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 CCh 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 NHs 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. Additional Index Words: nitrogen losses, diurnal pH variations, (NH 4)2SC>4, urea, N use efficiency, carbonic acid equilibrium. Mikkelsen D. S., S. K. De Datta, and W. N. Obcemea. 1978. Ammonia volatilization losses from flooded rice soils. Soil Sci. Soc. Am. J. 42:725-730. TIRE AGRONOMIC significance of the high pH that de-J. velops in ponds or rice flood water has largely been neglected as a factor in the direct volatilization loss of NH 3. Biologists have long been aware that the pH of water in natural systems rise to values as high as pH 10 in a regular diurnal pattern, which is determined by the chemical equilibria that exist between all aquatic organisms, their CO 2 metabolism, and the various solutes, solids, and gases in water systems. Park et al. (6) reviewed the diurnal pH variations in a variety of natural waters showing changes of as much as 3.5 pH units, rising in midday when the-photosynthetic process is actively withdrawing CO 2 from the ecosystem and falling at night when respiratory activities liberate free CO 2 into the water. The correlation between water pH and the carbonic acid system in natural waters is complex (8), but in its simplified form can be characterized by the proton condition [H+] = [HCO 3-] + 2 [CO 3 2-] + [OH-] and equilibrium concentrations-log [H + ] =-log [HC0 3-] = 5.65;-log [CO 2 aq.] =-log [H 2 CO 3 ] = 5.0 (as a non volatile acid);-log [H 2 C0 3 ] = 7.8;-log [CO 3 2-] = 8.5. 'Contribution from the Dept, of Agron., The Int. Rice Res. Inst., Los Banos, Laguna (mail address: IRRI, The relationship between pH and the mole fraction of dissolved carbonic acid, HCO 3 ~ and CO 3 2 ~, from data of Saruhashi (7) is shown in Fig. 1. The pH values different from those in a pure CO 2 system relate to either alkalinity or the mineral acidity inherent in the water, biochemical variances among the aquatic biota, and such factors as temperature, pressure, and air movement. Of particular interest in the shallow-water systems found in flooded rice culture is the CO 2 balance existing among the various submersed aquatic plant species, particularly algal forms, which develop quickly into a large biomass. The generalized reaction involving the plant biochemistry of CO 2 (i.e.) Photosynthesis n CO 2 + n H 2 O v. ". .=^ (CH 2 O) n + n O 2 2 Respiration " is very significant. The photosynthetic process decreases the net concentration of [CO 2. (aqueous)] + [H 2 CO 3 ] during favorable daylight periods but when respiratory activity exceeds photosynthesis both H 2 CO 3 , acidity, and the total concentration of dissolved carbonic acid increase. A number of complex relationships exist in the system. Reactions of CO 2 with alkaline earth minerals, carbonates, divalent cations, and with various components of the external environmental system affect the solubility of gases, their dissociation, and the relative activity of the entire system. Water pH in flooded rice is also affected to some extent by the soil type, its pH, electrical conductivity, previous cropping history, and soil management practices such as puddling. The quality of the irrigation water, its origin either from rainfall, wells, runoff , or streams and its silt load, may also influence the initial pH of paddy water. Ammonia and its ionized form (NH 4 +) are readily identifiable products of the decomposition of soil organic matter, and the micro and macro plant residues occurring in natural waters. Use of N fertilizers on rice also contributes large concentrations of dissolved NH 4 +-N salts. Am-monium-form fertilizers may dissociate directly, or like urea may decompose by catalytic hydrolysis to produce NH 4 + ions in water. Ammonium ions, loosely bound to water molecules, predominate in water at a pH above 7.2. With increasing hydroxyl-ion concentrations in the water, ionized NH 4 + increasingly converts to nonionized NH 3 , which may escape from the water as a gas. Losses from flooded soil systems are affected by a number of variables in a manner similar to direct NH 3 volatilization losses from upland soils. Particularly critical is a high level of solar radiation, the nature and numbers of aquatic plants, indirectly the CO 2 balance between pho-tosynthesis and respiration, and other factors such as the influence of temperature on CO 2 solubility in the water and carbonic acid equilibria. Losses of volatile NH 3 from flooded rice soils reported by MacRae and Ancajas (4) are calculated as 1% of the ammonium sulfate applied and as much as 19% of the urea 725