| Popis: |
Soils containing appreciable amounts of 2:1 clay minerals frequently contain a substantial portion of their total N as nonexchangeable NH; trapped within the interlayers of these clay minerals. Moreover, NH; 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; and its eventual accumulation in forms suitable for plant uptake in two alluvial soils of the lower Mississippi floodplain. Experiments involving 15 N-labeled NH 4 + , initially present solely as nonexchangeable 15 NH 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 15 N was recovered as microbial biomass N (12 %) and as exchangeable NH 4 + -N (36 %). The 15 NH 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 15 NH 4 + . In contrast, the addition of 4000 mg of mannitol-C kg −1 to stimulate biological activity enhanced the release of fixed 15 N and stimulated its subsequent nitrification. Experiments using two C-amended soils containing 90 or 180 mg of fixed 15 NH 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; 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; 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 |
