Global change affects biogeochemical cycles, and soils play a key role in these cycles [1
]. Soil erosion is a threat to soil sustainability and scientific research is looking for new solutions to reduce and control the accelerated soil erosion rates in agricultural lands [3
]. Soil erosion is a worldwide problem that needs solutions to achieve sustainable production of fibers and food for humankind and to maintain geochemical, hydrological, and erosional cycles in a stable condition [5
]. Soils play a key role to achieve the sustainable development goals (SDG) that the United Nations defined [7
Research on soil erosion has been focused on basins, catchments, and hillslope scales [8
]. However, it is also vital to understand soil erosion processes at the pedon and aggregate scales to find specific remediation strategies and achieve sustainable management of the soil system. Within this objective, it is relevant to determine the driving factors of soil erosion [10
]. At the pedon scale, factors such as organic carbon [11
], porosity [13
], soil texture [14
], bulk density [16
], and aggregate stability [17
] have been found to be factors in controlling soil erosion.
Aggregate stability is an important soil property that determines the cohesion between soil particles, but there is a lack of information about which control measures can effectively improve cohesion in agricultural fields [18
]. It is known that a cover of vegetation or litter will improve the stability of the aggregates [20
], and this will protect the ecosystem service provided by soil since soil erosion will be also reduced [21
]. However, many farmers are resistant to adopt straw or mulch cover [22
]. Thus, it is necessary to find solutions to mitigate the acceleration of soil erosion rates in agricultural lands and enhance infiltration [24
] that will be well-accepted by the farmers. For this reason, integrated studies in which soil conservation practices are based on rainfall erosivity are meaningful.
Recent studies about soil erosion control measures paid attention to the use of mulches [25
], cover crops [26
], or geotextiles [27
] as they reduce the rainfall erosivity and improve soil conditions which result in a lower soil erodibility. Another set of strategies is based on the management: plant species and grazing pressure in rangelands [29
], land abandonment [30
] or land preparation and vegetation covers [31
In this way, polymers have been also reported as useful tools to reduce soil erosion rates and increase soil fertility [32
]. Specifically, polyacrylamide (PAM) and polyvinyl alcohol (PVA) have been used in erosion studies since the 1950s [34
]. It is emphasized as a general result that even when the polymers are applied to the soil surface at low doses, they may have significant positive effects on the improvement of aggregates and structural stability [35
]. It was reported that flocculent materials reduce the adverse effects of disruptive and destructive forces by increasing binding forces among granules [37
]. Several decades ago, Vleeschauwer et al. [38
] studied the effect of PAM on soil physical properties, determining that its application was able to increase porosity and water infiltration rates into the soil profile, positively affecting aggregate stability. Chiellini et al. [39
] examined the adsorption of PVA by montmorillonite, quartz sand, and farm soil. Their findings demonstrated that PVA adsorption is increased when montmorillonite content increases, whereas quartz sand did not absorb PVA. They also demonstrated that PVA adsorbed on montmorillonite exhibits a much slower mineralization rate than the non-absorbed PVA remaining in solution. Slower mineralization rates of absorbed PVA inhibit its biodegradation.
These potential benefits of polymer application are significantly affected by their complex properties (molecular weight, load type, and charge density) and soil properties (texture, organic matter content, clay mineralogy, soil solution composition, and concentration) [40
]. One of the soil properties affecting polymer activity is the aggregate size. The aggregate size group dominating an environment can vary according to slope positions. Kussainova et al. [43
] observed that soil aggregates larger than 60.3 mm dominate on footslopes, and that aggregates smaller than 2 mm are predominant in summits and backslopes. However, in agricultural fields, tillage can affect soil aggregate sizes and, subsequently, bulk density can vary along the hillslope [44
]. Therefore, erosion sensitivity based on soil structural stability also varies in time and according to land cover and management [45
]. Aggregate formation in these agricultural ecosystems drastically changes over the seasons, and the average resistance duration of the macro-aggregates can be around 27 days, with a minimum resistance of 5 days [47
In order to obtain the most effective result from polymer application, the most appropriate polymer type, application form, and dosage must be determined for each soil. To reduce soil erosion, the effectiveness of each polymer applied on the soil will be closely related to the dynamics of aggregates disintegration, which is highly influenced by rainfall erosivity as it was demonstrated by other authors in non-cultivated soils [48
The main aim of this study was to investigate the effects of PAM and PAV applied to soil aggregates with different sizes on soil detachment to understand the initial soil erosion mechanisms. The experiments were carried out under indoor simulated rainfall experiments to achieve the highest accuracy under controlled laboratory conditions.
shows the time to runoff generation results in a bar graph with the different aggregate sizes. Polymer applications partially delayed runoff generation compared to the control plot. In addition, surface runoff starts earlier under sequential rainfall in all the erosion pans and different soil aggregate sizes. After the first run, runoff is generated the earliest at 35 s in the pan in which are placed <1 mm aggregates with no polymer application, meanwhile, for the largest aggregate size (>6.4 mm) runoff started the latest at 338 s with PAM.
