NITROGEN MINERALIZATION IN HUMID TROPICAL FOREST SOILS - MINERALOGY, TEXTURE, AND MEASURED NITROGEN FRACTIONS

Chemical and physical stabilization of soil organic matter by the interaction of organic compounds with soil clay minerals may affect N mineralization in tropical forest soils. Nitrogen mineralization rates during long-term incubation were determined on soils of several mineralogies collected from native forests in Costa Rica, Colombia, Peru, and Brazil. Amounts of N mineralization were related to soil mineralogy, texture, and N fractions in order to determine the effects of these soil components on N mineralization and to evaluate measurements of these components for their use as potential N availability indices. Allophanic soils produced higher cumulative levels of mineralized N and had higher initial levels of several N fractions than soils with smectitic, oxidic, and kaolinitic mineralogies. This mineralogical effect was attributed to greater initial N stabilization in allophanic soils followed by greater N release after the addition of nutrient solution during incubation. Total soil N had the strongest correlation with N mineralization among the N measurements considered in this study. The active N pool, as calculated by a two-pool regression model of cumulative mineralized N, had the highest positive relationships with NH4+-N evolved In a l-wk anaerobic incubation and soluble N. The stable N pool had the strongest correlations with total N and light-fraction N. The relationships between soluble N or NH4+-N evolved after a 1-wk anaerobic incubation and N mineralization were affected by soil mineralogy, which suggests the need for further evaluation of potential N availability indices for the effects of soil mineralogy.