# The Impacts of the Hydrological Regime on the Soil Aggregate Size Distribution and Stability in the Riparian Zone of the Three Gorges Reservoir, China

^{1}

^{2}

^{3}

^{*}

## Abstract

**:**

_{MWD}, PAD

_{GMD}, PAD

_{ASR}, and the fractal dimension demonstrated a reverse trend. It can thus be deduced that the hydrological regime of the TGR significantly modified the aggregate size distribution and dramatically reduced the aggregate stability, which may provide a crucial basis for assessing the soil erosion in similar riparian zones.

## 1. Introduction

^{2}. Laterally, it covers a vertical height of 30 m from 145 m to 175 m. Temporally, its water level fluctuates anti-seasonally between the base level of 145 m during the wet season (May to September) and the peak level of 175 m during the dry season (October to April), suggesting that the winter flooding regime of the TGR is opposite to the seasonal summer flooding regime of the natural river. Besides its anthropogenic-regulated water level fluctuation, its altered hydrological regime is also influenced by rainfall and waves, reflecting a variable flooding regime (e.g., flooding timing, magnitude, frequency, intensity, and duration) along its elevation gradient. These changes have resulted in highly spatial–temporal heterogeneities in the geomorphological, geochemical, and ecological processes that interact with the diverse bedrocks, soil types, topography, vegetation species, and human activities in the riparian zone of the TGR [21].

## 2. Materials and Methods

#### 2.1. Study Area

#### 2.2. Field Sampling

#### 2.3. Soil Properties Analysis

#### 2.4. Soil Aggregate Analysis

_{MWD}, PAD

_{GMD}, and PAD

_{ASR}values, the weaker aggregate stability. In contrast, the larger the D value, the weaker aggregate stability, while this is the opposite for the PAD

_{D}. Detailed equations for the related indicators are presented below:

_{i}is the mean aperture of the adjacent i and i + 1 sieves; n is the number of sieves; m

_{i}is the mass of the aggregates on the ith sieve; m is the total mass of the aggregates; MWD

_{d}and MWD

_{w}are the mean weight diameters of the dry-sieved and wet-sieved aggregates, respectively; GMD

_{d}and GMD

_{w}are the geometric mean diameters of the dry-sieved and wet-sieved aggregates, respectively; m

_{j}is the mass of the aggregates larger than 0.25 mm; ASR

_{d}is the mass proportion of the dry-sieved aggregates larger than 0.25 mm; ASR

_{w}is the mass proportion of the wet-sieved aggregates larger than 0.25 mm; w

_{i}is the mass of the aggregates with a size less than X

_{i}; X

_{max}is the mean diameter of the aggregates at the top sieve; and D

_{d}and D

_{w}are the fractal dimensions of the dry-sieved and wet-sieved aggregates, respectively.

#### 2.5. Statistical Analysis

## 3. Results

#### 3.1. Soil Physical and Chemical Properties

^{−3}in CK to 1.56 g·cm

^{−3}in LI, while the total porosity revealed a reduction from 52.31% in CK to 41.00% in LI. The soils were alkaline with pH values ranging from 8.19 to 8.59. The bulk density, total porosity, and pH were significantly different between the soils in the upland hillslopes and the soils in the riparian zone (p < 0.05), whilst the differences were not statistically significant among the three sampling transects within the riparian zone (p > 0.05). The soil organic carbon (SOC) was significantly different among the four sampling transects and it reflected a dramatic dwindling of 2.03 g·kg

^{−1}, 4.23 g·kg

^{−1}, and 8.43 g·kg

^{−1}in the riparian soils of SI, MI, and LI in comparison to CK, respectively. These results suggested that the hydrological regime of the TGR varied in the soil basic properties of the riparian zone, and that the modification of the SOC was particularly prominent.

