Soil CO₂ efflux from two mountain forests in the eastern Himalayas, Bhutan: components and controls

Abstract. The biogeochemistry of mountain forests in the Hindu Kush Himalaya range is poorly studied, although climate change is expected to disproportionally affect the region. We measured the soil CO₂ efflux (R_s) at a high-elevation (3260 m) mixed forest and a lower-elevation (2460 m) broadleaf forest in Bhutan, eastern Himalayas, during 2015. Trenching was applied to estimate the contribution of autotrophic (R_a) and heterotrophic (R_h) soil respiration. The temperature (Q₁₀) and the moisture sensitivities of R_h were determined under controlled laboratory conditions and were used to model R_h in the field. The higher-elevation mixed forest had a higher standing tree stock, reflected in higher soil C stocks and basal soil respiration. Annual R_s was similar between the two forest sites (14.5 ± 1.2 t C ha⁻¹ for broadleaf; 12.8 ± 1.0 t C ha⁻¹ for mixed). Modelled annual contribution of R_h was  ∼  65 % of R_s at both sites with a higher heterotrophic contribution during winter and lower contribution during the monsoon season. R_h, estimated from trenching, was in the range of modelled R_h but showed higher temporal variability. The measured temperature sensitivity of R_h was similar at the mixed and broadleaf forest sites (Q₁₀ 2.2–2.3) under intermediate soil moisture but decreased (Q₁₀ 1.5 at both sites) in dry soil. R_s closely followed the annual course of field soil temperature at both sites. Covariation between soil temperature and moisture (cold dry winters and warm wet summers) was likely the main cause for this close relationship. Under the prevailing weather conditions, a simple temperature-driven model was able to explain more than 90 % of the temporal variation in R_s. A longer time series and/or experimental climate manipulations are required to understand the effects of eventually occurring climate extremes such as monsoon failures.