Soil respiration (SR) in forests contributes significant emissions of carbon from terrestrial ecosystems into the atmosphere. Soil respiration is highly sensitive to environmental changes because it is affected by many factors, including soil temperature, soil moisture, microbial community, surface litter and vegetation type. Indeed, a small change in SR may have large impacts on the global carbon balance, further influencing feedbacks to climate change. Thus, detailed characterization of SR responses to changes in environmental conditions is needed to accurately estimate carbon dioxide emissions from forest ecosystems. However, data for such analyses are still limited, especially in tropical forests of Southeast Asia where various stages of forest succession exist due to previous land-use changes. In this preliminary study, we measured SR and some environmental factors including soil temperature (ST), soil moisture (SM) and organic matter content (OM) in three successional tropical forests in both wet and dry seasons. We also analyzed the relationships between SR and the three environmental variables. Results showed that SR was higher in the wet season and in older forests. While no response of SR to ST was found in younger forest stages, SR of the old-growth forest significantly responded to ST, plausibly due to the non-uniform forest structure, including gaps, that resulted in a wide range of ST. Across forest stages, SM was the limiting factor for SR in the wet season whereas SR significantly varied with OM in the dry season. Overall, our results indicated that the responses of SR to environmental factors were mediated by seasons and forest succession. These findings call for further investigations on SR and its variations with environmental factors in tropical forests with detailed temporal and spatial scales, particularly in Southeast Asia where patches of successional stages dominate.

We made the measurements in the wet season (June and September 2020) and in the dry season measurements (February and March 2021); hereafter described as ‘wet season’ and ‘dry season’, respectively. In each forest stage, we established a 1-ha plot and divided it into 20×20 m subplots as shown in Figure A2. Then, we randomly selected six sampling points within the 1-ha plot and measured all study variables concurrently at each point during 1000–1500 h on sunny days. For SR, we used a portable photosynthesis system (TARGAS-1, PP Systems, Amesbury, MA, USA) connected to a soil respiration chamber (SRC-2 Soil Respiration Chamber, PP Systems, Amesbury, MA, USA). In this process, the SR rate, measured in g CO₂ m⁻² h⁻¹, was calculated by measuring the rate of increase of CO₂ concentration in the chamber over a period which was set to 60 seconds. Before taking measurements, we installed a soil collar, whose cross-sectional area was 78 cm², on each selected sampling point at 5-cm depth in the soil, leaving it for at least one hour prior to the SR measurement. Before putting the soil respiration chamber on the soil collar, we removed small living plants and coarse litter from the soil surface within the collar to avoid measuring their respiration (Zhou et al., 2007; Peng et al., 2014). Simultaneously, ST was measured using a probe (STP-2 Soil Temperature Probe, PP Systems, Amesbury, MA, USA) at 10-cm depth near the soil collar. Soil moisture was measured at 5-cm depth from the soil surface using a probe (SM150T, DeltaT Devices, London, UK). For each sampling point, all measurements of SR, ST and SM were repeated three times and then averaged to represent each sampling point. For the soil analyses, we collected three 3.2-cm diameter soil core samples from each study plot at 10-cm soil depth in the wet season (September 2020) and the dry season (February 2021). We used a total organic carbon analyzer (Multi N/C 3100, Analytik Jena) to obtain OM values.