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

Chemical and physical stabilization of soil organic matter by the inte raction of organic compounds with soil clay minerals may affect N mine ralization in tropical forest soils. Nitrogen mineralization rates dur ing long-term incubation were determined on soils of several mineralog ies collected from native forests in Costa Rica, Colombia, Peru, and B razil. Amounts of N mineralization were related to soil mineralogy, te xture, 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. Allophan ic soils produced higher cumulative levels of mineralized N and had hi gher initial levels of several N fractions than soils with smectitic, oxidic, and kaolinitic mineralogies. This mineralogical effect was att ributed to greater initial N stabilization in allophanic soils followe d by greater N release after the addition of nutrient solution during incubation. Total soil N had the strongest correlation with N minerali zation among the N measurements considered in this study. The active N pool, as calculated by a two-pool regression model of cumulative mine ralized N, had the highest positive relationships with NH4+-N evolved In a l-wk anaerobic incubation and soluble N. The stable N pool had th e strongest correlations with total N and light-fraction N. The relati onships between soluble N or NH4+-N evolved after a 1-wk anaerobic inc ubation and N mineralization were affected by soil mineralogy, which s uggests the need for further evaluation of potential N availability in dices for the effects of soil mineralogy.