Microbial respiration measurement
The fresh soil samples from all forests for incubation were adjusted to 60% of water holding capacity and pre-incubated for 10-day at 20 °C to activate microorganisms and minimize the “pulse effect”. After equilibration, approximately 100 g (dried weight) of soil was placed into 500-ml rectangular plastic containers. These containers had pierced lids that enabled gas exchange, but minimized evaporation and soil drying. Soils were incubated for 72 hours at the corresponding MAT of their origin sites. When MAT of soil origin sites was lower than 10 °C, soils were incubated at 10 °C. To measure Rm, we connected each 500-ml rectangular soil container (without the lid) to an infrared gas analyzer (Li-Cor 820) in a close-loop configuration. The respired CO2 was measured by the Li-Cor 820 at every 24 hours. During incubation, all soil samples were maintained with 60% of water holding capacity by regularly weighing the soil containers and adding distilled water to compensate for water loss.