#### 3.2. Size Distribution of Mechanically Stable Aggregates

#### 3.3. Size Distribution of Water-Stable Aggregates

#### 3.4. Aggregate Stability along the Elevation Gradient

_{MWD}, PAD

_{GMD}, PAD

_{ASR}, and PAD

_{D}), is presented in Figure 3. On the one hand, the MWD, GMD, and ASR of both the mechanically stable aggregates and water-stable aggregates showed similar successive decreases to the decrease in the elevation gradient, whilst the D

_{d}and D

_{w}changed in the order of CK < SI < MI < LI. Compared to CK, the MWD

_{d}of SI, MI, and LI decreased by 6.60%, 6.67%, and 8.28%, and the MWD

_{w}of SI, MI, and LI dropped by 3.27%, 11.73%, and 54.12%, respectively. Similarly, the GMD

_{d}of SI, MI, and LI declined by 13.07%, 14.89%, and 19.03% in comparison to that of CK, and the GMD

_{w}of SI, MI, and LI went down by 5.61%, 23.73%, and 70.52% in comparison that of CK. Meanwhile, the highest ASR

_{d}(98.78%) and ASR

_{w}(92.75%) appeared in CK, while the ASR

_{d}and ASR

_{w}of LI were the lowest, accounting for 95.25% and 73.18%, respectively. Additionally, the D

_{d}demonstrated successive increments and grew from 1.82 in CK to 2.23 in LI and the D

_{w}increased from 2.41 in CK to 2.63 in LI. On the other hand, the PAD indexes showed a reverse trend to the corresponding aggregate stability indicators. The PAD

_{MWD}increased dramatically from 3.12% in CK to 53.20% in LI and the PAD

_{GMD}increased from 15.59% in CK to 71.63% in LI. Likewise, the PAD

_{ASR}presented a dramatic rise, in that the PAD of LI was approximately four times higher than that of CK. In contrast, the PAD

_{D}gradually declined from 32.08% in CK to 18.17% in LI. Regarding the four indicators of aggregate mechanical stability (i.e., MWD

_{d}, GMD

_{d}, ASR

_{d}, and D

_{d}), there were no significant differences between SI and MI, while significant differences appeared between CK and LI (p < 0.05). As for the four indicators of aggregate water stability (i.e., MWD

_{w}, GMD

_{w}, ASR

_{w}, and D

_{w}), there were no significant differences between CK and SI, while the differences were statistically significant between LI and the other three groups (p < 0.05). In terms of the aggregate destruction indexes, the PAD

_{MWD}, PAD

_{ASR}, and PAD

_{D}in LI were significantly different from those in MI, SI, and CK (p < 0.05), while an insignificant difference for PAD

_{GMD}only appeared between MI and SI. These results indicated that the hydrological regime reduced the aggregate mechanical stability and water stability, and that the longer the inundation lasted, the more unstable the soil aggregates were.

#### 3.5. Determinants of Soil Aggregate Stability

_{d}and D

_{w}showed a reverse significant correlation with the soil properties. On the other hand, there were statistically significant correlations between the aggregate destruction indexes (i.e., PAD

_{MWD}, PAD

_{GMD}, PAD

_{ASR}, and PAD

_{D}) and the soil properties. The PAD

_{MWD}, PAD

_{GMD}, and PAD

_{ASR}were positively correlated with sand and BD, whist these indexes were negatively correlated with silt, clay, and SOC. Additionally, the correlations between the PAD

_{D}and soil composition (i.e., sand, silt, and clay) were statistically insignificant (p > 0.05), while the significant correlation coefficients between the PAD

_{D}and BD, as well as the SOC, were −0.63 and 0.55, respectively. Except for a significant correlation between the SOC and PAD

_{D}, an extremely significant correlation appeared between the SOC and the other aggregate stability indicators and aggregate destruction indexes. These results demonstrated that the soil composition, bulk density, and soil organic carbon had significant effects on the aggregate stability, and that the SOC was the most important factor controlling this aggregate stability.

