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

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 15 N tracer tech- niques. Termite-based rates of N mineralization by Cubitermes umbratus and Cubitermes ugandensis in soil microcosms amended with 15 NH4 ? 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 15 NO3 - at approx- imately 10% of the mineralization rate, C. umbratus also formed N2 at similar rates. The formation of labelled N2 in microcosms amended with 15 NH4 ? 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 15 N abundance in the ammonia pools of the posterior hindgut compartments manifests additional hitherto unknown metabolic pro- cesses in this gut region. Collectively, our results not only reinforce the concept of nitrogenous soil compo- nents as an important dietary resource for soil-feeding termites, but also allow us to predict that N mineral- ization and nitrate ammonification activities in the termite gut should positively affect the dynamics of N in tropical soil.