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

Oxygen transport through the air spaces (aerenchyma tissue) of the stem and roots of aquatic macrophytes into the root zone supports nitrification of NH 4 + , with the NO 3 − 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 applied 15 NH 4 + ‐N to 15 N 2 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 15 N 2 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 15 N 2 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.