Ameliorating soil structure for the reservoir riparian: The influences of land use and dam-triggered flooding on soil aggregates

https://doi.org/10.1016/j.still.2021.105263Get rights and content

Highlights

  • Soil aggregate stability was dominated by dam-triggered flooding in the riparian.

  • Artificial mixed forests were mostly effective in improving SAS under low flooding intensity.

  • Natural grasslands were highly effective in stabilizing aggregates under high flooding intensity.

  • Land use strategies should consider flooding regime and soil erosion control in the riparian zone.

Abstract

Soil aggregates are considered to be strongly influenced by land use and hydrological regime. Yet, the relative importance of land use and flooding intensity on soil aggregate stability (SAS) in riparian zones remains unclear. Hence, the objective of this study was to examine the effects of land use and flooding intensity on SAS, as well as the aggregate-associated organic carbon (C) and nitrogen (N) contents in the riparian zone along the Three Gorges Reservoir of Yangtze River, China. In 2019, during the reservoir discharge period when the reservoir is at the lowest water level, SAS and aggregate-associated organic C and N contents were investigated along a gradient of the flooding intensity across riparian zones with different restoration practices, including natural grasslands, artificial mixed forests consisting of flood tolerant plants (mulberry and metasequoia), and transplanted lotus. Croplands (i.e., corn, paddy and peanut fields) at different elevations above sea level were used as references. The results showed that artificial mixed forests were more effective in improving the SAS than natural grasslands at elevations above 173 m, while natural grasslands were more effective in improving the SAS than artificial mixed forests and lotus at elevations below 173 m. However, riparian soil stability cannot be simply explained by land use management only, but rather the coupling effects of land uses and flooding intensity. Variation partitioning analysis showed that dam-triggered flooding intensity (21.5%) contributed more than land use (15.5%) in explaining riparian SAS. Also, the soil organic carbon (SOC) and total nitrogen (TN) had significantly positive correlations with the proportion of large macroaggregates (>2 mm) and SAS, indicating that maintaining higher levels of SOC and TN via the development of land use strategies commensurate with the flooding intensity was conducive to the formation and stability of soil aggregates. Meanwhile, the relative contribution of microaggregates (<0.053 mm) to bulk SOC or N content decreased with elevation. Therefore, the finding provides new insights into the focus of C sequestration, and suggests that C assessment should be shifted to microaggregates in the riparian zone. Overall, our study highlights that different land use strategies should be conducted for different elevations to promote SAS, aggregate-associated C and N accumulation in the riparian zone, which can provide a reference for ecological restoration in areas experiencing similar conditions.

Introduction

Soil aggregates are the basic units of soil structure controlling the dynamics of soil organic carbon (SOC) and nutrient cycling (Six et al., 2004, Liu et al., 2014). Organic materials are the main cementing agents in the formation and stabilization of aggregates (Tang et al., 2011). Soil aggregate stability (SAS) is a significant indicator to evaluate soil quality (Dou et al., 2020), which is affected by a series of biotic and abiotic factors, especially the concentrations of clay minerals (Schweizer et al., 2019), organic matter (Six et al., 2004), Fe and Al oxides (Zhao et al., 2017), soil microorganisms (Wu et al., 2020), wetting and drying cycles (Najera et al., 2020), land use management strategies (Guo et al., 2020), among others.

The extent, frequency and duration of water level fluctuations (WLFs) in lakes and rivers are dominant driving forces for the functioning of riparian ecosystems. Even small fluctuations can have prominent biological effects in riparian areas, especially in areas with low water levels, which can result in significant changes in the physical processes and biological productivity of soil (Leira and Cantonati, 2008). Additionally, WLFs are one of the key forces that form drying and wetting cycles, which inevitably have great impacts on soil aggregates and the availability of aggregate-associated C and nitrogen (N) by changes to the formation and stabilization conditions (Six et al., 2004, Graf-Rosenfellner et al., 2016, Ye et al., 2019). Mega reservoirs, such as the Three Gorges Reservoir (TGR) in China, are designed for flood control, power generation, and shipping purposes, often with remarkable WLFs since the full of damming operations in 2010. Water level fluctuates between 145 m in summer and 175 m in winter (Fig. 2), which forms riparian zones in the TGR with a total area of 349 km2 (Zhu et al., 2020).

