| The factors regulating nitrogen (N) cycling in streams draining Konza Prairie Biological Station, Kansas were analyzed to examine the influence of agricultural activity and non-anthropogenic factors. Patterns in water-column total nitrogen (TN) concentrations were driven largely by changes in nitrate (NO3−) concentrations, and a gradient of increasing NO3− occurred from pristine upland reaches to the more agriculturally influenced lowland reaches. Upstream, relatively pristine sites had significantly lower nitrification and denitrification rates than downstream, more agriculturally influenced, sites. Water-column ammonium (NH4+) and nitrate (NO3− ) concentrations were correlated with whole stream nitrification rates, but not whole stream denitrification rates. Nitrification rates were positively correlated with denitrification rates, being coupled at the whole stream level, but not the substratum level. A significant positive relationship was observed between substrata nitrification rates and dissolved oxygen (O2) concentration in all seasons except summer, when concentrations were super saturated. Nitrogen uptake demonstrated Michaelis-Menton kinetics when associated with all substrata, and was 10 fold higher for NH4+ relative to NO3−. Nitrification and denitrification response to variable NH4+, NO3 −, and O2 concentrations differed with substrata type. Substantial temporal variability (>30%) was predicted in uptake, nitrification, and denitrification due to natural variation in water-column NH4 +, NO3−, and O2 concentrations. These data suggest that natural processes lead to less than 10 fold variation in NO3− concentrations in relatively pristine prairie streams, whereas variation due to agricultural inputs is >100 fold. Data also suggest that whole stream nitrification and denitrification may be decoupled at the reach level by factors that decrease substrata heterogeneity in stream channels. |
