In-depth analysis of NO fluxes in tropical forest soils of the Congo Basin combining isotope and functional gene analysis.

Primary tropical forests generally exhibit large gaseous nitrogen (N) losses, occurring as nitric oxide (NO), nitrous oxide (NO) or elemental nitrogen (N). The release of NO is of particular concern due to its high global warming potential and destruction of stratospheric ozone. Tropical forest soils are predicted to be among the largest natural sources of NO; however, despite being the world's second-largest rainforest, measurements of gaseous N-losses from forest soils of the Congo Basin are scarce. In addition, long-term studies investigating NO fluxes from different forest ecosystem types (lowland and montane forests) are scarce. In this study we show that fluxes measured in the Congo Basin were lower than fluxes measured in the Neotropics, and in the tropical forests of Australia and South East Asia. In addition, we show that despite different climatic conditions, average annual NO fluxes in the Congo Basin's lowland forests (0.97 ± 0.53 kg N ha year) were comparable to those in its montane forest (0.88 ± 0.97 kg N ha year). Measurements of soil pore air NO isotope data at multiple depths suggests that a microbial reduction of NO to N within the soil may account for the observed low surface NO fluxes and low soil pore NO concentrations. The potential for microbial reduction is corroborated by a significant abundance and expression of the gene nosZ in soil samples from both study sites. Although isotopic and functional gene analyses indicate an enzymatic potential for complete denitrification, combined gaseous N-losses (NO, N) are unlikely to account for the missing N-sink in these forests. Other N-losses such as NO, N via Feammox or hydrological particulate organic nitrogen export could play an important role in soils of the Congo Basin and should be the focus of future research.