Detailed regional predictions of N₂O and NO emissions from a tropical highland rainforest

Abstract. Tropical forest soils are a significant source for the greenhouse gas N₂O as well as for NO, a precursor of tropospheric ozone. However, current estimates are uncertain due to the limited number of field measurements. Furthermore, there is considerable spatial and temporal variability of N₂O and NO emissions due to the variation of environmental conditions such as soil properties, vegetation characteristics and meteorology. In this study we used a process-based model (ForestDNDC-tropica) to estimate N₂O and NO emissions from tropical highland forest (Nyungwe) soils in southwestern Rwanda. To extend the model inputs to regional scale, ForestDNDC-tropica was linked to an exceptionally large legacy soil dataset. There was agreement between N₂O and NO measurements and the model predictions though the ForestDNDC-tropica resulted in considerable lower emissions for few sites. Low similarity was specifically found for acidic soil with high clay content and reduced metals, indicating that chemo-denitrification processes on acidic soils might be under-represented in the current ForestDNDC-tropica model. The results showed that soil bulk density and pH are the most influential factors driving spatial variations in soil N₂O and NO emissions for tropical forest soils. The area investigated (1113 km²) was estimated to emit ca. 439 ± 50 t N₂O-N yr⁻¹ (2.8–5.5 kg N₂O-N ha⁻¹ yr⁻¹) and 244 ± 16 t NO-N yr⁻¹ (0.8–5.1 kg N ha⁻¹ yr⁻¹). Consistent with less detailed studies, we confirm that tropical highland rainforest soils are a major source of atmospheric N₂O and NO.