Nitrogen transformations and loss in flooded soils and sediments

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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.