In Table 2
, total runoff (R), sediment yield (Sy), and sediment yield mobilized by splash (Sp-Sy) results are added. Under first rainfall, the highest R values are measured for soil aggregates smaller than <1 mm in all the experiments. Specifically, PAM shows the highest runoff amount (45.1 mm) and PVA the lowest one (42.4 mm). The highest Sy values vary from 110 (<6.4 mm) to 2926 g m−2
(>1 mm) for the control plots, from 86.5 to 2071 g m−2
(>1 mm) in PAM and from 53 (>6.4 mm) to 2310 g m−2
(>1 mm) in PVA. PVA produces the lowest Sy values (113 g m−2
) and the highest one is registered in the control plot (197 g m−2
). Sp-Sy shows the highest values again in the control plot (9 g m−2
), but the lowest is registered on the PAM plot (4.0 g m−2
In Table 3
, soil erosion results after the sequential simulated rainfall are included. The highest runoff amounts are found for soil aggregates smaller than <1 mm in all the experiments, but differences are lower than in the first run. However, the control plot shows the highest R, reaching 48 mm. In contrast, PAM and PVA show lower values, reaching 37 and 39 mm, respectively. In the control plot, the highest Sy values are registered, and values for all aggregate sizes are two and three magnitudes greater. For both PAM and PVA, sediment yield registers higher values than after the first run, but lower than the control one (160 and 145 mm, respectively). Sp-Sy shows similar values for all the plots (with and without polymer applications). The lowest values are found on the PVA plot (145 mm).
To observe the differences among applications and initial aggregate size groups, an ANOVA test was conducted and results are presented in Table 4
. R values are significant for the polymer applications for the first (p
< 0.01) and sequential rainfall (p
< 0.001). Initial aggregate size also affects runoff under both runs (p
< 0.001). Sy values show under both rainfall simulations highly statistical significance (p
< 0.001). Under first rainfall, the effect of applications on Sy is not statistically significant, but under sequential rainfall statistically significant differences can be found (p
< 0.05). Sp-Sy under the first run does not show statistical significance among applications, but it is significant among initial aggregate size groups (p
< 0.05). In contrast, both results are statistically significant after the second run.
PAM is the most effective application to reduce R generated under the first rainfall (Table 5
). However, the effectiveness of PVA is not different from the control plot. Under sequential rainfall, results follow the order no polymer<PVA<PAM in terms of effectiveness in reducing R, being different from each other statistically. When compared to no polymer applications, the polymers succeed in reducing Sy in comparison to the control plot, but there is no statistical difference between the polymers. Finally, no significant statistical difference is found in Sp-Sy after the first run, but the difference is significant after the second run. Polymers were demonstrated to be successful in reducing Sp-Sy.
Under the first run, the most effective initial aggregate sizes where polymers reduced R was from 4–6.4 to >6.4 mm sizes, which are statistically similar, meanwhile, the least effective polymer application was for aggregates <1 mm (Table 5
). There is statistically no difference between Rs from pans of mixed aggregates (all) and from 1 to 2 mm size. Under sequential rainfall, all of initial aggregate size groups show statistical differences from each other in terms of their effects on R. After conducting the first rainfall, Sy from 2 to >6.4 mm showed a statistical relationship. The other sizes (<1 and 1–2 mm) register differences from each other and from the other ones. After the sequential rainfall, similar results are found. Under the first rainfall, the effects of polymers on all initial aggregate size groups in reducing Sp-Sy are statistically the same except for <1 mm. Under sequential rainfall, the most ineffective groups for reducing Sp-Sy are <1 mm and 1–2 mm, with no statistical difference among them (Table 6
Finally, a comparison of two runs by dependent sample t
-test in terms of R, Sy, and Sp-Sy are added in Table 7
. All three variables are affected by successive rainfall simulations at different statistical levels. In general, the highest soil and water losses occur after the second simulated rainfall.
The use of small portable rainfall simulators is considered as a valuable technique to estimate initial soil and water losses and is highly used to assess the effectiveness of soil erosion control measures. However, the use of mini-rainfall simulators is not common in the scientific literature. Small plots (<1 m2
) have been the subject of criticism by several authors [56
] related to the representativeness of these results to larger scales of plot size and rainfall intensity. It was decided to work with this simulator since it is standardized and hydrometeorological characteristics have been well-specified and defined by the company. Therefore, it can be used to easily obtain results that can be compared to results from future research, and the measurements and experiments can be repeated during other seasons and under field conditions. The rainfall intensity generated by the mini-rainfall simulator is influenced by the viscosity of the sprinkler water used and by the clogging of the capillary sprinkler heads. For this reason, the simulator was calibrated prior to using it. It has been confirmed that specific factors, soil properties, and solutions can be detected and quantified only at the pedon scale.
