Effects of sediment texture on in-stream nitrogen uptake

A comparative experiment was conducted in two cross sections with sandy and sandy loam sediment textures along an agricultural drainage stream in eastern China to address the effects of sediment texture on in-stream nitrogen uptake efficiency. Using dimerous chambers for in situ incubations, NO₃-N and NH₄-N uptake metrics (i.e., areal uptake rate and uptake velocity) and associated hydrochemical variables in the enclosed sediment–water column system were measured for 8 days and two nights across April–July in 2011 and March–June in 2012. For the investigated sites, in-stream uptake accounted for 2–45 and 9–36 % of the initial NH₄-N and NO₃-N within the enclosed water column, respectively. Although similar daytime, diel and day-to-day (daytime) variation patterns of NO₃-N or NH₄-N uptake metrics were observed for the two sites, the sandy loam sediments had average net NO₃-N and NH₄-N uptake efficiency ~50 % higher and ~40 % lower than for the sandy sediments, respectively. As NO₃-N was the dominant nitrogen form in the studied water columns (typical of agricultural drainage rivers), the sandy loam sediment site had an average of about 47 % higher net uptake efficiency for dissolved inorganic nitrogen (i.e., NO₃-N + NH₄-N). This study demonstrates that sediment texture has a considerable effect on spatial variation of nitrogen uptake along the river system. Changing sediment texture due to anthropogenic modifications on catchment land use and stream channels has the potential to change stream nitrogen cycling as well as altering nitrogen inputs and forms to downstream aquatic ecosystems.