## 4. Discussion

_{D}, significantly decreased, whilst the PAD

_{MWD}, PAD

_{GMD}, PAD

_{ASR}, D

_{d}, and D

_{w}dramatically increased. These results indicated that the hydrological regime of the TGR disintegrated the soil aggregates and reduced the aggregate stability. The longer the inundation was, the higher the disaggregation, and a lower aggregate stability was presented.

## 5. Conclusions

_{D}significantly decreased, whilst the PAD

_{MWD}, PAD

_{GMD}, PAD

_{ASR}, and D remarkably increased, suggesting that the hydrological regime reduced the aggregate stability and that the aggregate stability gradually decreased with an increase in the inundation duration. Furthermore, the statistical analysis showed that the differences in the MWD

_{w}, GMD

_{w}, PAD

_{GMD}, and D

_{w}were more significant within the elevation gradient than the other indicators, meaning that the MWD

_{w}, GMD

_{w}, PAD

_{GMD}, and D

_{w}were more sensitive to the hydrological regime and more suitable for indicating the aggregate stability in this study area. These findings may provide implications for the study of soil erosion in similar riparian zones, but further studies are still needed to focus on the mechanisms of aggregate breakdown and the relationships between aggregate stability and soil erosion.

## Author Contributions

## Funding

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## References

- Rabot, E.; Wiesmeier, M.; Schlüter, S.; Vogel, H.J. Soil structure as an indicator of soil functions: A review. Geoderma
**2018**, 314, 122–137. [Google Scholar] [CrossRef] - Munkholm, L.J.; Heck, R.J.; Deen, B. Soil pore characteristics assessed from X-ray micro-CT derived images and correlations to soil friability. Geoderma
**2012**, 181–182, 22–29. [Google Scholar] [CrossRef] - Le Bissonnais, Y. Aggregate stability and assessment of soil crustability: I. Theory and methodology. Eur. J. Soil Sci.
**1996**, 47, 425–437. [Google Scholar] [CrossRef] - Le Bissonnais, Y.; Arrouays, D. Aggregate stability and assessment of soil crustability and erodibility: II. Application to humic loamy soils with various organic carbon contents. Eur. J. Soil Sci.
**1997**, 48, 39–48. [Google Scholar] [CrossRef] - Nsabimana, G.; Hong, L.; Yuhai, B.; de Dieu Nambajimana, J.; Jinlin, L.; Ntacyabukura, T.; Xiubin, H. Soil aggregate disintegration effects on soil erodibility in the water level fluctuation zone of the Three Gorges Reservoir, China. Environ. Res.
**2023**, 217, 114928. [Google Scholar] [CrossRef] [PubMed] - Rohošková, M.; Valla, M. Comparison of two methods for aggregate stability measurement—A review. Plant Soil Environ.
**2004**, 50, 379–382. [Google Scholar] [CrossRef] - Saygın, S.D.; Cornelis, W.M.; Erpul, G.; Gabriels, D. Comparison of different aggregate stability approaches for loamy sand soils. Appl. Soil Ecol.
