AVAILABILITY OF 15N-LABELED NONEXCHANGEABLE AMMONIUM TO SOIL-MICROORGANISMS

Soils containing appreciable amounts of 2:1 clay minerals frequently contain a substantial portion of their total N as nonexchangeable NH4+ trapped within the interlayers of these clay minerals. Moreover, NH4+ applied as fertilizer to these soils may be rapidly fixed. A laboratory study was conducted to assess the role of soil microorganisms in the release of clay-fixed NH4+ and its eventual accumulation in forms suitable for plant uptake in two alluvial soils of the lower Mississippi floodplain. Experiments involving N-15-labeled NH4+, initially present solely as nonexchangeable (NH4+)-N-15, showed that indigenous heterotrophic microorganisms readily assimilated this N when the availability of a C substrate created demand. When Commerce silt loam initially containing 180 mg of clay-fixed (NH4+)-N-15-N kg(-1) was incubated for 28 days, 48% of fixed N-15 was recovered as microbial biomass N (12%) and as exchangeable NH4+-N (36%). The (NH4+)-N-15 released was not rapidly refixed by clays, even though only small amounts were nitrified. Fumigation to inhibit soil biological activity resulted in a substantial reduction in the release of fixed (NH4+)-N-15. In contrast, the addition of 4000 mg of mannitol-C kg(-1) to stimulate biological activity enhanced the release of fixed N-15 and stimulated its subsequent nitrification. Experiments using two C-amended soils containing 90 or 180 mg of fixed N-15(4+)-N kg(-1) showed that 64 to 96% of this N was released in 28 days. Measures of microbial respiration indicated that recently fixed NH4+ was 23 to 46% as available as NH4+ added directly to soil as a neutral salt and considerably more available than native fixed NH4+. The conclusion that heterotrophic microorganisms play a principal role in the release of recently fixed NH4+ was supported by experiments showing that addition of N in this form stimulated microbial activity and growth. These findings suggest that the activity of heterotrophic microorganisms may conserve soil N by promoting the release of recently fixed NR(4)(+), primarily near the surface of roots that supply C to these organisms.