southern Ontario Stream Sediments1

The transformation of nitrate was investigated in diverse stream sediments from six areas of Southern Ontario, Canada. Laboratory Experiments conducted with intact 0‐5 cm depth sediment cores overlain with aerated 5 mg/l nitrate‐N solution reveald considerable nitrate depletion during a six‐day period as a resuit of denitrification and nitrate reduction to ammonium. Nitrification and ammonification also occurred simultaneously with nitrate reduction in many stream sediments. Mixed 0‐5 cm depth sediments collected from 81 stream sites were used to examine the relationship between microbial nitrate removal and sediment characteristics. Rates of nitrate‐N loss from aerated 5 mg/l nitrate‐N solutions overlying these sediments ranged between 11 and 171 mg/m 2 /day for a 24‐hour incubation period. Rates of nitrate loss for the 24‐hour period exhibited a significant positive correlation (r=0.72) with sediment ammonium content. Organic carbon, total nitrogen and sediment texture also had significant but weak correlations with nitrate loss.