**2012**, 54, 1–6. [Google Scholar] [CrossRef] - Seybold, C.; Herrick, J. Aggregate stability kit for soil quality assessments. Catena
**2001**, 44, 37–45. [Google Scholar] [CrossRef] - Barthès, B.; Roose, E. Aggregate stability as an indicator of soil susceptibility to runoff and erosion; validation at several levels. Catena
**2002**, 47, 133–149. [Google Scholar] [CrossRef] - Wu, X.; Wei, Y.; Wang, J.; Wang, D.; She, L.; Wang, J.; Cai, C. Effects of soil physicochemical properties on aggregate stability along a weathering gradient. Catena
**2017**, 156, 205–215. [Google Scholar] [CrossRef] - Cosentino, D.; Chenu, C.; Le Bissonnais, Y. Aggregate stability and microbial community dynamics under drying–wetting cycles in a silt loam soil. Soil Biol. Biochem.
**2006**, 38, 2053–2062. [Google Scholar] [CrossRef] - Utomo, W.; Dexter, A. Changes in soil aggregate water stability induced by wetting and drying cycles in non-saturated soil. J. Soil Sci.
**1982**, 33, 623–637. [Google Scholar] [CrossRef] - Hochman, D.; Dor, M.; Mishael, Y. Diverse effects of wetting and drying cycles on soil aggregation: Implications on pesticide leaching. Chemosphere
**2021**, 263, 127910. [Google Scholar] [CrossRef] [PubMed] - Peng, X.; Hallett, P.D.; Zhang, B.; Horn, R. Physical response of rigid and non-rigid soils to analogues of biological exudates. Eur. J. Soil Sci.
**2011**, 62, 676–684. [Google Scholar] [CrossRef] - Rahman, M.T.; Guo, Z.C.; Zhang, Z.B.; Zhou, H.; Peng, X.H. Wetting and drying cycles improving aggregation and associated C stabilization differently after straw or biochar incorporated into a Vertisol. Soil Tillage Res.
**2018**, 175, 28–36. [Google Scholar] [CrossRef] - Tang, Q.; Collins, A.L.; Wen, A.; He, X.; Bao, Y.; Yan, D.; Long, Y.; Zhang, Y. Particle size differentiation explains flow regulation controls on sediment sorting in the water-level fluctuation zone of the Three Gorges Reservoir, China. Sci. Total Environ.
**2018**, 633, 1114–1125. [Google Scholar] [CrossRef] - Cui, J.; Tang, X.; Zhang, W.; Liu, C. The Effects of Timing of Inundation on Soil Physical Quality in the Water-Level Fluctuation Zone of the Three Gorges Reservoir Region, China. Vadose Zone J.
**2018**, 17, 180043. [Google Scholar] [CrossRef] - Bao, Y.; He, X.; Wen, A.; Gao, P.; Tang, Q.; Yan, D.; Long, Y. Dynamic changes of soil erosion in a typical disturbance zone of China’s Three Gorges Reservoir. Catena
**2018**, 169, 128–139. [Google Scholar] [CrossRef] - Bao, Y.; Yu, D.; Tang, Q.; He, X.; WEi, J.; Hu, Y.; Li, J. Combined Effects of Hillslope-Concentrated Flows and Riverine Stream Waves on Soil Erosion in the Reservoir Riparian Zone. Water
**2021**, 13, 3465. [Google Scholar] [CrossRef] - Tang, Q.; Bao, Y.; He, X.; Fu, B.; Collins, A.L.; Zhang, X. Flow regulation manipulates contemporary seasonal sedimentary dynamics in the reservoir fluctuation zone of the Three Gorges Reservoir, China. Sci. Total Environ.
**2016**, 548–549, 410–420. [Google Scholar] [CrossRef] - Bao, Y.; Gao, P.; He, X. The water-level fluctuation zone of Three Gorges Reservoir—A unique geomorphological unit. Earth Sci. Rev.
**2015**, 150, 14–24. [Google Scholar] [CrossRef] - Pires, L.F.; Auler, A.C.; Roque, W.L.; Mooney, S.J. X-ray microtomography analysis of soil pore structure dynamics under wetting and drying cycles. Geoderma
**2020**, 362, 114103. [Google Scholar] [CrossRef] - Amézketa, E. Soil Aggregate Stability: A Review. J. Sustain. Agric.
