Bioavailability of Nonexchangeable Ammonium in Soils.

The agriculturally important soils of the lower Mississippi alluvial plain typically display surprisingly high levels of nitrogen (N) fertility given the low amounts of organic matter and readily available forms of N characteristically present in these soils. The principal goal of this research was to investigate N dynamics in these soils with special attention devoted to the possible role of clay-fixed NH$\sb4\sp+$ in N fertility. Characterization of the N distribution in 9 soil profiles (0-150 cm) representative of the principal agricultural soils of the lower Mississippi River alluvial plain indicated that most of the N in surface horizons was in the form of organic N (85%), but appreciable amounts ($\sim$15%) were present as nonexchangeable NH$\sb4\sp+$ entrapped within clay lattices. Nonexchangeable NH$\sb4\sp+$ accounted for a substantially larger portion (15-60%) of total N in most subsurface horizons. Regression analysis indicated that the portion of total N recovered as organic N was most closely related to organic C content and the amounts of 2:1 clay minerals present. Experiments to assess the possibility that the activity of soil microorganisms influence the release of clay-fixed NH$\sb4\sp+$ showed that heterotrophic microorganisms play a principal role. Experiments using Commerce silt loam enriched to contain 180 mg $\sp{15}$N nonexchangeable NH$\sb4\sp+$ showed that 48% of this N was immobilized into microbial biomass N after 28 d and an additional 36% had accumulated in forms suitable for plant uptake. In the absence of microbial activity, only 20% of clay-fixed NH$\sb4\sp+$ became available to plants in 28 d. During the course of this investigation it became evident that the methods commonly used to determine clay-fixed NH$\sb4\sp+$ in soils did not recover a substantial portion of recently fixed NH$\sb4\sp+$. Because recently fixed NH$\sb4\sp+$ appeared to play a principal role in the N fertility of soils, a rapid, accurate and precise method was developed to quantitatively measure this fraction as well as native fixed NH$\sb4\sp+$. This method will be of great utility in future efforts to devise optimum fertilization programs for soils that rapidly fix appreciable amounts of fertilizer N into clay lattices