Through our experiments, we were able to confirm that the mini-rainfall simulator is useful to evaluate different soil erosion control measures (polymers). Therefore, this study can be included in the body of research that focuses on soil erosion research under laboratory conditions with direct application to field conditions as the accuracy we achieved in the laboratory was the highest [58
Aggregate stability is one of the factors that enhances resistance to soil erosion [19
] but little is known about how to improve this stability in agricultural soils with different aggregate sizes. We observed in our study that aggregate sizes show different responses to raindrop impact under high rainfall intensities. The dose was selected according to results of preliminary experiments and a literature search of comparable studies [61
Splash effect impact on soil aggregates is not a well-known process, though its relevance is often highlighted in the literature [48
]. Our results demonstrated that the most resistant soil aggregates are the largest sizes compared to the smallest ones (Table 7
). In agricultural fields, the stability of macroaggregates is affected by management practices and mineral soil components added as manure or mulching that change their hydrophilic nature [66
]. In this context, the use of polymers such as PAM and PVA was demonstrated to be effective in increasing structural stability, decreasing runoff generation, splash, and soil losses. The potential benefits of polymers are influenced significantly by their complex: molecular weight, load type, and charge density [42
By using a mini-rainfall simulator on different soil aggregate sizes, we detected that soil aggregation is developed in a hierarchical order. The micro-aggregates are generated and then will be joined into new macroaggregates, acting as building blocks for larger aggregates [71
]. However, as for our tested soils, if this aggregation is not possible because of the tillage or extreme rainfall events, soil erosion will take place inevitably. Therefore, the application of polymers will be a determinant factor to improve aggregation [28
]. In macroaggregates, where capillary roots and various hyphae play a role as binders, which also act as aggregates. This effect is temporary and these roots and hyphae serve as cores of these micro-aggregates inside the macroaggregates [16
]. Due to the temporary binding characteristic of these roots and hyphae, aggregates cannot hold together indefinitely and disintegrate into fragments. These fragments covered with a glue-like material formed during fragmentation are covered by clay particles and a new structure of macroaggregate forms in which a micro-aggregate is involved [72
]. Thus, PAM and PVA effects applied to the aggregates with different initial sizes have varied, since the durability of each aggregate size is different.
Small sized (micro-sized) inorganic and organic stabilizer compounds can be integrated into the micro-aggregates, stabilizing them, but bigger roots and hyphae play a part as binders in the stabilization of the macroaggregates. This can be because the stomas in micro-aggregates are micropores and the stomas between micro-aggregates are bigger pores (macropores) [73
]. Because the binding agent acts in the soil system according to the stoma size, stoma size distribution largely affects size and stability. Stoma size distribution is an active feature of the soil in aggregation and fragmentation, which is why PAM and PVA activity on structural stability have differed with the change in initial sizes of aggregates in the study.
In the study conducted by Levy and Miller [74
], it was discovered that anionic polymers penetrate the stomas more easily and are absorbed in the inner surface since the stomas between big aggregates will be wide. Researchers have noted that the durability of these aggregates would be long as the applied polymers have a chance to penetrate bigger aggregates from both inside and outside. In a study carried out under low-kinetic energy rainfalls [75
] under laboratory conditions, soil degradation has been achieved at the same level in different textured soils, although the resistance of soil to shredding is different in distinct textured soils [76
We also conclude that one of the most important key factors that can modify the stability of the soil aggregate as other authors mentioned is the antecedent soil moisture content before conducting the experiments [47
]. In this study, the activation of the runoff and soil particle yield after the sequential rainfall was higher than after the first rainfall, which, in fact, it could be attributable in that the soil is not completely dry out between two rainfall events. Moreover, some soil aggregates start to be affected and disintegrated. After that, aggregates were most susceptible to the separation prior to performing the sequential rainfall, since the reconnection, which is likely to occur due to drying, did not occur in this period.
In the future, we consider that some future lacks should be also filled in order to improve the knowledge about the impact of polymers on soil the initial soil erosion and their effectivity: (i) increasing the plot size and testing different soils with other soil texture and organic matter contents; (ii) assessing changes of soil erosion resistance after polymer application at short- and medium-terms; (iii) performing an economic analysis of the viability to apply polymers at the larger scales in agricultural fields; and (iv) comparing their effects to other natural control soil erosion measures such as straw, grass cover, or catch crops.
From the point of view of new challenges to be researched to achieve proper management, we suggest using a combination of two strategies to control soil and water losses. This will increase the efficiency of the strategy from an environmental and pure economic point of view. We must research in the future not only polymer impact on runoff and erosion but also how straw—such as Rodrigo-Comino et al., [78
] demonstrate—is very efficient in soil erosion control but can be even more in combination with the use of polymers that will reduce soil erodibility. This approach should be also applied to the use of chipped pruned branches, no-tillage, geotextiles, grass strips, etc. and other strategies that with the combination with the polymers will contribute to reducing the extreme soil losses in agriculture and forest land [79