In-Stream Nutrient Retention in Agricultural Catchments

In northeastern Austria, marshlands have been turned into the most productive arable land of the country. As a result, most headwater streams show structurally degraded channels, lacking riparian buff er zones, which are heavily loaded with nutrients from the surrounding crop fi elds. Th e present study examines whether longitudinally restricted riparian forest buff ers can enhance the in-stream nutrient retention in nutrient-enriched headwater streams. We estimated nutrient uptake from pairwise, shortterm addition experiments with NH4, NH4, PO4, and NaCl within reaches with riparian forest buff ers (RFB) and degraded reaches (DEG) of the same streams. Riparian forest buff ers originated from the conservation of the pristine vegetation or from restoration measures. Hydrologic retention was calculated with the model OTIS-P on the basis of conductivity breakthrough curves from the salt injections. A signifi cant increase in surface transient storage was revealed in pristine and restored RFB reaches compared with DEG reaches due to the longitudinal steppool pattern and the frequent occurrence of woody debris on the channel bed. Ammonium uptake lengths were signifi cantly shorter in RFB reaches than in DEG reaches, resulting from the higher hydrologic retention. Uptake velocities did not diff er signifi cantly between RFB and DEG reaches, indicating that riparian forest buff ers did not aff ect the biochemical nutrient demand. Uptake of NH4 was mainly driven by autotrophs. Net PO4 uptake was not aff ected by riparian forest buff ers. Th e study shows that the physical and biogeochemical eff ects of riparian forest buff ers on the in-stream nutrient retention are limited in the case of highly eutrophic streams. Eff ects of Riparian Forest Buff ers on In-Stream Nutrient Retention in Agricultural Catchments