『Abstract
　Soil-feeding termites are abundant and play important roles in the biogeochemical processes in tropical soils. Previous studies indicated that they preferentially utilize the peptidic components of soil organic matter as a nutrient resource. Here, we determined the corresponding mineralization fluxes and elucidated other N transformation processes that occur during soil gut passage using ¹⁵N tracer techniques. Termite-based rates of N mineralization by Cubitermes umbratus and Cubitermes ugandensis in soil microcosms amended with ¹⁵NH4⁺ were 6.6 and 9.2 nmol N day^-1 (g fresh wt)^-1, which means that the soil peptides fuel about 20 and 40％ of the respiratory activity of these insects. Considering the areal biomass of soil-feeding termites in humid savannahs, soil-feeding termites should mineralize about 3％ of the total N in their food soil per year. In addition to producing ammonia from ingested ¹⁵NO3^- at approximately 10％of the mineralization rate, C. umbratus also formed N2 at similar rates. The formation of labelled N2 in microcosms amended with ¹⁵NH4⁺ seems to be at least partially due to nitrification activity in the soil; evidence for the formation of nitrate in the posterior hindgut remains inconclusive. However, the so far unexplained increase of ¹⁵N abundance in the ammonia pools of the posterior hindgut compartments manifests additional hitherto unknown metabolic processes in this gut region. Collectively, our results not only reinforce the concept of nitrogenous soil components as an important dietary resource for soil-feeding termites, but also allow us to predict that n mineralization and nitrate ammonification activities in the termite gut should positively affect the dynamics of N in tropical soil.

Keywords: Mineralization; Nitrification; Denitrification; Nitrate ammonification; Termites; Soil macrofauna; ¹⁵N tracer』

Introduction
Materials and methods
　Termites
　Soil analysis
　Concentrations of ammonia and nitrate in the intestinal tract
　Microcosm experiments
　Determination of isotope ratios by GC-IRMS
　Calculations of N transformation rates
Results
　Dynamics of inorganic nitrogen species in soil, gut, and nest material
　Nitrogen mineralization rates
　Nitrification and denitrification
　Nitrate ammonification activities
　Localization of labelled ammonia in the gut
Discussion
　Mineralization of soil organic nitrogen
　Localization of N mineralization in the gut
　Localization of nitrification activities
　Fate of nitrate during soil gut passage
　Ecological implications: effects of termites on soil N transformations
Conclusions
Acknowledgments
References