**1999**, 14, 83–151. [Google Scholar] [CrossRef] - Hussein, J.; Adey, M. Changes in microstructure, voids and b-fabric of surface samples of a Vertisol caused by wet/dry cycles. Geoderma
**1998**, 85, 63–82. [Google Scholar] [CrossRef] - Peng, X.; Horn, R.; Smucker, A. Pore Shrinkage Dependency of Inorganic and Organic Soils on Wetting and Drying Cycles. Soil Sci. Soc. Am. J.
**2007**, 71, 1095–1104. [Google Scholar] [CrossRef] - Ran, Y.; Ma, M.; Liu, Y.; Zhou, Y.; Sun, X.; Wu, S.; Huang, P. Hydrological stress regimes regulate effects of binding agents on soil aggregate stability in the riparian zones. Catena
**2021**, 196, 104815. [Google Scholar] [CrossRef] - Ran, Y.; Ma, M.; Liu, Y.; Zhu, K.; Yi, X.; Wang, X.; Wu, S.; Huang, P. Physicochemical determinants in stabilizing soil aggregates along a hydrological stress gradient on reservoir riparian habitats: Implications to soil restoration. Ecol. Eng.
**2020**, 143, 105664. [Google Scholar] [CrossRef] - Nsabimana, G.; Bao, Y.; He, X.; Nambajimana, J.d.D.; Wang, M.; Yang, L.; Li, J.; Zhang, S.; Khurram, D. Impacts of Water Level Fluctuations on Soil Aggregate Stability in the Three Gorges Reservoir, China. Sustainability
**2020**, 12, 9107. [Google Scholar] [CrossRef] - Nsabimana, G.; Bao, Y.; He, X.; Nambajimana, J.d.D.; Yang, L.; Li, J.; Uwiringiyimana, E.; Nsengumuremyi, P.; Ntacyabukura, T. Soil aggregate stability response to hydraulic conditions in water level fluctuation zone of the Three Gorges Reservoir, China. Catena
**2021**, 204, 105387. [Google Scholar] [CrossRef] - He, X.; Bao, Y.; Nan, H.; Xiong, D.; Wang, L.; Liu, Y.; Zhao, J. Tillage pedogenesis of purple soils in southwestern China. J. Mt. Sci.
**2009**, 6, 205–210. [Google Scholar] [CrossRef] - Zhang, S.-J.; Tang, Q.; Bao, Y.-H.; He, X.-B.; Tian, F.-X.; Lü, F.-Y.; Wang, M.-F.; Anjum, R. Effects of seasonal water-level fluctuation on soil pore structure in the Three Gorges Reservoir, China. J. Mt. Sci.
**2018**, 15, 2192–2206. [Google Scholar] [CrossRef] - Yoder, R. A direct method of aggregate analysis of soils and a study of the physical natural of erosion losses. J. Am. Soc. Agron.
**1936**, 28, 337–351. [Google Scholar] [CrossRef] - Kemper, W.; Rosenau, R. Aggregate Stability and Size Distribution. In Methods of Soil Analysis, Part 1: Physical and Mineralogical Methods; Klute, A., Ed.; ASA and SSA: Madison, WI, USA, 1986; pp. 425–442. [Google Scholar]
- Zheng, T.; Yang, J.; Zhang, J.; Tang, C.; Liao, K.; Liu, Y. Factors contributing to aggregate stability at different particle sizes in ultisols from Southern China. J. Soils Sediments
