Dissertation Abstract

Factors controlling spatial and temporal patterns of nitrogen cycling in tallgrass prairie streams.

Bernot, Melody J  2001  

, Kansas State University.

 
The factors regulating nitrogen cycling in streams draining Konza Prairie Biological Station, Kansas, were analyzed to examine the influence of agricultural activity and non-anthropogenic factors. Patterns of total nitrogen concentrations in the water-column were driven largely by changes in nitrate concentration. A gradient of increasing nitrate occurred from pristine upland reaches to the more agriculturally influenced lowland reaches. The relatively pristine upstream sites had significantly lower nitrification and denitrification rates than the downstream, more agriculturally influenced, sites. Water-column ammonium and nitrate concentrations were correlated with whole stream nitrification rates, but not whole stream denitrification rates. Nitrification rates were positively correlated with denitrification rates at the whole stream level, but not at the substratum level. A significant positive relationship was observed between substrata nitrification rates and dissolved oxygen concentration in all seasons except summer, when concentrations were super saturated. Nitrogen uptake followed Michaelis-Menton kinetics when associated with all substrata, and was 10-fold higher for ammonium relative to nitrate. Nitrification and denitrification responses to variable ammonium, nitrate, and oxygen concentrations differed with substrata type. Substantial temporal variability (< 30%) was predicted in uptake, nitrification, and denitrification due to natural variation in water-column ammonium, nitrate, and oxygen concentrations. These data suggest that natural processes lead to less than 10-fold variation in nitrate 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.