Spatial variations of nitrogen trace gas emissions from tropical mountain forests in Nyungwe, Rwanda

Abstract

Globally, tropical forest soils represent the second largest source of N₂O and NO. However, there is still considerable uncertainty on the spatial variability and soil properties controlling N trace gas emission. Therefore, we carried out an incubation experiment with soils from 31 locations in the Nyungwe tropical mountain forest in southwestern Rwanda. All soils were incubated at three different moisture levels (50, 70 and 90 % water filled pore space (WFPS)) at 17 °C. Nitrous oxide emission varied between 4.5 and 400 μg N m⁻² h⁻¹, while NO emission varied from 6.6 to 265 μg N m⁻² h⁻¹. Mean N₂O emission at different moisture levels was 46.5 ± 11.1 (50 %WFPS), 71.7 ± 11.5 (70 %WFPS) and 98.8 ± 16.4 (90 %WFPS) μg N m⁻² h⁻¹, while mean NO emission was 69.3 ± 9.3 (50 %WFPS), 47.1 ± 5.8 (70 %WFPS) and 36.1 ± 4.2 (90 %WFPS) μg N m⁻² h⁻¹. The latter suggests that climate (i.e. dry vs. wet season) controls N₂O and NO emissions. Positive correlations with soil carbon and nitrogen indicate a biological control over N₂O and NO production. But interestingly N₂O and NO emissions also showed a positive correlation with free iron and a negative correlation with soil pH (only N₂O). The latter suggest that chemo-denitrification might, at least for N₂O, be an important production pathway. In conclusion improved understanding and process based modeling of N trace gas emission from tropical forests will benefit from spatially explicit trace gas emission estimates linked to basic soil property data and differentiating between biological and chemical pathways for N trace gas formation.