Effect of residence time on the kinetics of nonexchangeable ammonium release from illite and vermiculite
Diedrich Steffens and Donald L. Sparks
The fraction of nonexchangeable ammonium (NH4 +) can play an important role in N cycling of soils as a sink (fixation) or a source (release) of NH4 +. Recently fixed nonexchangeable NH4 + especially seems to be a significant source for N release. The aim of our study was to determine the effect of residence time on the kinetics of nonexchangeable NH4 + release from illite and vermiculite. Calcium-saturated illite and vermiculite, containing NH4 + that was “fixed” for one and 60 d, were extracted with a H-resin for 0.25 to 384 h. Both clay minerals “fixed” significantly more NH4 + in 60 d than in 1 d, but vermiculite “fixed” more NH4 + than illite. The kinetics of nonexchangeable NH4 + release from illite and vermiculite were well described by the Elovich equation and by a heterogeneous diffusion model. In vermiculite the percentage of nonexchangeable NH4 + release decreased from 84% to 78% when the time of fixation increased from 1 to 60 d. In illite time of residence has not influenced the complete release of newly fixed NH4 +.
Key words: ammonium / fixation / clay minerals / nitrogen / kinetics
It is well established that nonexchangeable ammonium (NH4 + ) can play an important role in the cycling or dynamics of soil N and in the efficiency of applied fertilizer N (Scherer, 1993; Nommik and Vahtras, 1982). Measurements of the N fluxes between different N pools in an Alfisol derived from loess have shown that considerable amounts of N mineralized from the biomass were fixed in the clay minerals and were released under field conditions. Without mineral N application 200 kg nonexchangeable NH4 + -N ha-1 were released to winter wheat (Triticum aestivum L.) from the topsoil layer (0-0.3 m depth), whereas a mineral N application in form of a mixture of NH4 + -NO3-urea decreased the release of nonexchangeable NH4 + from 200 to 120 kg N ha-1 (Nieder et al., 1995). Soon (1998) reported that 150 and 19 kg nonexchangeable NH4 + - N ha-1 were released during crop growth in 1994 and 1995, respectively. These recently published results and other studies indicate the importance of nonexchangeable NH4 + as a slow release N source (Kowalenko, 1978; Kowalenko and Ross, 1980; Mengel and Scherer, 1981; Drury and Beauchamp, 1991). The release of nonexchangeable NH4 + is not the result of a surface chemical process from primary or secondary NH4 + bearing clay minerals such as micas, vermiculite or smectites, but a diffusion-controlled release mechanism (Steffens and Sparks, 1997) which is similar to the release of nonexchangeable K+ from soils and minerals (Martin and Sparks, 1982; Sparks and Huang, 1985). A net release of nonexchangeable NH4 + requires low concentrations of NH4 + and K+ in the soil solution (Smith et al., 1994). It occurs more in cultivated than in fallow soils (Scherer, 1993) since crops take up large amounts of K+ and NH4 + , which decreases the K+ and NH4 + concentrations in the rhizosphere and enhances the release of nonexchangeable NH4 + (Mengel et al., 1990). However, plant species have a different potential for mobilizing nonexchangeable NH4 + . Rape (Brassica napus L.) depleted the concentration of nonexchangeable NH4 + in the root vicinity more than ryegrass (Lolium perenne L.) and red clover (Trifolium pratense L.), since rape roots released more protons than the roots of ryegrass and red clover which promoted NH4 + release (Scherer and Ahrens, 1996).
A major problem in defining and in estimating the bioavailability of nonexchangeable NH4 + is that nonexchangeable NH4 + is often comprised of two types with different binding energies. One type is defined as the indigenous or native nonexchangeable NH4 + and the other is defined as recently or newly fixed nonexchangeable NH4 + (Schachtschabel, 1961). The native nonexchangeable NH4 + is less readily released than the recently fixed nonexchangeable NH4 + (Allison et al., 1951; Black and Waring, 1972; Kowalenko and Cameron, 1978; Kudeyarov, 1981; Niederbudde, 1983; Scheffer and Schachtschabel, 1984). For example, Kowalenko and Ross (1980) reported that the release of nonexchangeable NH4 + was relatively slow after an initial rapid release. The initial release was ascribed to recently fixed NH4 + and the slow release was attributed to the release of indigenous fixed NH4 + . Steffens and Sparks (1997) studied the kinetics of nonexchangeable NH4 + release from different soils using a H-resin extraction method. It was shown that the release of nonexchangeable NH4 + from the soils ranged from 4 to 25% of the total nonexchangeable NH4 + with the subsoils releasing less nonexchangeable NH4 + than the topsoils. This was ascribed to a higher amount of indigenous nonexchangeable NH4 + in the subsoils which was less easily released. It appears that the location of NH4 + in the clay interlayer spaces and the time of contact between clay minerals and NH4 + (residence time) may influence the quantity and the kinetics of nonexchangeable NH4 + release. Accordingly, the objectives of our investigations were to determine the effect of time on NH4 + fixation in illite, vermiculite and montmorillonite and to investigate the effect of residence time on the kinetics of nonexchangeable NH4 + release from illite and vermiculite.