Due to prolonged inundation in the riparian zone during the winter, serious environmental issues and the deterioration of ecological function have emerged and are increasingly concerning (Langer et al., 2008, Ye et al., 2014). Therefore, restoration of the riparian soil stability has become an important issue. Normally, artificial vegetation plantation and natural vegetation regeneration are recognized as important strategies to recover the structure and functionality of degraded ecosystems (Benayas et al., 2009, Trujillo-Miranda et al., 2018, Hu et al., 2021), and have been promoted in recent years as potential approaches to restore and protect the riparian ecosystem (Ye et al., 2014). Previous studies have demonstrated that both of the strategies can enhance the stability of soil aggregates, as well as the contents of aggregate-associated C and N (An et al., 2010, Erktan et al., 2016, Zeng et al., 2018). As compared with artificial vegetation plantation, natural vegetation restoration requires a long period to restore the functions of an ecosystem. Hence, many developing countries have prioritized artificial vegetation plantation for fragile ecosystems (Watson et al., 2000, Cao et al., 2021). However, it remains unclear whether artificial vegetation plantation or natural vegetation restoration is best to promote SAS of a fragile riparian ecosystem under drastic hydrological changes regulated by a river dam. Thus, it is necessary to compare soil aggregates under different land use strategies, which is crucial for successful vegetation recovery in a riparian zone (Ye et al., 2014).

Under the WLF conditions of the TGR, soil erosion through periodic exposure and submersion, as well as the large number of ecological restoration projects and disorganized land utilization, may have important influences on the formation and stabilization of soil aggregates in the riparian zone of the TGR. Jiang et al. (2015) reported that WLFs strongly influence the SAS of mulberry forestlands in the riparian zone of the TGR, while Qian et al. (2018) found that SAS and soil aggregate-associated organic C content decreased following land use conversion from abandoned to cultivated lands in the riparian zone. Previous studies have documented the effects of independent external factors (e.g., WLFs or land use) on SAS and aggregate-associated C and N contents. However, few studies have concentrated on synthesizing the effect of land use and flooding intensity on SAS and aggregate-associated C and N contents, particularly in the riparian ecosystems, where the soil structure and nutrient dynamics are more sensitive to changes to external factors than normal terrestrial ecosystems (Cui et al., 2018). In addition, the dominant driving mechanisms of SAS in the riparian zone of the TGR must be identified.

The three main land use strategies in the riparian zone of the TGR include restoration-oriented natural grassland, restoration-oriented artificial flood-tolerant vegetation planting and production-oriented land farming. Since riparian zones of the TGR are ecologically fragile due to drastic hydrologic changes, and natural grassland restoration requires a long period of time to restore ecosystem functions, we hypothesized that the dam-triggered flooding intensity could affect more on SAS than land uses, and artificial flood-tolerant vegetation planting would be the most effective in maintaining SAS, aggregate-associated C and N among the three land use strategies in the short term (i.e., less than a decade). To test the hypothesis, our study aims: (1) to investigate SAS and aggregate-associated C and N among the three land use strategies; (2) to quantify the influences of land uses and dam-triggered flooding intensity on SAS.

Section snippets

Site description

The study sites are located at Dalangba (31°08′30″N, 108°30′50″E), Baijiaxi (31°08′30″N, 108°31′20″E) and Wuyangwan (31°11′30″N, 108°28′0″E), representing typical land use in the riparian zone of the Pengxi River, southeast of the Kaizhou District, Chongqing, southwestern China (Fig. 1, Tables S1 and S2). The Pengxi River, a main branch of the Yangtze River, is in the upper reaches of the TGR. The water level of the Pengxi River, which is regulated by the Three Gorges Dam, fluctuates annually

Basic soil properties

Soil properties, including soil bulk density (SBD), pH, SOC, total nitrogen (TN), C:N ratio, and soil textural class, are presented in Table 2 and Table S2. All study sites are classified as silt loamy soils. Although the soil type, soil textural class, the topographic and meteorological conditions are similar along the same elevation (i.e., hydrologic regime) among the three study areas, different land uses resulted in significant differences in soil properties.

Along the 173 m elevation

Responses of SAS to land use strategies

Differences in land use strategies were reflected in the vegetation structure and species composition. SOC is considered one of the main binding agents favoring soil aggregation (Pinheiro et al., 2004). The intrinsic characteristics of vegetation structure and species composition change along with changes in land use (Luo and Zhou, 2006, Zhu et al., 2020), which affects the SOC content, thus directly or indirectly impacting SAS (Abiven et al., 2009, Erktan et al., 2016, Zhao et al., 2017, Qian

Conclusions

We studied the impacts of multiple land use strategies and flooding intensity on soil aggregates and its aggregate-associated SOC, N content in reservoir riparian ecosystems. The results showed that artificial mixed forests were most effective in improving the SAS above 173 m, while natural grasslands were most effective in improving the SAS at elevations below 173 m, implying that different land use strategies should be adopted for different elevations to promote vegetation restoration and

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

We appreciate the valuable comments from the editors and anonymous reviewers which greatly improved the manuscript. This study was financially supported by the National Natural Science Foundation of China (41771266, 41401243), Research Fund of State Key Laboratory of Soil and Sustainable Agriculture, Nanjing Institute of Soil Science, Chinese Academy of Sciences (No. Y812000005), the Youth Innovation Promotion Association of the Chinese Academy of Sciences (No. 2017391), and Chongqing Buearu of

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