Reply on RC2

Abstract

Tropical montane forest store high amounts of soil organic carbon. However, global warming may affect these carbon stocks by enhancing soil organic carbon respiration. Better insight into temperature response of soil organic carbon respiration can be obtained from in and ex situ warming studies. In situ warming via translocation of intact soil mesocosms was carried out along an elevational transect ranging between ca. 1250 m a.s.l. in the Kibale Forest to ca. 3000 m a.s.l. in the Rwenzori Mountains. Samples from the same transect were also warmed ex situ to determine temperature sensitivity. The ex situ results revealed that along the natural climate gradient in the elevational transect, specific heterotrophic CO2 respiration decreased linearly by 1.01 ± 0.12 μg C h−1 g−1 SOC per 100 m of elevation increase. Similarly, the temperature sensitivity increased from 1.50 ± 0.13 in the lowest elevation clusters to 2.68 ± 0.25 in the highest elevation cluster, showing a linear decrease of 0.09 ± 0.03 per 100 m of elevation increase. Additionally, the 13C depletion factor of the respired CO2 decreased linearly by 0.23 ± 0.04 ‰ per 100 m of elevation increase. The results indicate an increased recalcitrance and decreased mineralisation of soil organic carbon with elevation driven by decreasing soil temperature and pH. Subsequently, after two years of in situ warming (0.9 to 2.8 °C), specific heterotrophic soil organic carbon respiration tends to be lower for warmed as compared to control soil. Further, in warmed soils, 13C and content of soil organic carbon relatively increased and decreased, respectively. This indicates increased mineralisation and depletion of readily available carbon during two years of warming. In conclusion, our results suggest that climate warming may trigger enhanced losses of soil organic carbon from tropical montane forests, due to a combination of a higher temperature sensitivity of mineralisation and soil organic carbon content.