Nitrogen transformations and loss in flooded soils and sediments

Abstract

This paper presents a critical review on nitrogen (N) transformations functioning in flooded soils and sediments, and their role in N loss from the system. Nitrogen transformations reviewed include (1) ammonification (organic ? ammonium N) (2) nitrification (ammonium ? nitrate), (3) ammonia volatilization (ammonium N ? NH₃), (4) denitrification (nitrate ? gaseous products N₂O and N₂). Transport processes reviewed were ammonium and nitrate diffusion. Inorganic N in flooded soils and sediments and waters is converted to gaseous forms by two processes, i.e., (1) nitrification‐denitrification; and (2) NH₃ volatilization. The former process seems to be more predominantly involved in the ammonium N loss from the sediments, whereas the latter process occurs under specialized conditions, mainly in the overlying waters. The processes involved in converting organic N to gaseous end products include ammonification of organic N to ammonium N, upward diffusion of ammonium N into the aerobic soil layer, oxidation of ammonium N in the aerobic soil layer, downward diffusion of nitrate N into the anaerobic soil layer, and reduction of NO₃‐N to the gaseous end products such as N₂ and N₂O. Under certain conditions where nitrification rate is slower than the flux of ammonium N from the .anaerobic soil layer, ammonium N may diffuse into the overlying waters. If the optimum conditions such as high pH and high temperature exist in the water column, ammonium N can be lost through volatilization. Based on the literature data, it can be concluded that ammonium N diffusion and nitrification functioning at slower rates was found to be limiting in controlling N loss from flooded soils and sediments, while nitrate diffusion and denitrification functioning at a rapid rate were found to be nonlimiting in controlling N loss during the nitrification‐denitrification sequence. Ammonium diffusion was dependent on the ammonium N removal mechanisms (nitrification, and NH₃ volatilization) in the aerobic soil layer and floodwater to establish a concentration gradient. If ammonium N removal mechanisms do not occur in the aerobic soil layer or floodwater, there will be very little or no ammonium N loss from the flooded soils or sediments.