Nitrogen mineralization, denitrification, and nitrate ammonification by soil-feeding termites: a 15N-based approach

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 N-15 tracer techniques. Termite-based rates of N mineralization by Cubitermes umbratus and Cubitermes ugandensis in soil microcosms amended with (NH4)-N-15 (+) 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 (NO3)-N-15 (-) at approximately 10% of the mineralization rate, C. umbratus also formed N-2 at similar rates. The formation of labelled N-2 in microcosms amended with (NH4)-N-15 (+) 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 N-15 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.