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 NH+4 trapped within the interlayers of these clay minerals. Moreover, NH+4 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 NH+4 and its eventual accumulation in forms suitable for plant uptake in two alluvial soils of the lower Mississippi floodplain. Experiments involving 15N-labeled NH+4, initially present solely as nonexchangeable 15NH+4, 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 15NH+4-N kg−1 was incubated for 28 days, 48% of fixed 15N was recovered as microbial biomass N (12%) and as exchangeable NH+4-N (36%). The 15NH+4 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 15NH+4. In contrast, the addition of 4000 mg of mannitol-C kg−1 to stimulate biological activity enhanced the release of fixed 15N and stimulated its subsequent nitrification. Experiments using two C-amended soils containing 90 or 180 mg of fixed 15NH+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 NH+4 was 23 to 46% as available as NH+4 added directly to soil as a neutral salt and considerably more available than native fixed NH+4. The conclusion that heterotrophic microorganisms play a principal role in the release of recently fixed NH+4 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 NH+4, primarily near the surface of roots that supply C to these organisms.