**2018**, 19, 1342–1354. [Google Scholar] [CrossRef] - Van Bavel, C. Mean weight-diameter of soil aggregates as a statistical index of aggregation. Soil Sci. Soc. Am. J.
**1950**, 14, 20–23. [Google Scholar] [CrossRef] - Mazurak, A. Effect of gaseous phase on water-stable synthetic aggregates. Soil Sci.
**1950**, 69, 135–148. [Google Scholar] [CrossRef] - Tyler, S.; Wheatcraft, S. Fractal scaling of soil particle-size distribution: Analysis and limitation. Soil Sci. Soc. Am. J.
**1992**, 56, 362–369. [Google Scholar] [CrossRef] - Zhang, B.; Horn, R. Mechanisms of aggregate stabilization in Ultisols from subtropical China. Geoderma
**2001**, 99, 123–145. [Google Scholar] [CrossRef] - Steiger, J.; Tabacchi, E.; Dufour, S.; Corenblit, D.; Peiry, J.L. Hydrogeomorphic processes affecting riparian habitat within alluvial channel-floodplain river systems: A review for the temperate zone. River Res. Appl.
**2005**, 21, 719–737. [Google Scholar] [CrossRef] - Ran, Y.; Wu, S.; Zhu, K.; Li, W.; Liu, Z.; Huang, P. Soil types differentiated their responses of aggregate stability to hydrological stresses at the riparian zones of the Three Gorges Reservoir. J. Soils Sediments
**2020**, 20, 951–962. [Google Scholar] [CrossRef] - Hu, B.; Wang, Y.; Wang, B.; Wang, Y.; Liu, C.; Wang, C. Impact of drying-wetting cycles on the soil aggregate stability of Alfisols in southwestern China. J. Soil Water Conserv.
**2018**, 73, 469–478. [Google Scholar] [CrossRef] - Ye, C.; Butler, O.M.; Chen, C.; Liu, W.; Du, M.; Zhang, Q. Shifts in characteristics of the plant-soil system associated with flooding and revegetation in the riparian zone of Three Gorges Reservoir, China. Geoderma
**2020**, 361, 114015. [Google Scholar] [CrossRef] - Ye, C.; Zhang, K.; Deng, Q.; Zhang, Q. Plant communities in relation to flooding and soil characteristics in the water level fluctuation zone of the Three Gorges Reservoir, China. Environ. Sci. Pollut Res. Int.
**2013**, 20, 1794–1802. [Google Scholar] [CrossRef] - Zhu, Z.; Chen, Z.; Li, L.; Shao, Y. Response of dominant plant species to periodic flooding in the riparian zone of the Three Gorges Reservoir (TGR), China. Sci. Total Environ.
**2020**, 747, 141101. [Google Scholar] [CrossRef] [PubMed] - Bronick, C.J.; Lal, R. Soil structure and management: A review. Geoderma
**2005**, 124, 3–22. [Google Scholar] [CrossRef] - Six, J.; Bossuyt, H.; Degryze, S.; Denef, K. A history of research on the link between (micro)aggregates, soil biota, and soil organic matter dynamics. Soil Tillage Res.
**2004**, 79, 7–31. [Google Scholar] [CrossRef] - Zhong, R.; He, X.; Bao, Y.; Tang, Q.; Gao, J.; Yan, D.; Wang, M.; Li, Y. Estimation of soil reinforcement by the roots of four post-dam prevailing grass species in the riparian zone of Three Gorges Reservoir, China. J. Mt. Sci.
**2016**, 13, 508–521. [Google Scholar] [CrossRef] - Peng, X.; Yan, X.; Zhou, H.; Zhang, Y.Z.; Sun, H. Assessing the contributions of sesquioxides and soil organic matter to aggregation in an Ultisol under long-term fertilization. Soil Tillage Res.
**2015**, 146, 89–98. [Google Scholar] [CrossRef] - Tisdall, J.M.; Oades, J.M. Organic matter and water-stable aggregates in soils. J. Soil Sci.
**1982**, 33, 141–163. [Google Scholar] [CrossRef] - Zhong, R.; Hu, J.; Bao, Y.; Wang, F.; He, X. Soil nutrients in relation to vertical roots distribution in the riparian zone of Three Gorges Reservoir, China. J. Mt. Sci.
**2018**, 15, 1498–1509. [Google Scholar] [CrossRef]

