Afforestation changes soil organic carbon stocks on sloping land: The role of previous land cover and tree type

https://doi.org/10.1016/j.ecoleng.2020.105860Get rights and content

Highlights

  • Afforestation of steep slopes sequesters more SOC than afforestation of gentle slopes.

  • Afforestation on sloping cropland significantly increases SOC stock in upper soil.

  • Afforestation on sloping grassland decreases SOC stock except for very steep slopes.

  • Tree type has a limited effect on changes in SOC after afforestation on sloping land.

Abstract

Afforestation contributes to soil organic carbon (SOC) sequestration and the restoration of degraded soil. However, the changes in SOC stock following afforestation on sloping land are poorly understood. Here we conduct a quantitative review that synthesizes 239 observations from 31 studies to evaluate the effects of slope gradients on the changes in SOC stock in three soil layers (0–20 cm, 20–40 cm and 40–60 cm) following afforestation, considering previous land cover (cropland and grassland) and tree type (broadleaf and conifer). In the topsoil layer, slope gradient has a significant effect on the changes in SOC stock: slope gradient positively relates to SOC stock change and significantly predicts the SOC stock change after controlling for temperature, precipitation, stand age, and initial SOC stock. Soil gradient explains 12.5% of the variance. We find a significant decrease in SOC stock after the afforestation of gentle slopes with a gradient lower than 10% but a significant increase in SOC stock on slopes with a gradient higher than 30%. Afforestation on sloping cropland results in an increase in SOC stock, especially in the upper soil, while afforestation on grassland results in an increase in SOC stocks only on very steep slopes (gradient >60%). Tree type has a limited effect on changes in SOC after afforestation on sloping land. However, conifers show a greater SOC sequestration potential in deeper soils and on steep slopes with a gradient higher than 60%, while broadleaf trees sequester more SOC in topsoil and on slopes with a gradient of 30–60%. The present study suggests that to increase SOC sequestration, priority should be given to afforestation with conifers on steep cropland.

Introduction

Soil organic carbon (SOC) is one of the largest components of the terrestrial ecosystem's carbon pool (Scharlemann et al., 2014). The loss of SOC through soil erosion is a growing source of atmospheric CO2 (Lal, 2019). SOC plays an important role in maintaining soil structure and aggregation, making soils less prone to degradation (Gaiser and Stahr, 2013; Nabiollahi et al., 2018). Land use changes from natural forest to cultivated land result in severe soil erosion, especially on sloping land (Borrelli et al., 2017; Nabiollahi et al., 2018). Soil erosion leads to the removal of fine soil particles and organic carbon through runoff (Khan et al., 2013), which is more pronounced on steeper slopes than gentler slopes (Yohannes et al., 2015). Therefore, more SOC losses may be occur in slopes with greater gradients (Nabiollahi et al., 2018). However, studies report inconsistent results about the relationship between SOC stocks and slope gradient. For instance, a negative relationship was found by Mora et al. (2014) and Yohannes et al. (2015), a positive relationship was described by Chaplot et al. (2010), Fang et al. (2012), Wang et al. (2012a) and Zhao et al. (2015), and no significant relationship was reported by Wang et al. (2012b).

Plants mitigate soil erosion through their greater soil water holding capacity and by mechanically stabilizing the soil with their roots (Macci et al., 2012; Giadrossich et al., 2019). According to Borrelli et al. (2017), the average soil erosion in croplands is 77 times higher than the rate of soil erosion in natural forests. However, averages hide the fact that SOC stocks change with slope gradient and land cover (Yuan et al., 2018; Shi et al., 2019). For example, (i) for farmland, gently sloped land has greater SOC than more steeply sloped land. This is explained by the tendency of soil erosion (and therefore SOC loss) to increase with slope gradient (Fu et al., 2009; Karchegani et al., 2012; Shi et al., 2019). (ii) For grassland, slope gradient either shows no significant relationship with SOC stock (Zhang et al., 2018; Zhu et al., 2019), or a moderate slope (<15%) results in an increase in SOC accumulation (Fissore et al., 2017). (iii) For forestland, some researchers found that SOC is greater in steeply sloped land than in more gently sloped land (Ajami et al., 2016; Fang et al., 2018), while others found the opposite (Conforti et al., 2016; Nanko et al., 2017).

The inconsistent relationship between SOC stocks and slope gradients could be due to the influence that land cover has on SOC sequestration ability. SOC change is a balance between organic C input from above- and below-ground plant sources and organic C losses as a result of decomposition, erosion, and leaching (Olson et al., 2014; Hou et al., 2019a). Forests increase SOC inputs because trees produce more litterfall and have larger roots than crops and grass. Forests also decrease soil erosion and the mineralization of SOC which occur when soil is disturbed by tillage. On the other hand, slope is an important factor that determines what vegetation type is planted or left growing in fields. For example, trees, which usually sequester more SOC than crops, tend to be preserved (or planted) on more steeply sloping land, while crops (which usually sequester less SOC than trees) tend to be grown on more gently sloping land (Wang et al., 2012a; Liu et al., 2016).

