Ammonia Volatilization Losses from Flooded Rice Soils

Abstract The pH of the flood water in rice fields is largely determined by the chemical equilibria that exist between the CO 2 balance achieved by the aquatic biota and the various solutes, solids, and gases in the water. Water pH values undergo diurnal changes, increasing by midday to values as high as pH 9.5–10 and decreasing as much as 2–3 pH units during the night. The pH of shallow flood water is greatly affected by the total respiration activity of all the heterotrophic organisms and the gross photosynthesis of the species present. Ammonium form fertilizers broadcast into a high pH water are highly susceptible to direct NH 3 volatilization losses. Nitrogen losses from fertilizer broadcast into flood water on a fertile, neutral‐pH Maahas clay were as high as 20% of the amount applied, but losses varied depending upon water pH, the nitrogen source, and rate, time, and method of application. Losses from an acid Luisiana clay, where the flood water was not conducive to algal growth and did not exceed pH 6.8, produced NH 3 volatilization losses consistently less than 1% of the total N applied. Placement of N fertilizer in the soil at depths of 10–12 cm reduced NH 3 volatilization losses to less than 1% of the total N applied.