**Figure 2.**The water level hydrograph and water level duration curve in the Three Gorges Reservoir (

**a**); the flow hydrograph and flow duration curve in the Three Gorges Reservoir (

**b**); and schematic diagram of sampling design (

**c**). Data on reservoir water level and flow were obtained from the China Three Gorges Corporation (http://www.ctg.com.cn/, accessed on 31 December 2016).

**Figure 3.**Aggregate stability indicators and aggregate destruction indexes at four sampling transects in the riparian zone of the TGR: (

**a**) mean weight diameter of mechanically stable aggregates and water-stable aggregates and percentage of aggregate destruction of mean weight diameter; (

**b**) geometric mean diameter of mechanically stable aggregates and water-stable aggregates and percentage of aggregate destruction of geometric mean diameter; (

**c**) aggregate stability rate of mechanically stable aggregates and water-stable aggregates and percentage of aggregate destruction of aggregate stability rate; and (

**d**) fractal dimension of mechanically stable aggregates and water-stable aggregates and percentage of aggregate destruction of fractal dimension. CK (none inundation, 175–180 m); SI (short-term inundation, 172–175 m); MI (moderate inundation, 156–172 m); and LI (long-term inundation, 145–156 m). Bars indicate standard deviation. Different letters (a, b, and c) indicate significant differences among four sampling transects at the p < 0.05 level (LSD).

**Figure 4.**Correlation analysis between soil basic properties and aggregate stability. MWD

_{d}and MWD

_{w}are mean weight diameters of mechanically stable and water-stable aggregates, respectively; GMD

_{d}and GMD

_{w}are geometric mean diameters of mechanically stable and water-stable aggregates, respectively; ASR

_{d}and ASR

_{w}are the mass proportions of the mechanically stable and water-stable aggregates larger than 0.25 mm; D

_{d}and D

_{w}are fractal dimensions of mechanically stable and water-stable aggregates; PAD

_{MWD}, PAD

_{GMD}, PAD

_{ASR}, and PAD

_{D}are percentages of aggregate destruction of MWD, GMD, ASR, and D; BD, bulk density; and SOC, soil organic carbon. The orange color means positive correlation, while the blue color means negative correlation; the lighter color and smaller circle means a lower correlation coefficient, while the darker color and bigger circle means a higher correlation coefficient.

**Table 1.**Basic information on the sampling transects along the elevation gradient in the riparian zone of the Three Gorges Reservoir.

Sampling Transects | Inundation Year (Year) | Inundation Month | Inundation Day (Day) | Inundation Height (m) | Soil Type | Vegetation Cover (%) | Slop Gradient (°) |
---|---|---|---|---|---|---|---|

CK | 0 | 0 | 0 | 0 | Entisol | 72 | 3–5 |

SI | 6 | October–January | 3–97 | 0–3 | Entisol | 70 | 3–5 |

MI | 8 | September–April | 97–249 | 3–19 | Entisol | 67 | 4–7 |

LI | 10 | September–May | 249–365 | 19–30 | Entisol | 63 | 5–9 |

**Table 2.**Soil physicochemical properties of the sampling transects along the elevation gradient in the riparian zone of the TGR.

Sampling Transects | Sand (%) | Silt (%) | Clay (%) | BD (g·cm ^{−3)} | Total Porosity (%) | pH | SOC (g·kg ^{−1}) |
---|---|---|---|---|---|---|---|

CK | 8.17 ± 0.81 c | 88.18 ± 0.44 a | 3.65 ± 0.43 a | 1.26 ± 0.21 b | 52.31 ± 8.08 a | 8.19 ± 0.05 b | 12.59 ± 1.51 a |

SI | 10.32 ± 0.63 c | 86.52 ± 0.45 a | 3.16 ± 0.19 a | 1.42 ± 0.12 a | 46.23 ± 4.65 b | 8.59 ± 0.06 a | 10.56 ± 1.22 b |

MI | 16.65 ± 2.90 b | 80.15 ± 2.86 b | 3.20 ± 0.25 a | 1.49 ± 0.15 a | 43.65 ± 5.49 b | 8.55 ± 0.24 a | 8.36 ± 1.06 c |

LI | 23.39 ± 5.56 a | 74.21 ± 4.93 c | 2.40 ± 0.72 b | 1.56 ± 0.13 a | 41.00 ± 4.95 b | 8.59 ± 0.05 a | 4.16 ± 0.99 d |

**Table 3.**Differential size distribution of mechanically stable aggregates at four sampling transects in the riparian zone of the TGR.