Afforestation is viewed as an effective method to reduce soil loss and contribute to SOC sequestration (Wang et al., 2012b; Xiong et al., 2018; Li et al., 2019). However, the ability of forests to sequester SOC is influenced by many factors, such as climatic conditions, soil texture, tree type, etc. Models have mainly focused on the effects of climatic conditions, initial SOC stock and stand age for predicting SOC stock changes (e.g. Don et al., 2011; Deng et al., 2014a, Deng et al., 2014b; Hou et al., 2019a). However, many afforestation projects have been undertaken on sloping land because sloping land is less productive and its afforestation has greater environmental benefits (Wang et al., 2019). For example, the world's biggest afforestation program, China's Grain for Green, gives priority to afforesting land with a slope greater than 25° (equivalent to 43.86%) (Delang and Yuan, 2016).

Most previous studies focused on the effects of slope positions and aspects (e.g. Wei et al., 2010; Liu et al., 2019; Zhu et al., 2019) or on the changes in the soil's physical and chemical properties after vegetation restoration, but paid less attention to the effects of slope gradient on changes in SOC stock following afforestation. Given the importance of slope gradient on soil erosion and SOC loss, in this paper we look at the relationship between slope gradient and change in SOC stock after afforestation. The amount of soil erosion, SOC sequestration and soil conservation are also affected by the type of trees planted (e.g. broadleaf trees and conifers) (Li et al., 2012; Hou et al., 2019a; Olsson et al., 2019; Shinohara et al., 2019). The effect of tree type on SOC sequestration of sloping land is complex and has received insufficient attention. For example, on the one hand, SOC stock tends to decrease after afforestation with coniferous trees but increase after afforestation with broadleaf trees, since conifer needles decompose at a slower rate than broadleaf litters (Hou et al., 2019a). On the other land, coniferous trees have greater soil and water retention abilities than broadleaf trees, therefore, there is lower soil erosion rate on slopes planted with conifers (Qin and Shangguan, 2019; Shinohara et al., 2019). These opposing trends call for further exploration on the relationship between tree types and SOC sequestration rates on sloping land.

Given the limited understanding of the effects of slope gradient and the role of land cover and tree type on SOC sequestration, this study aims to examine: 1) how change in SOC stock after afforestation is related to slope gradient; 2) to what extent the slope gradient can affect SOC stock changes after controlling for temperature, precipitation, stand age and initial SOC stock; and 3) how the previous land cover and the type of trees influence SOC sequestration after the afforestation of sloping land. Our findings may contribute to a better understanding of SOC stock and dynamics following afforestation on sloping land and further inform afforestation policies (Hou et al., 2019b).

Section snippets

Data sources and compilation

The academic articles included in this study were identified by searching Google Scholar and the Web of Science with the keywords “afforestation”, “soil organic carbon” and “slope” (cut-off date: 1st April 2019). We included only articles published in international refereed journals. To avoid bias in the selection of the literature, we chose only research articles that fulfil the following conditions. First, the study had to report the SOC value of at least the topsoil layer (0–20 cm) or,

Descriptive statistics

Table 1 shows the preliminary data screening of all studied variables. Skewness and Kurtosis values are within the acceptable range of ±1 except for the values for annual average precipitation, slope and SOC stock change (Table 1). Values that were slightly out of the acceptable range (<20%) were used in the analysis. Results show data are generally normally distributed and the assumption of homogeneity of variance is not violated.

Descriptive statistics, including means and standard deviations

Effects of slope gradient on SOC sequestration after afforestation

The potential for SOC sequestration after afforestation varies with slope gradient. Our study shows SOC sequestration is greater after afforestation on steeper slopes than on gentler slopes, which is in line with the results of Ajami et al. (2016), Fang et al. (2018) and Zhang et al. (2020). Soil erosion leads to SOC loss by the removal of the light organic components of the soil and the alteration of the soil's physical and biological properties (Olson et al., 2016). Greater slope gradient

Conclusion

Afforestation not only contributes to soil and water conservation on sloping land but also increases the SOC input from tree litterfall and root biomass, which is an effective method of increasing SOC sequestration and retention (Olson, 2010; Xiong et al., 2018). The present study shows afforesting steeply sloping land can sequester more SOC than afforesting slightly sloping land and there is a significant decrease in SOC stock after afforestation on gentle slopes (gradient lower than 10%). In

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

The work described in this paper has been fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. 12305116).

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