Comparison of mineral and organic nitrogen forms in regulated and restored sections of a small lowland river

Abstract

The lower, ending part of a small lowland Rudnia River (RR) in NE Poland was restored in 1999 and transformed into a meandering river. In this study, a few years after the natural connection between the main channel and its historic floodplain was restored, we examined the impact of river channel restoration on nitrogen dynamics and its biogeochemistry. Data were collected over a period of a year at monthly intervals from two sections of the river located on the regulated and restored sections. Spatial and temporal variations in the occurrence of mineral 4 2 3 (NH , NO , NO ) + − − and organic forms of nitrogen were investigated. Much higher average concentrations of organic nitrogen forms were found in the water from the restored part of the river than in the regulated one. Mineral nitrogen contribution to the total nitrogen (TN) pool was the highest, and the total organic nitrogen (TON) to total inorganic nitrogen (TIN) ratio exceeded 2 mg·dm in the restored river section. The seasonal trend in concentration of organic nitrogen forms for the restored section was higher and statistically significantly different from the data recorded at the regulated river section. Geological, hydrological, and biogeochemical factors caused elevated organic nitrogen concentrations in the water at the restored part. The most important environmental parameters for nitrogen dynamics are geological structure of river valley, formation and condition of river banks ecotone zone, and water retention time in the river channel. Results from this study suggest that properly restored lowland river floodplains can be effective nitrate sink for mineral forms of nitrogen.