Kinetics of Nonexchangeable Ammonium Release from Soils

The rate of nonexchangeable NH/ release from soils can have a significant effect on N dynamics and environmental quality. Nonexchangeable NH/ is that N fraction which is fixed in the interlayers of clay minerals. The objective of this study was to determine the kinetics of nonexchangeable NH4 release from two topsoils from Germany [Giessen Ap (Aqualf) and Hungen Ap (Alfisol)] and from two subsoils, one from Germany [Hungen C (Alfisol)] and one from Kentucky [Shrouts Bt (Hapludalf)]. Calcium-saturated, freeze-dried soils were extracted with H-resin for 0.25 to 384 h. Sodiumethylmercurythio-salisylate (Thimerosal) was added to the soil/H-resin suspension to inhibit microbiological ammonification. The kinetics of nonexchangeable NH4* release from the soils were biphasic and were best described by Elovich and heterogeneous diffusion models. The release of nonexchangeable NH4 as a percentage of total nonexchangeable NH4 in the soils ranged from 4 to 25% and was lower in subsoils than in topsoils. This was ascribed to the higher levels of indigenous nonexchangeable NH4 in the subsoils, which is more tightly held than recently fixed nonexchangeable NH4. F OR NONEXCHANGEABLE AMMONIUM (NH|) Can be an important reservoir of N in soils. Schachtschabel (1961) defined nonexchangeable NH/ as that fraction that is fixed in the inorganic fraction of the soil components, such as mica, vermiculite, and illite clay minerals, and that is not exchangeable with neutral salt solutions. Nonexchangeable NH.J" can also be defined as that N fraction that is analyzed after potassium hypobromite oxidation, leaching with 0.5 M KC1, and digestion with a mixture of 5 M HF/1 M HC1 (Silva and Bremner, 1966). Indigenous or native nonexchangeable NH/ is that NH| that was fixed during the genesis of silicates. The recently or newly fixed nonexchangeable NH/ is defined as that NH^ that was fixed after the native nonexchangeable NH| and resulted from mineralization of soil organic matter and organic and mineral fertilizers (Schachtschabel, 1961). Indigenous nonexchangeable NH^ is much less easily released than recently fixed nonexchangeable NH|, since the former may be trapped more in the center of the interlayers, while recently fixed NH/ may be fixed more in the peripheral zone of the interlayers (Scheffer and Schachtschabel, 1984). The amount of nonexchangeable NH^f in soils is greatly affected by the clay content and by the types of clay minerals that are present. Vermiculite, mica, and illite, and to some extent smectites (montmorillonite, beidellite), are the clay minerals that have the greatest capacity to fix NH/. Said (1973) reported that soils from Sudan, in which montmorillonite was the most abundant clay mineral, contained only small amounts of nonexchangeD. Steffens, Inst, for Plant Nutrition, Justus-Liebig-University, Suedanlage 6 G-35390 Giessen, Germany; and D.L. Sparks, College of Agricultural Sciences, Univ. of Delaware, Dep. of Plant and Soil Sciences, 147 Townsend Hall, Newark, DE 19717-1303. Received 25 Sept. 1995. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 61:455-462 (1997). able NH/ ranging between 30 and 60 mg N kg ' soil. Scherer and Weimar (1994) applied slurry to different soils and they showed that soils that contained 14% montmorillonite and 13.6% vermiculite in the clay fraction fixed more NH| than soils that had 30% montmorillonite and 14% vermiculite in the clay fraction. Beidellite, in contrast to montmorillonite, is a high NH/ and K fixing smectite, due to isomorphic substitution in the tetrahedral layer (Niederbudde et al., 1983; Feigenbaum et al., 1994). Shales and granites can contain indigenous nonexchangeable NH^. Stevenson (1959) found that nonexchangeable NHl in shales varied from 330 to 420 mg N kg" and ranged from 5 to 27 mg N kg" in granites. Schachtschabel (1961) reported that marsh soils in Northern Germany contained between 150 and 850 mg nonexchangeable NrU-N kg" soil. Sparks et al. (1979) found that the Shrouts soils (fine, mixed, mesic Typic Hapludalf) in Kentucky contained in average of 365, 463,433, 469, and 543 mg N kg" nonexchangeable NH| in the Ap, B21t, B22t, C, and Cr horizons, respectively. The high levels of indigenous nonexchangeable NH^ in these soils were ascribed to the shale parent materials. Baethgen and Alley (1986) found that nonexchangeable NH4 ranged from 600 to 3000 kg N ha" in the A and B horizons of Virginia soils. In general, nonexchangeable NH/, as a percentage of total soil N, increases with increasing soil depth as clay content increases (Sparks et al., 1979; Nette and Resch, 1992). There are dynamic, equilibrium reactions between nonexchangeable, exchangeable, and solution NH|. Fixation and release depends on the level of NH^ in the soil solution, the type of clay minerals present in the soil, and wetting and drying (Nommik and Vathras, 1982). As levels of solution NH^ increase, fixation and adsorption of NH/ on exchange sites of clay minerals and organic matter also can increase. Likewise, with higher levels of nonexchangeable NH|, release could occur and exchangeable NH/ would increase in the soil solution. Fixation is usually faster than release of NH| (Drury and Beauchamp, 1991). Drury et al. (1989) found that 18 to 23% of added NH4 was fixed after a 15-d incubation period in soils with high vermiculitic contents. Kowalenko (1978) found that 66% of the NH4 that was fixed in 1.7 d was released in 86 d. Drury and Beauchamp (1991) showed that «10% of NH4 fixed in 30 d was released in 30 d. Nonexchangeable NH| can be an important source of slow release N for plants and microbes and perhaps could even impact groundwater quality. Investigations by Mengel and Scherer (1981) on western German loess soils and by Li et al. (1990) on Chinese loess soils have shown that considerable amounts of nonexchangeable NH| were released during the vegetative growth period of crops, while NH4 fixation occurred after crop harvest. Nonexchangeable NH| was also shown to be important