Litter and microclimate controls on soil heterotrophic respiration after converting seasonal rainforests to rubber plantations in tropical China
Introduction
Deforestation is known as the second largest source of the elevated atmospheric carbon dioxide. Carbon loss through deforestation and degradation of tropical forests was estimated at 0.8 Pg to 1.0 Pg C yr−1 in the last decades (Bacciniet al., 2012; Harris et al., 2012). Southeast Asia is one of the global deforestation hot spots where almost 250,000 hectares of natural forest was converted to rubber plantation (RP) between 2005 and 2010 (Ahrends et al., 2015). Rubber and other economic crops have been the major land-use changes in this region. The area of RP in Southeast Asia reached 10.4 million ha by 2017, accounting for 89% of world total RP area (FAOSTAT, 2019). Since the first rubber establishment in the 1950s, the area of RPs has increased up to 24.2% of land cover by 2014 in Xishuangbanna, Southwest of China (Chen et al., 2016).
It is recognized that soil carbon sequestration ability in RPs is much lower than that of natural forests (Jiang et al., 2017; Yang et al., 2016). This reduction in soil organic C may result from either reduced litter input or accelerated decomposition due to changed litter quality, microclimate, or soil activity. Litter production was reported to be lower in RP than in seasonal rain forest (SR) (Zhang and Zou, 2009; Zhang et al., 2013; Kotowska et al., 2016). Litter quality is lower in RP than in SR as indicated by a higher C/N ratio and aromatic carbon fractions (Zhang and Zou, 2009; Zhang et al., 2013). However, thinner floor layer in RP than SR (Zhang and Zou, 2009; Lan et al., 2020) suggests a cause of either reduced litter inputs or accelerated litter decomposition due to improved soil physical environment and soil activity, rather than litter decomposability in RP.
Soil is responsible for 60–90% of the carbon dioxide (CO2) released into the atmosphere in terrestrial ecosystems (Goulden et al., 1996). Soil CO2 efflux is the outcome of the metabolic processes of plant roots (autotrophic respiration, Ra) and microbial communities (heterotrophic respiration, Rh). The Ra refers to CO2 efflux derived from roots and the associated symbiotic organisms, Rh refers to microbial respiration of litter and root exudates as well as the transformed soil organic carbon (Sulaman et al., 2005; Kuzyakov, 2005). Because Rh could account for up to 70% of the total soil respiration (Rs) in forest stands (Sulaman et al., 2005; Fernández-Alonso et al., 2018), it is therefore important to determine how Rh responds to land-use changes from native forests to monoculture agroforestry systems (Barba et al., 2016; Fernández-Alonso et al., 2018).
The conversion of native forest to monoculture RP may result in changes in plant litter input, soil biochemical properties such as microbial biomass and soil labile organic carbon, or in environmental conditions that include microclimate and soil water content (Liu et al., 2011, Meijide et al., 2018; Song et al., 2017; Lan et al., 2020), and consequently alter soil respiration and soil carbon stock. Temperature is the essential factor controlling soil metabolic activities; thus, the kinetics of CO2 efflux had been modeled by various temperature-dependent functions. Many hypotheses were postulated to link the response of Rh to soil temperature (Tuomi et al., 2008; Tremblay et al., 2018; Salazar et al., 2019). However, the temporal variability of Rh observed in field experiments captures not only its dependence on soil temperature, but also reflects the influence on decomposition caused by seasonal changes in substrate quality and quantity, soil moisture and soil community (Gonzalez et al., 2001; Hou et al., 2005; Fernández-Alonso et al., 2018; Huang et al., 2020), as well as their interactions (Wall et al., 2008). Strong variations in seasonality of modeled soil Rh was commonly observed, suggesting an apparent limitation to soil metabolic activity beyond that could be explained by temperature. Furthermore, homeostasis of native soil microbial community would suggest Rh dependency on soil organisms, which were also greatly constrained by soil water and temperature (Fernández-Alonso et al., 2018).
We conducted this study to elucidate the influence of aboveground litter input, forest soils and its organisms, and forest microclimate and water content on soil Rh and its temperature sensitivity following conversion of natural forest to RPs. A 15-month reciprocal soil and litter translocation mesocosm experiment were performed in Xishuangbanna of SW China. In the root-exclusion treatment it was assumed that Ra was completely suppressed and hence solely Rh occurred that the CO2 efflux derived from the microbial decomposition of soil organic matter and litter. We collected in-situ data on soil Rh, litterfall, soil labile organic carbon, soil microbial biomass, and soil temperature and moisture from these mesocosms in SR and RP. We hypothesized that reduction of TOC in RPs was resulted from (1) reduced production of above-ground rubber litter, or (2) accelerated soil heterotrophic respiration and altered temperature sensitivity of TOC due to improved physical environmental conditions such as temperature and moisture or soil biochemical properties such as microbial biomass and soil labile organic carbon.
Section snippets
Site description
Study sites were located in the Xishuangbanna Tropical Botanical Garden (XTBG, 21°54′N, 101°46′E), near the monitoring plots of the Chinese Ecological Research Network supported by the Chinese Academy of Sciences. The annual mean temperature in the region is 21.8°C and the annual mean precipitation is 1493 mm (Cao et al., 2006). This region is strongly influenced by the tropical monsoons that bring warm and humid air from the northeastern Indian Ocean during the months of May through October.
Controls on heterotrophic respiration
Soil respiration rates varied greatly, ranging from 1.8 µmol m−2 s−1 to 14.3 µmol m−2 s−1 at the SR site and from 3.6 to 28.1 µmol m−2 s−1 at the RP site. Soil respiration rate without litters was on average about 68% of that with litters. Annual Rh was apparently greater in RP than SR sites, and in treatment with litter than without litter inputs, suggesting that physical environment and the presence of litter had significant influences on annual Rh (Fig. 1). The average Rh of soils incubated
Discussion
Reduction in TOC following conversion from SR to RP may have resulted from one or more of these factors: improved physical environment for decomposition, improved quality of organic C for decomposition, reduced litter input, and increased biotic activity in decomposition. In this study, soil Rh in RP site was overall higher than that in SR site, whereas aboveground litter input and soil origin had no significant influences on Rh. Higher C/N ratio (Zhang et al., 2013) of RP litter (42.5±1.0)
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research was financially supported by the Asia-Pacific Network for Global Change Research (ARCP2008-19NMY).
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