Dynamics of Nitrification in Soils Using a Miscible Displacement Technique

An apparatus developed to study miscible displacement processes in porous materials was used to study the rate of nitrification in soils. The water content of Guelph loam was maintained at the one-third bar value and the flow rate was controlled by means of a constant head technique or a constant volume pump. Different concentrations of (NH4)2SO4 solutions were leached at constant flow rates through unsaturated soil columns and effluent samples were analyzed for NH4-, NOrand NO3~-N (periodic checks were made for pH). On one core, Ca, Mg, K, and Na were also determined in the effluent. Adsorption of NH< during the initial lag phase was followed by a period of rapid oxidation of NH4to NOrand NOr-N. The addition of 5 Mg/g soil of N-Serve was found to inhibit nitrification for a period of several weeks. N STUDIES on soils under laboratory conditions have usually used either the incubation or the perfusion technique. The incubation method has a number of shortcomings (7) and laboratory results must be correlated with field data. The perfusion technique permits a direct and continuous investigation of nitrification without disturbing the soil column; however, the change of physical and chemical properties of the perfusate which is recycled through the soil sample as well as the soil itself may affect the nitrification process. Both methods study nitrification in a closed system; the soil is actually an open system (8). The method used here is basically a direct leaching technique and is similar to the continuous flow procedure adopted by Macura and Malek (9). However, with the present method, accurate control of the water content of the soil sample and the flow rate is obtained using modern soil physics' techniques. In the miscible displacement technique or the continuous flow method the solution is continuously leached through the soil column but not recycled and products of nitrification are continuously removed from the active sites. This is somewhat analogous to the removal of nutrients by plant roots or natural leaching. This method permits nitrification to be studied in an open system thus giving a better approximation of natural conditions. The high concentrations and large amounts of N used in this study are not particularly realistic in view of field considerations; experiments with lower concentrations or 1 Contribution from Dep. of Soil Science, Univ. of Guelph, Guelph, Ontario, Canada. The authors would like to acknowledge support from the Ontario Dep. of Agr. and Food. Presented before Div. S-3, Soil Sci. Soc. of Amer. , Aug. 24, 1966, at Stillwater, Okla. Received Oct. 27, 1966. Approved Feb. 20, 1967. 2 Graduate Student, Professor and Associate Professor of Soil Science, and Associate Professor of Microbiology, respectively. The senior author is now graduate student, Dep. of Water Science and Engineering, Univ. of California, Davis. small slug applications of high concentrations would be more representative of field conditions. MATERIALS AND METHODS Soil—Guelph loam was air-dried, passed through a 2-mm sieve and stored until used. The general properties of this soil are listed in Table 1. Apparatus and experimental treatments—The basic apparatus and technique was described by Elrick et al. (5). The soil column was approximately 4 cm in length and 10 cm in diameter and contained about 400 g of soil, depending upon the bulk density desired. The columns were packed uniformly, placed in the pressure chamber, and saturated under vacuum with distilled water or (NH4)2SO4 solution. Core no. 4 was thoroughly mixed with 5 ng N-Serve/g soil before packing. After saturation, the soil column was immediately placed under J^ bar pressure to remove the excess water and was allowed to come to equilibrium. Then the experiment was initiated by leaching (NH4)zSO4 solution through the unsaturated column using the technique described by Elrick et al. (5). The effluent was collected automatically by a fraction collector. The experimental conditions as well as the different treatments of the soil core are summarized in Table 2. All experiments were conducted at 25C. Method of analysis—The effluent was analyzed for NH4+-N, NOa~-N and N02~-N using a Technicon Auto Analyzer (5). Calcium,magnesium, potassium, and sodium were determined by using an Atomic Absorption Spectrophotometer (4). The pH of the effluent was also determined. RESULTS AND DISCUSSION Initial and final soil effluent pH—The initial and final soil and effluent pH are presented in Table 3. The decrease in soil pH, measured after the experiment terminated, was closely related to the concentration of the last (NH4)2SC>4 solution leached. This was expected because of the complete conversion of NH4-N to NOs~-N. The ions Ca~*", Mg, K, and Na were leached from the soil core resulting in a relatively high effluent pH. Hydrogen ions, released during nitrification, were exchanged with the native basic cations, thus lowering the soil pH. The observed increase in soil pH of the soil core pretreated with N-Serve cannot be explained. Distribution of NH^-N, NO^-N and NOf-N in the effluent—The data obtained are presented in Fig. 1 to 6. The concentration of the different forms of N in parts per million is plotted against time and pore volumes (one pore volume is the total volume of water or solution contained by the soil core). The horizontal dashed line indicates the influent NH4+-N concentration. As listed in Table 2, the experiments can be divided into three groups. Enriched soil core—Core no. 1 was first saturated with 100 ppm N of (NH4)2S04 solution and then incubated under 3 2-chloro-6(trichloromethyl) pyridine, supplied by Dow Chemi cal of Canada Ltd., Sarnia, Ont. 585 (383) Published May, 1967