Spatial and temporal patterns of nitrogen concentrations in pristine and agriculturally-influenced prairie streams

Long-term data on nitrogen chemistry of streams draining Konza Prairie Biol ogical Station (Konza), Kansas were analyzed to assess spatial and temporal patterns and examine the influence of agricultural activity on these patte rns. Upland watersheds of Konza are predominantly tallgrass prairies, but a gricultural fields and riparian forests border the lower reaches of the str eams. We have up to 11 years of data in the relatively pristine upland reac hes and 4 years of data on wells and downstream reaches influenced by ferti lized croplands. Seasonal and spatial patterns in total nitrogen (TN) conce ntrations were driven largely by changes in the nitrate (NO3-) concentratio ns. A gradient of increasing NO3- concentrations occurred from pristine upl and stream reaches to the more agriculturally-influenced lowland reaches. N itrate concentrations varied seasonally and were negatively correlated with discharge in areas influenced by row-crop agriculture (p = 0.007). The NO3 - concentrations of stream water in lowland reaches were lowest during time s of high precipitation, when the relative influence of groundwater drainag e is minimal and water in the channel is primarily derived from upland prai rie reaches. The groundwater from cropland increased stream NO3- concentrat ions about four-fold during low-discharge periods, even though significant riparian forest corridors existed along most of the lower stream channel. T he minimum NO3- concentrations in the agriculturally influenced reaches wer e greater than at any time in prairie reaches. Analysis of data before and after introduction of bison to four prairie watersheds revealed a 35% incre ase of TN concentrations (p < 0.05) in the stream water channels after the introduction of bison. These data suggest that natural processes such as bi son grazing, variable discharge, and localized input of groundwater lead to variation in NO3- concentrations less than 100-fold in prairie streams. Ro w-crop agriculture can increase NO3- concentrations well over 100-fold rela tive to pristine systems, and the influence of this land use process over s pace and time overrides natural processes.