Disentangling In‐Stream Nitrate Uptake Pathways Based on Two‐Station High‐Frequency Monitoring in High‐Order Streams

Abstract

AbstractIn‐stream nitrate (NO3−) uptake in rivers involves complex autotrophic and heterotrophic pathways, which often vary spatiotemporally due to biotic and abiotic drivers. High‐frequency monitoring of NO3− mass balance between upstream and downstream measurement sites can quantitatively disentangle multi‐path NO3− uptake dynamics at the reach scale. However, this approach remains limited to a few river types and has not been fully explored for higher‐order streams with varying hydro‐morphological and biogeochemical conditions. We conducted two‐station 15‐min monitoring in five high‐order stream reaches in central Germany, calculating the NO3−‐N mass balance and whole‐stream metabolism based on time series of NO3−‐N and dissolved oxygen, respectively. With thorough considerations of lateral inputs, the calculated net NO3−‐N uptake rates () differed substantially among campaigns (ranging from −151.1 to 357.6 mg N m2 d−1, with cases of negative values representing net NO3−‐N release), and exhibited higher during the post‐wet season than during the dry season. Subtracting autotrophic assimilation (, stoichiometrically coupled to stream metabolism) from , represented the net balance of heterotrophic NO3−‐N uptake ( > 0, the dominance of denitrification and heterotrophic assimilation) and NO3−‐N release ( < 0, the dominance of nitrification/mineralization). This rarely reported uptake pathway contributed substantially to patterns, especially during post‐wet seasons; moreover, it appeared to exhibit various diel patterns, and for > 0, diel minima occurred during the daytime. These findings advance our understanding of complex reach‐scale N‐retention processes and can help develop future modeling concepts at the river‐network scale.