Drought and soil erosion are two environmental problems on loess hillslopes. Runoff, which forms when the rainfall intensity exceeds the infiltration capacity of the soil, is the direct cause of slope erosion by detaching and transporting displaced soil particles while flowing across the soil surface. Crust formation at the surface of cultivated soils exposed to the impact of raindrops is a common phenomenon, which is formed by the combined effect of physical disintegration of soil aggregates on the surface by the impact energy of raindrops and physicochemical dispersion of soil clays. It can disperse cultivated soils aggregates into micro-aggregates, reduce infiltration rate and increase runoff and erosion [1
], which causes negative effects on the survival and growth of plants in arid and semiarid regions. Therefore, improving soil physical characteristics and reducing runoff by increasing rainfall infiltration can be considered a vital measurement for controlling water and soil loss.
Some studies indicate that soil amendments such as gypsum and polymers can improve the stability of soil structure by electrostatic absorption of polymer molecules on the clay particles [4
], thereby preventing crust formation, increasing infiltration and reducing surface runoff as well as soil erosion [5
]. The application of various synthetic polymers and surfactants has been reported in the technical literatures. Botha et al. [6
] investigated the influence of polymer PVA on the liquid–solid contact angles of a fine sandy soil. Pini and Vigna [7
] found the formation of soil micro-aggregates by using two uncharged polymers, PVA and detrans, to study the interaction of water soluble stabilizing agents with soil particles. Floyd [8
] studied PVAc emulsion as a soil conditioner. Three types of polyurethane were used in soil stabilization to improve the erosion resistance [9
], and the result indicated that the polyurethanes improved both strength and erosion resistance significantly.
Among these soil conditioners, polyacrylamide (PAM) was widely used and was a particularly effective polymer in improving soil aggregation, increasing water infiltration and preventing erosion [10
]. About 400,000 ha of agricultural land has been treated each year with PAM in the USA [11
]. Many researchers have indicated that the application of anionic polymer could significantly improve soil physical properties, namely increasing water-stable aggregates, reducing tensile strength, bulk density, surface compaction and clay dispersion, and delaying runoff formation and decreasing erosion and runoff [12
]. However, Yu et al. [14
] found that spreading granular PAM alone was only beneficial for reducing erosion but not for maintaining high infiltration rate (IR), and only the addition of gypsum with granular PAM was effective for increasing infiltration rate through simulated rain. Other researches also showed lower rates of PAM application decreased runoff while the higher rates increased runoff [15
]. The effect of PAM was influenced by the clay content of soil. Increasing the clay content of soil improved the effect of PAM treatments, and negative impact on some coarser texture soils may exist [13
]. Vacher et al. [13
] found that application of PAM on the soil surface on steep slopes did not reduce runoff significantly, but application of polyacrylamide with gypsum was effective for runoff reduction.
Previous studies indicate that soil amendments are a promising way to improve soil physical properties and reduce soil erosion. However, the effect of soil amendments depends on its physiochemical properties, the mode of application and its quantity which also varies from time and soils. Tang et al. [16
] showed that the negative effects of soil amendments were observed on barren or salty soils, and the application rate, either low or high contents of soil amendments, hardly improved soil properties and reduced soil erosion. Other researchers also treated the soil with different integrated chemicals to obtain better results. For example, in order to find better polymers, Wu et al. [17
] applied polypropylene acid (PPA), polythene alcoholic (PTA) and urea-formaldehyde poly-condensate (UR) to China’s loess areas through indoor laboratory experiments and outdoor artificial rainfall simulations. The results indicated that the amendment applications could decrease the erosive forces of raindrops, increase the water-stable soil aggregates contents, reduce surface crusting and improve rainfall infiltration. Additionally, in order to pursue better polymers, Liu et al. [18
] studied the effect of new polymer NPD on sheet erosion of experimental loess slopes through simulated rainfall. The results indicated that NPD effectively delayed the onset, reduced volume and sediment content of the runoff by significantly increasing the shear strength and the content of large aggregates from soil surface. Therefore, it is necessary to develop new and effective macromolecular polymers to regulate rainfall infiltration in order to reduce surface runoff and soil loss. Jag S and Jag C162 are two new natural polymer derivatives in our research, as SOLVAY polymers that are extracted from bean embryos. These are green chemicals showing no irritation and no known adverse effects on aquatic species on which they were tested.
The objectives of this research were to: (i) determine the effects of Jag S and Jag C162, two new natural polymer derivatives, at different rates on soil loss and rainfall infiltration; (ii) reveal the mechanisms responsible for their effects by analyzing the proportions of water-stable soil aggregates of different sizes and shear strength after Jag S and Jag C162 were spread under simulated rainfall on experimental loess hillslope.