Sampling Transects | Mass Percentage of Aggregates at Different Sizes (%) | |||||
---|---|---|---|---|---|---|

>5 mm | 5–2 mm | 2–1 mm | 1–0.5 mm | 0.5–0.25 mm | <0.25 mm | |

CK | 79.15 ± 3.47 a | 13.28 ± 2.24 b | 4.31 ± 0.82 b | 1.30 ± 0.27 b | 0.74 ± 0.13 b | 1.22 ± 0.01 b |

SI | 70.73 ± 1.30 b | 15.23 ± 1.76 ab | 6.81 ± 0.15 a | 2.74 ± 0.13 a | 2.49 ± 0.35 ab | 2.00 ± 0.14 b |

MI | 69.87 ± 2.03 b | 14.61 ± 0.68 ab | 6.89 ± 0.51 a | 2.73 ± 0.27 a | 2.85 ± 0.45 a | 3.05 ± 0.57 ab |

LI | 64.25 ± 1.24 b | 17.90 ± 0.54 a | 8.26 ± 0.44 a | 2.82 ± 0.21 a | 2.02 ± 0.29 ab | 4.75 ± 0.09 a |

**Table 4.**Differential size distribution of water-stable aggregates at four sampling transects in the riparian zone of the TGR.

Sampling Transects | Mass Percentage of Aggregates at Different Sizes (%) | |||||
---|---|---|---|---|---|---|

>5 mm | 5–2 mm | 2–1 mm | 1–0.5 mm | 0.5–0.25 mm | <0.25 mm | |

CK | 77.35 ± 1.51 a | 8.00 ± 0.68 b | 3.03 ± 0.30 b | 2.58 ± 0.33 c | 1.80 ± 0.08 b | 7.25 ± 0.33 b |

SI | 68.35 ± 2.11 a | 11.73 ± 0.53 b | 4.28 ± 0.35 b | 4.40 ± 0.48 c | 3.30 ± 0.36 ab | 7.95 ± 0.70 b |

MI | 64.29 ± 1.29 b | 10.79 ± 0.51 b | 5.10 ± 0.28 b | 4.97 ± 0.32 b | 3.40 ± 0.20 b | 11.45 ± 0.62 b |

LI | 21.91 ± 2.95 c | 13.98 ± 1.10 a | 11.58 ± 1.05 a | 13.78 ± 0.84 a | 11.94 ± 1.43 a | 26.83 ± 2.92 a |

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content. |

© 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Zhang, S.; Chen, T.; Bao, Y.; Tang, Q.; Li, Y.; He, X.
The Impacts of the Hydrological Regime on the Soil Aggregate Size Distribution and Stability in the Riparian Zone of the Three Gorges Reservoir, China. *Water* **2023**, *15*, 1791.
https://doi.org/10.3390/w15091791

**AMA Style**

Zhang S, Chen T, Bao Y, Tang Q, Li Y, He X.
The Impacts of the Hydrological Regime on the Soil Aggregate Size Distribution and Stability in the Riparian Zone of the Three Gorges Reservoir, China. *Water*. 2023; 15(9):1791.
https://doi.org/10.3390/w15091791

**Chicago/Turabian Style**

Zhang, Shujuan, Tianyi Chen, Yuhai Bao, Qiang Tang, Yongtao Li, and Xiubin He.
2023. "The Impacts of the Hydrological Regime on the Soil Aggregate Size Distribution and Stability in the Riparian Zone of the Three Gorges Reservoir, China" *Water* 15, no. 9: 1791.
https://doi.org/10.3390/w15091791