Converting evergreen broad-leaved forests into tea and Moso bamboo plantations affects labile carbon pools and the chemical composition of soil organic carbon
Graphical abstract
Introduction
In recent decades, a large number of natural forests have been globally converted to intensively-managed plantations for economic reasons due to an increasing demand for timber (Park, 2015, Lin et al., 2018). This involves intensive management practices, including fertilization, deep ploughing and removal of the understory vegetation, which could significantly change the physico-chemical properties of the soil (Li et al., 2014). Previous studies have shown that conversion of evergreen broad-leaved forests (EBFs) to plantations and the subsequent intensive management had a significant impact on soil nutrient content and carbon sequestration (Dai et al., 2018a, Dai et al., 2018b, Fang et al., 2017, Li et al., 2014, Zhao et al., 2020). Land-use change as a result of human activity is the most important factor to affect soil carbon content and stability (Gispert et al., 2017, Meng et al., 2019), highlighting the importance of investigating these following land-use conversion and intensive management practices.
Land use conversion from natural forests to more economical plantations impacts soil nutrients and labile soil organic carbon (SOC) pools because of differences between the management practices thereof (Paudel and Sah, 2015, Huang et al., 2008, Wu et al., 2019a, Wu et al., 2019b) and varying carbon input and quality due to leaf litter and root exudation (Wang et al., 2010, Fu et al., 2014a, Fu et al., 2015, Macinnis-Ng and Schwendenmann, 2015). These differences can significantly affect the storage and chemical composition of SOC. For example, slow decomposition of root litter may contribute to a longer residence time for organic carbon (Poirier et al., 2018), and after conversion from natural forests to both broad-leaved and coniferous plantations, the the resulting change in litter quantity can lead to a reduction in SOC (Chen et al., 2005). In addition, Kashem et al. (2015) found that conversion from forests to Acacia plantations significantly increased the SOC further contributing to an increase in litter. Management practices such as cultivation, tillage and understory vegetation removal also alter soil properties and SOC (Liu et al., 2005, Xue et al., 2015;).
Labile C pools (e.g. water soluble organic carbon (WSOC), light fraction organic carbon (LFOC), heavy fraction organic carbon (HFOC), SOC in aggregates with different particle sizes and humus carbon (HC)) are sensitive to land-use conversion or alteration in management practices (Arevalo et al., 2009, Zhang et al., 2009, Oliveira et al., 2016, Wang et al., 2017, Hu et al., 2018). Due to differing vegetation types and management practices, SOC density in Zhejiang province decreased from southwest to northeast (Dai et al., 2018c). In comparison with the SOC in surface soil (5–10 cm), an excess of 50% of carbon stock was found in subsoil (below 20 cm) (Salome et al., 2010) and the SOC in subsoil was more stable than that in surface soil. Therefore, it is important to monitor SOC density throughout the soil profile after conversion from forests to plantations. Land-use conversion also significantly affects the chemical composition of SOC and 13C-nuclear magnetic resonance (13C NMR) has been increasingly used to investigate the stability of SOC by obtaining the chemical composition of SOC (Li et al., 2010, Wang and Zhong, 2016). Therefore, exploring the response of SOC components after land-use change will elucidate the mechanisms involved.
Tea plantations in China account for 63% of the total global distribution, with an annual output of more than 2 million tons (Ren et al., 2015). Bamboo plantations in China cover more than 6 million hectares, accounting for approximately 25% of the total area worldwide (Zhou et al., 2019). In the past few decades, evergreen broad-leaved forests (EBFs) have been converted to tea plantations (TPs) and Moso bamboo plantations (MBPs) due to the commercial importance and value of tea and bamboo and their byproducts. Subsequent intensive management practices have included deep ploughing, application of chemical fertilizers, which influence soil physico-chemical properties and soil microbial communities (Chen et al., 2019, Sun et al., 2019). The objectives of this study were (1) to analyze the effects of conversion from EBFs to intensively managed tea plantations (TPs) and Moso bamboo plantations (MBPs) on the labile organic carbon pools and the chemical composition of SOC, (2) to investigate the land-use conversion effects on soil organic carbon density (SOCD), and (3) to explore the relationships among the organic carbon pools.
Section snippets
Experimental site
This study was carried out in Meixi town (30°28′N,119°24′E), east of An’ji county, Zhejiang Province, China (Fig. S1). The region has a subtropical marine monsoon climate with an average annual temperature of 15.1 °C and average annual precipitation of 1286 mm. Annual daylight hours and frost-free days are 1770 h and 234 d, respectively. The soils at the study site were classified as Ferralsols using the FAO soil classification system (World Reference Base for Soil Resources (WRB), 2006).
Experimental design and soil sampling
In
Effects of land-use change on soil physico-chemical properties
Soil bulk density (BD) decreased in both the 10–30 and 30–60 cm soil layers (Fig. 1) with the conversion of EBFs to TPs and MBPs. Over the duration of the intensive management, there is no significant difference in BD between treatments of TP and MBP, respectively. Soil pH in different land-use types ranged from 4.0 to 4.7 (Fig. 2a) and regardless of soil layers, an increasing trend was evident after conversion (Fig. 2a). Compared with EBF, soil pH in MBP10 and MBP40 were significantly
Land-use conversion and intensive management effect on soil physico-chemical properties
The results revealed that land-use conversion from EBF to TPs and MBPs significantly increased the soil pH (Fig. 2a). In contrast, Qin et al. (2014) found that the conversion of a rice paddy to a Lei bamboo plantation caused a decrease in the pH of the soil. The contrary results were probably related to the different land-use changes. The presence of humus in the EBF which decomposed to produce acidic conditions, lowering soil pH when compared to the plantations may be a possible explanation.
Conclusion
Converting EBFs to TPs and MBPs and after intensive management increased the pH. The land-use change decreased the concentrations of SOC, WSOC, LFOC, HFOC and HC (P < 0.05), reduced O-alkyl C and carbonl C contents, but increased alkyl C, Aromatic C contents, aromaticity as well as the A/O-A ratio (P < 0.05). Our results demonstrated that land-use conversion and subsequent intensive management had negative effects on the content of SOC and labile carbon pools. Therefore, in order to enhance the
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.
Acknowledgments
This study was financially supported by National Natural Science Foundation of China (No. 41201323) and Zhejiang Key Scientific and Technological Innovation Group (No. 2009R50033).
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These two authors contributed equally to this work.