DOI: 10.5061/dryad.t4b8gtj2p
title: Soil respiration in a successional tropical forest in Thailand

description: 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.
description: 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
 CO<sub>2</sub> m<sup>−2</sup>
 h<sup>−1</sup>, was calculated by measuring the rate of
 increase of CO<sub>2</sub> 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
 cm<sup>2</sup>, 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.