Nitrification‐denitrification at the plant root‐sediment interface in wetlands

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Abstract
Oxygen transport through the air spaces (aerenchyma tissue) of the stem and roots of aquatic macrophytes into the root zone supports nitrification of NH4+, with the NO3− formed diffusing into the adjacent anaerobic zone where it undergoes denitrification. To test this hypothesis, we conducted a growth chamber study to determine the transformation of applied15NH4+‐N to 15N2 in the root zone of three aquatic macrophytes: rice (Oryza sativa L.), pickerel weed (Pontederia cordata L.), and soft rush (Juncus effusus L.). Detection of gaseous 15N2 in the air above the floodwater of the soil column with aquatic plants provided direct evidence of nitrification‐denitrification in the root zone, while such losses were not measurable for soil columns without plants. Air spaces in aquatic plants can also function as conduits for denitrified gases from anaerobic sediments to the atmosphere. Maximal 15N2 flux due to this process was 102, 113, and 122 mg N m−2 d−1 for soft rush, rice, and pickerel weed. This N loss mechanism has important agronomic and ecological consequences.