2. Materials and Methods
2.1. Soil and Polymers
The soil samples for testing were collected from Ansai County of the Loess Plateau (a typical region with hills and gullies). Ansai (109°19’ E, 36°51’ N) located in northern Shaanxi Province has a mean annual temperature of 8.8 °C and an annual precipitation of 500 mm. The soil was a silt loam (USDA) collected from the farming layer with a depth about 25 cm. The contents of organic matter, clay, silt and sand were 0.5%, 8.7%, 54.7% and 36.6% respectively. The d50 was 0.037 mm. The soil was air dried, crushed, well mixed and then passed through a 10-mm sieve to remove weeds and stones. The polymeric compounds tested were natural polymer derivatives, neutral polysaccharide (Jag S) and cationic hydroxypropyl polysaccharide (Jag C162), as SOLVAY polymers that are extracted from bean embryos, and both are easy to dissolve in water. Jag S and Jag C162 are both polysaccharide; however, Jag S is neutral, and Jag C162 consists of cationic hydroxypropyl and polysaccharide, respectively. In addition, lab studies showed that they were green chemicals that have shown no irritating or adverse effects on aquatic species, and were harmless to humans and can be used for preparations of personal care formulations.
Experiments were conducted in the Simulated Rainfall Hall of the State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau at the Institute of Soil and Water Conservation (Chinese Academy of Science and Ministry of Water Resources in China). A rainfall simulator system with a side-sprinkler was used to apply simulated rainfall. This rainfall simulator can be set to rainfall intensity ranging from 0.5 to 3.5 mm/min, by adjusting water pressure and nozzle sizes. The fall height of raindrop from the top to soil slope surface is 16 m. The uniformity of simulated rainfall is higher than 80%. The kinetic energy of raindrop to strike soil slope surface at rainfall intensities from 1 to 2 mm/min is about from 365 to 847 J/h·m2, and the diameters of raindrops range from about 0.25 to 0.375 mm.
Experimental plots were constructed with metal frames of 1.2 m (length) × 0.4 m (width) × 0.25 m (depth), with adjustable slope gradients by a movable base. A metal outlet at the lower end allowed the collection of runoff samples. In the bottom of the plots, natural sand to a depth of 5 cm and overlaid with permeable gauze was used to drain the infiltration water. The soil was packed to a depth of 20 cm in four 5-cm layers at a bulk density of 1.2 g/cm3 (measured by a cutting ring in a compacted state). Before packing, the water content of the soil was adjusted to 14%, the typical level during the flood season on the Loess Plateau when most erosion occurs. After the soil was packed, Jag S and Jag C162 solutions of 1, 3 and 5 g/m2 were prepared in 2 L of water to produce final rates of 0.024%, 0.072% and 0.12%, respectively. These solutions were uniformly sprayed on the surfaces of the plots and the control plot was sprayed with equal water (2 L). The simulated rainfall experiments began approximately 15 h later. Four rates (0, 1, 3, and 5 g/m2), three rainfall intensities (1, 1.5, and 2 mm/min), and a slope gradient of 15°, which was setup based on the middle slope of cultivated land ranging mainly from 5° to 25° in the Loess Plateau, were tested with two replicates. The duration of all simulated rainfall events was 40 min.
For each treatment, runoff samples were collected 1 and 3 min after the onset of runoff and then every 3 min until the end of the experiment. The runoff volumes were measured with a graduated cylinder [18
], and the sediments were dried at 105 °C for 10 h and weighed to estimate the soil erosion rate by calculating the sediment weight per unit area per unit time and amount of rainfall infiltration by the principle of water balance, while the rainfall infiltration rate was defined as infiltration amount per unit time. The cumulative infiltration (mm) was defined as the sum of the infiltration rate (mm/min) multiplied by time in all the time during rainfall process, and the cumulative erosion modulus (kg/m2
) was defined as the sum of the erosion rate (kg/m2
·min) multiplied by time in all the time during rainfall process. Aggregate size distribution on the surface (0–1 cm) were measured by wet sieving [19
] after rainfall in the classes of >5, 2–5, 1–2, 0.5–1, 0.25–0.5 and <0.25 mm. Each class of aggregates was dried and weighed. Three samples were measured for each treatment and averaged. After each simulated rainfall, six measurements of the shear strength of the soil surface were also taken using a 14.10 Pocket Vane Tester (Eijkelkamp, Giesbeek, The Netherlands). Because shear strength is closely related to water content, we measured the water content continuously after each simulated rainfall. Soil water content was measured by alcohol burning method [20
], and three samples were taken from surface 1 cm soil. The final shear strength was measured when the water content dropped to 22–25% after air drying. All data were analyzed using SPSS (Chicago, IL, USA) by one-way ANOVA and least significant difference (LSD) tests. The significant level was 0.05.
