Soil organic carbon storage capacity positively related to forest succession on the Loess Plateau, China
Introduction
Soils play an important role in the global carbon cycle (Post et al., 1990, Schlesinger and Andrews, 2000, Scurlock and Hall, 1998). Soil carbon storage is roughly twice that of atmospheric storage (Davidson et al., 2000). Consequently, soil as either a carbon sink or source has become a focus of research in the scientific debate on global climate change (Knorr et al., 2005, Qiu et al., 2010, Schlesinger and Lichter, 2001). Local land-use and land-cover change can play a key role in both ecological and environmental changes thereby, contributing to global climate change (IPCC (Intergovernmental Panel on Climate Change), 2007, Wilson et al., 2003). Soil degradation is a major threat to the sustainable use of soil ecosystems because it decreases both the actual and potential vegetation cover (Cheng et al., 2012, Jia et al., 2005). Consequently, improving the physical and chemical properties of degraded soil is particularly important for sustainable soil ecosystems (Jia et al., 2005).
Soil organic carbon (SOC) is an essential part of soil physical and chemical properties (Schoenholtz et al., 2000). In addition, it is a key element in the process of trapping atmospheric CO2 in terrestrial ecosystems through primary production (Post and Kwon, 2000). SOC loss caused by the conversion of natural to cultivated vegetation is well documented (Yan et al., 2012). Globally, 24% of the SOC stock has been lost through the conversion of forestland to cropland (Murty et al., 2002) and 59% through the conversion of pastureland to cropland (Guo and Gifford, 2002). In contrast, where cropland is withdrawn from farming and converted into natural vegetation, SOC accumulates and is locked up for greater periods of time due to the slower turnover rates associated with natural vegetation (Degryze et al., 2004, Post and Kwon, 2000, Zhang et al., 2010). The stock of SOC can be increased by preventing soil erosion (Lal, 2002), increasing organic matter inputs (Smith, 2008), and decreasing both weathering and microbial breakdown (Lal, 2005, Post and Kwon, 2000, Smith, 2008). Guo and Gifford (2002), for example, concluded that SOC storage increased by a remarkable 53% when crop was allowed to convert to secondary forest. However, Vesterdal et al. (2002) observed that afforestation on former arable land did not lead to an increase in SOC within three decades, but rather affected the redistribution of SOC in the soil profile. Understanding the pattern of SOC sequestration following the conversion of cropland to perennial vegetation is important not only to provide information for ecosystem management practices alone but also to support international policies on the mitigation of greenhouse gas emissions (Lal, 2002, Lal, 2004, Post and Kwon, 2000).
It is well known that succession can lead to the recovery of deteriorated soil properties (Jia et al., 2005, Zhao et al., 2010). For this reason, it is essential to understand the process of succession in secondary forests in the central part of the Loess Plateau in China where the vegetation cover of the plateau tends to be poor and sparse (Cheng et al., 2012, Jia et al., 2005) and, where simultaneously the plateau suffers from extreme soil erosion resulting in severe soil degradation (Deng et al., 2012, Liu et al., 2007).
Unsustainable land-use practices are one of the most important causes contributing to soil degradation (Jia et al., 2005). Consequently, the Chinese government has instituted various erosion mitigation measures on the Loess Plateau (SFA, China State Forestry Administration, 2002), especially the conversion of cropland into forest- and grassland. In the surroundings of the study area, some cropland has been restored to natural vegetation. Recently, Chinese scientists have increased the attention paid to the succession of secondary forests in the Ziwuling Range (Cheng et al., 2012, Jia et al., 2005, Wang et al., 2010, Zhao et al., 2010). While a lot of research has focused on changes in the aboveground vegetation of secondary forests in the central part of the Loess Plateau (Wang et al., 2010, Zou et al., 2002), few studies have focused on changes to the soil itself (Jia et al., 2005), although studies have already been done on soil properties in other regions of the Loess Plateau (Li et al., 2013, Wang et al., 2012). Furthermore, little investigation has been carried out on soil nutrient dynamics at the different succession stages of these secondary forests.
Changes in land use caused by revegetation probably enhance the carbon sequestration capacity of terrestrial ecosystems on the Loess Plateau. Naturally regenerating forests as carbon sinks is no doubt attributed to increases in their biomass carbon storage (Canadell and Raupach, 2008, Fang et al., 2001) capacity. Furthermore, soil carbon sequestration probably occurs more slowly (Vesterdal et al., 2002, Zhou et al., 2006). Thus, the study hypothesized that the stock of SOC varied with forest vegetation age through succession. Therefore, the objectives of the study were to investigate: (1) the dynamics of soil organic carbon storage with the succession of secondary forests, (2) the effects of vegetation succession on the level of SOC storage, and (3) those factors affecting SOC storage.
Section snippets
Study area
The study was conducted on the Lianjiabian Forest Farm of the Heshui General Forest Farm of Gansu (35°03′–36°37 N′, 108°10′–109°18′E, 1211–1453 m a.s.l.), located in the hinterland of the Loess Plateau, the Ziwuling forest region, covering a total area of 23,000 km2 (Fig. 1). The altitude of the region's hilly and gully landforms averages 1500 m a.s.l., their relative height difference is about 200 m, the area's annual temperature is 10 °C, annual rainfall is 587 mm, accumulative temperature is 2671 °C,
SOC, TN and C/N
Generally, the SOC in the different soil layers increased gradually along with vegetation restoration (Fig. 3a) and differed significantly among the different restoration stages. The SOC in the 0–60 cm soil layer significantly increased at the early stage of succession (< 50 yr) and tended to be stable at the later stage of forest (> 50 yr) (Fig. 4a). The SOC in the 0–20 cm soil layer had not significantly varied after 20 years of restoration (P > 0.05) (Fig. 3a), but had differed significantly with
Discussion
Depending on land use, soil acts either as a carbon source or as a carbon sink. Changes in land use change the function of source or sink (Guo and Gifford, 2002). The study indicated that the Cs increased in the earliest stage of converting cropland to natural vegetation (< 50 yr), which indicates soil as a CO2 sink during restoration. This is probably because vegetation recovery facilitated SOC accumulation from biomass input (Tang et al., 2010). The accumulation of nutrients and organic matter
Conclusions
The impact of long-term vegetation restoration on the soil carbon pool was significant from grassland to forest. The storage of soil organic carbon increased with vegetation restoration. The Cs in the 0–60 cm soil significantly increased at the early stage of restoration (0–50 yr) and tended to be stable at the later stage of forest (50–150 yr). The Cs was higher in the upper soil than in the lower soil at the different restoration stages, however, the SOC storage mainly changed in the lower soil.
Acknowledgments
The study was funded by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA05060403), the Scholarship Award for Excellent Doctoral Student granted by the Ministry of Education, and supported by the Key Research Program of the Chinese Academy of Sciences (KZZD-EW-04).
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