Ditch Sediment Contribution to Remove Geogenic Nitrogen in a Reclaimed Area (Po River Delta, Italy)

Networks of drainage ditches play a crucial role in processing N loads along the land–river–sea system. This study investigated the N removal capacity of ditch sediments in a reclaimed area with paleo-marsh peat lenses releasing geogenic N to surface waters (Po River Delta, Italy). Sediment–water fluxes of O2, inorganic N and N2O, as well as denitrification and DNRA (dissimilatory nitrate reduction to ammonium) rates, were measured in spring and summer via incubations of intact sediment cores sampled at two sites having different pedological characteristics (silty clay loam and organic silty clay). The NO3− removal rate (247–534 µmol N m−2 h−1) was primarily due to denitrification in both seasons, with a low amount recycled into NH4+ by DNRA (6–11%) and negligible conversion to N2O (< 7‰). Under dark conditions, denitrification rates were higher in summer than in spring, driven by higher temperatures that stimulated microbial activity and oxygen consumption. Under light conditions, particularly in spring, coupled nitrification–denitrification was enhanced by microphytobenthos whose photosynthetic activity turned the sediment into a net oxygen source. Enrichment experiments suggested that denitrification was primarily controlled by NO3− availability whose production from nitrification can be limited in organic-rich sediments by oxygen shortage. Although the agricultural ditch system plays a significant role in dissipating N loads, removing NH4+ produced from the peat layers requires larger treatment areas or longer retention times, particularly in summer when the risk of eutrophication in coastal zones is greatest. Future management and restoration strategies for lowland watersheds must take into account these internal geogenic sources, since they can circumvent standard nutrient reduction measures.