Many researchers have shown that polymers significantly improve soil physical properties. The viscosity of polymers can improve soil structure and increase the stability of aggregates [23
]. Aggregates and their stability determine the sizes and stability of soil pores. Water-stable aggregates >0.25 mm can increase soil permeability and are an important indicator of anti-erosion [24
], but also indicate soil quality. High aggregate stability can maintain more appropriate pore spaces for permeability and can resist erosion better. Aly and Letey [25
] found that the effect of a polymer often depended on its capacity to facilitate flocculation, which prompted the polymer molecules to adhere on the surface of soil particles and act as a bonding agent, keeping soil particles together against the destructive impacts of water drops, and to diminish the destruction of the aggregates on the soil surface [26
Our data showed that Jag S and Jag C162 could improve soil physical properties with their applications. The polymer molecule serves as a bridge between two soil particles in an aggregate by bonding with particles [27
], so they can strengthen the interactions between soil particles and enhance the stability of aggregates. Thus, the aggregate size classes increased, more for large aggregates than for small aggregates, and the aggregation of small particles into larger particles was promoted, consistently with the findings by Xu et al. [28
]. They reported that both synthetic and natural polymers had highly significant effects on aggregate size classes, especially for aggregates >2.0 mm, and could increase the mass of macro-aggregates and decrease the mass of micro-aggregates. The properties of polymers and their application rates both influenced their efficacy in soil [29
]. Higher Jag S and Jag C162 rates increased more aggregates >0.25 mm and higher shear strength due to stronger adhesions. The soil shear strength can also serve as an indicator of soil resistance capacity during erosion [30
Infiltration and erosion are closely associated with soil properties. Increases in aggregate sizes can lead to the maintenance of appropriate spaces for infiltration, thereby decreasing surface runoff. Changes in aggregate size distribution and shear strength can lead to the stabilization of the soil surface against shear-induced detachment [31
], thereby decreasing erosion [32
]. The applications of Jag S and Jag C162 changed the aggregate size distribution and shear strength which can effectively reduce soil crust formation, limiting surface runoff and runoff-induced erosion, similarly to polyacrylamide [33
]. In this experiment, Jag S and Jag C162 were effective in improving soil shear strength and aggregates, thereby increasing rainfall infiltration and reducing soil loss on experimental loess hillslope. The effects were better as the rates increased.
Applying the two polymers (Jag S and Jag C162) to the soil had a significant effect on the infiltration and the final IR at different rainfall intensities (1, 1.5 and 2 mm/min) on an experimental loess hillslope. In the control treatment, the soils generated the smallest final IR (0.65, 0.73, and 0.59 mm/min) and the largest runoff amounts because of crust formation. However, spraying Jag S and Jag C162 on the soil surface increased the final IR by as much as one to two times, with values ranging from 0.70 to 1.51 mm/min compared with untreated soils.
However, the results also showed that the effects of Jag S on the soil physical properties and soil erosion were lower than those of Jag C162. One possible explanation was that the effect of the polymer in stabilizing soil aggregates could be related to the capability of the polymer to move into aggregates. High rates of Jag S micro-molecules were hard to diffuse into the soil because of low viscosity, which limited the movement of Jag S molecules into soil aggregates, and too high rates of Jag S could jam the soil porosity so that the interaction between soil particles and Jag S was not sufficient, resulting in delaying the formation of more soil aggregates. In addition, a crust would form easily to decrease rainfall infiltration when the aggregates on the surface were broken by raindrops. Thus, the effect of Jag S on the soil physical properties and soil erosion was lower.
Similar observations of improving rainfall infiltration and aggregates with the application of PAM were reported by Mamedov et al. [34
], who found that the application of PAM resulted in increasing rainfall infiltration and aggregate stability compared with the control. Macromolecular polymers such as PAM can improve soil physical conditions. The polymers bind the soil particles and prevent dispersion due to their cohesiveness, thereby forming aggregates. These aggregates resist the effects of raindrops and soil crusting or sealing, which results in greater rainfall infiltration. Schamp et al. [35
] explained that polymers could enhance the stability of aggregates via adhesion and adsorption. Shainberg and Levy [12
] revealed that increasing aggregates’ stability could prevent soil crusting, and that polymer treatments could effectively decrease the formation of soil crusts by increasing aggregates’ size distribution and improving aggregates stability [27
]. Compared with the control, the application of Jag S and Jag C162 significantly increased the mass proportions of different sizes of soil aggregates, especially for the >0.25 mm aggregates. In addition, Jag S and Jag C162 acted as binding agents that stabilized the soil aggregates, resisted soil crusting and increased infiltration; the result was consistent with previous studies (just like PAM) [5