Soil Respiration Rate and Climatic Parameters from Mixed and Broad-leaved forest sites from Bhutan, Eastern Himalayas for 2015

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 (Rs) at a high elevation (3260 m) mixed, and a lower elevation (2460 m) broadleaved forest in Bhutan, eastern Himalayas, during 2015. Trenching was applied to<i> </i>estimate the contribution of autotrophic (Ra) and heterotrophic (Rh) soil respiration. The temperature (<i>Q₁₀</i>) and the moisture sensitivities of Rh were determined under controlled laboratory conditions and were used to model Rh 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 Rs was similar between the two forest sites (14.5 ± 1.2 t C yr^-1 broadleaved; 12.8 ± 1.0 t C yr^-1 mixed). Modelled annual contribution of Rh was ~ 65 % of Rs at both sites with a higher heterotrophic contribution during winter and lower contribution during the monsoon season. Rh, estimated from trenching, was in the range of modelled Rh but showed higher temporal variability. Measured temperature sensitivity of Rh was similar at the mixed and broadleaved forest site (<i>Q₁₀</i> 2.2- 2.3) under intermediate soil moisture but decreased (<i>Q₁₀</i> 1.5 at both sites) in dry soil. Rs closely followed the annual course of field soil temperature at both sites. Co-variation between soil temperature and moisture (cold-dry winters, warm-wet summers) likely was the main cause for this tight relationship. Under the prevailing weather conditions, a simple temperature-driven model was able to explain more than 90 % of the temporal variation in Rs. Longer time series and/or experimental climate manipulations are required to understand the effects of eventually occurring climate extremes such as monsoon failures.

兴都库什-喜马拉雅（Hindu Kush-Himalaya）山脉山地森林的生物地球化学研究仍较为薄弱，尽管气候变化预计将对该区域产生不成比例的影响。本研究于2015年在喜马拉雅东部不丹境内，针对一处高海拔（3260米）混交林与一处低海拔（2460米）阔叶林开展了土壤CO₂排放通量（soil CO₂ efflux，以下简称Rs）的原位测定。研究采用壕沟法（trenching）分离并估算自养呼吸（autotrophic respiration，以下简称Ra）与异养呼吸（heterotrophic respiration，以下简称Rh）的贡献量。通过受控实验室培养实验，测定了Rh的温度敏感性系数（Q₁₀）与土壤湿度敏感性，并将参数应用于野外Rh的模拟建模。高海拔混交林拥有更高的立木蓄积量，对应更高的土壤碳库与基础土壤呼吸速率。两处林分的年Rs通量相近：阔叶林为14.5 ± 1.2 t C yr⁻¹，混交林为12.8 ± 1.0 t C yr⁻¹。模拟结果显示，两处林分的Rh年贡献量均约为Rs的65%，且异养呼吸贡献占比在冬季更高、季风季更低。通过壕沟法估算得到的Rh值处于模拟Rh的区间范围内，但表现出更高的时间变异性。在中等土壤湿度条件下，混交林与阔叶林样地的Rh温度敏感性相近（Q₁₀为2.2~2.3）；而在干旱土壤中，两者的Q₁₀均降至1.5。两处样地的Rs均与野外原位土壤温度的年动态变化高度吻合，土壤温度与湿度的协同变化（冬季寒冷干旱、夏季温暖湿润）可能是这一紧密关联的主要驱动因素。在当前气象条件下，一个简单的温度驱动模型即可解释Rs超过90%的时间变异。未来需要更长时间序列的观测和/或人工气候调控实验，以理解诸如季风中断等极端气候事件的潜在影响。