Soil CO<sub>2</sub> efflux from two mountain forests in the Eastern Himalayas Bhutan: components and controls

Abstract

<p><strong>Abstract.</strong> 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 CO<sub>2</sub> efflux (Rs) at a high elevation (3260 m) coniferous, and a lower elevation (2460 m) broadleaved forest in Bhutan, eastern Himalayas, during 2014 and 2015. Both sites experienced typical monsoon weather (cold-dry winters, warm-wet summers) during the study. Trenching was applied to estimate the contribution of autotrophic (Ra) and heterotrophic (Rh) soil respiration. The temperature (<i>Q</i><sub>10</sub>) and the moisture sensitivities of Rh were determined under controlled laboratory conditions and were used to model Rh in the field. The higher elevation coniferous forest had a higher standing tree stock, reflected in higher soil C stocks and basal soil respiration (<i>R</i><sub>10</sub>). Rs was similar between the two forests (2015: 14.5 ± 1.2 t C yr<sup>−1</sup> broadleaved; 12.8 ± 1.0 t C yr<sup>−1</sup> coniferous). Modelled annual contribution of Ra was ~ 45 % at both forests with a low autotrophic contribution during winter and high contribution during the monsoon season. Ra, estimated from trenching, was lower and highly variable, indicating that trenching poorly performed at these forests/soils. Rs neatly followed the annual course of soil temperature (field <i>Q</i><sub>10</sub> between 4 and 5) at both sites. Co-variation between soil temperature and moisture likely was the main cause for the high <i>Q</i><sub>10</sub> obtained from field Rs. Temperature sensitivity of Rh was lower (<i>Q</i><sub>10</sub> ~ 2.3 at both sites). Under the preceding weather conditions, a simple temperature-driven model was able to explain more than 90 % of the temporal variation in Rs. To predict and understand how Rs responds to infrequently occurring extreme climate conditions such as monsoon failures, however, longer Rs time series are required for a better integration of interactions between soil temperature, moisture, Ra and Rh.</p>

Experimental throughfall reduction barely affects soil carbon dynamics in a warm-temperate oak forest, central China

“…Similarly low threshold values (~12 vol% soil moisture) for SR were observed at Harvard temperate hardwood forest 45 and in a boreal aspen forest 46 , respectively. Higher (~20 vol%) soil moisture thresholds for HR were reported for a Bhutan oak forest 47 and an oak forest in central Italy 48 .…”

Soil respiration in a subtropical forest of southwestern China: Components, patterns and controls

“…The relative contributions of R A and R H to total R S varied with forest types, climate and methods [ 26 – 29 ]. R A contributed 14%-73% to R S among global forest ecosystems [ 27 ].…”

“…This is due to the fact that R A and R H are dominated by different mechanisms [ 10 ]. R A is closely linked to root activity and photosynthesis, while R H is the respiratory product of soil organic matter decomposition that mainly controlled by substrate and temperature [ 26 ]. As a result, there is a significant increase in R A during the growing seasons in M .…”

Soil respiration in a tropical montane grassland ecosystem is largely heterotroph-driven and increases under simulated warming

“…Many studies have reported warminginduced increases in soil respiration in subtropical, temperate and boreal ecosystems (e.g. Buchmann 2000; Rustad et al 2001;Conant et al 2004;Bronson et al 2007;Lu et al 2013;Li et al 2016;Wangdi et al 2017), and it is estimated that warming has accounted for a 3% increase in soil respiration levels from 1989 to 2008 in tropical ecosystems as well (Bond-Lamberty and Thomson 2010). Warmer conditions can influence CO2 fluxes by affecting both autotrophic respiration, from plant roots and plant-associated symbionts such as arbuscular mycorrhizal fungi (AMF), and heterotrophic respiration due to microbial (fungal and bacterial) and animal decomposers.…”