Hydromulching is a new mulch product which has positive effects on soil moisture conservation and the inhibition of evaporation. However, current research on its soil moisture conservation effect has mainly adopted the field fixed-point observation method, rather than revealing the soil moisture conservation mechanism from the microstructure of the hydromulching. This paper applied spectrum analysis technology to observe the structure and micromorphological features of hydromulching, which will be able to reveal the mechanism of soil moisture conservation. Three types of hydromulching, based on laboratory preparation, were used to analyze the mechanism of soil moisture, using the infrared spectrometer (FTIR) and environmental scanning electron microscope (ESEM). The results showed that −OH generates intermolecular hydrogen bonds between carboxymethylcellulose sodium (CMC) and polyvinyl alcohol (PVA); the hydrogen bonds were formatted between the molecules of the two components, strengthening their compatibility, which increased the effectiveness of the hydromulching and created a dense mesh structure. ESEM observation results showed that preparation 1's hydromulching coverage had good effects, while preparations 2 and 3 performed poorly. Therefore, we can conclude that hydromulching effectively cements soil particles. Then it can generate the soil membrane structure to reduce evaporation and improve soil moisture conservation function.
Hydromulching (Bagley et al. 1977; Lentz & Sojka 1994) has many applications, chiefly as a soil moisture conservation material with the advantage of chemical- and water-saving coverage. It is an important measure to alleviate shortages of water resources in arid and semi-arid agriculture. Research exists on hydromulching and its effects (Orts et al. 2000; Chiellini et al. 2008), such as polyacrylamide (PAM) being successfully used in water saving irrigation. This had engendered interest in the use of other polymers with similar properties, but concerns had also arisen regarding the use of PAM in agricultural environments because the acryl amide and monomer were neurotoxins (Orts et al. 2000). Even PAM devoid of monomer (<0.05%) had not eliminated concerns, as the monomer can be used as a degradation product by degrading the amino group in the main chain of the polymer. Therefore, we had an increased interest in the final fate of polymers applied in agriculture and the possible ecotoxicity of their degradation products (Fritz et al. 2002).
Polyvinyl alcohol (PVA) and sodium carboxymethylcellulose (CMC) had shown a positive structuring effect on soil (Stefanson 1974, 1975; Oades 1976; Painuli & Pagliai 1990); moreover, they were nontoxic, biocompatible materials (Nie & Liu 2003; Zhang et al. 2006) that appeared suitable for application in open fields.
Previous studies (Yan 2002; Zhang et al. 2005; Tao & Luo 2009) indicated that hydromulching can increase soil's saturated hydraulic conductivity, water use efficiency, microbial and enzyme activity, and production and temperature. Furthermore, it could reduce soil erosion. However, research on the microscopic mechanism of water retention had been relatively rare. The successful application of X-ray diffraction and modern testing technology, infrared spectroscopy, differential thermal analysis, electron microscopy, nuclear magnetic resonance (NMR) and electron spin resonance can provide methods to analyze soil-related components and structures of hydromulching spray.
Soil micromorphology is a continuation of soil profile morphology in which a microscope is used to analyze core soil content and microstructure, including bone particles, fine soil material, pores, and microstructural spatial arrangement (Onofiok & Singer 1984; Cao 1996). Micromorphology can reveal soil's composition, micro-solid material distribution, shape, size, material composition, void shape and size, and material proportions.
Therefore, this paper aimed to analyze the hydromulching soil moisture mechanism using Fourier transform infrared spectroscopy (FTIR) and environmental scanning electron microscope (ESEM) methods. We also applied spectroscopy to reveal the mechanism of hydromulching, which can provide a reference for similar research in future.
EXPERIMENTAL METHODS AND MATERIALS
PVA and CMC were used as the main raw materials for the hydromulching preparation. The secondary raw materials were water, defoamers, crosslinking agents and glycerin. Three types of hydromulching were prepared and labeled, from 1 to 3, PVA:CMC = 1:3, 1:1, and 3:1, respectively. The preparation method was as follows. Pure CMC powder and PVA powder were mixed with water deionized according to the three ratios. For 3 hours, the mixtures remained at 80 °C in an autoclave with constant stirring at a speed of 100 r/min, producing an 8% solution. After the PVA and CMC were completely dissolved, the other auxiliary materials were added, and then the mixture was cooled for 30 minutes.
Experimental site and treatment
The research was conducted at the field experiment base of the Key Laboratory of Water-Saving Agriculture in Fuxin City, Liaoning (41°41′N–42°56′N and 121°01′E–122°56′E). The region is warm, with a continental monsoon climate featuring hot summers and cold winters. The annual average temperature is 7.2 °C, with an average of 2,865.5 hours of sunshine. The land is of a typical dry composition with an average annual precipitation of 480 mm, of which more than 60% occurs between June and August. The soil is cinnamon, and the texture is sandy. The three types of biodegradable hydromulching made up the trial materials.
MEASUREMENTS AND ANALYSIS
The hydromulching made from the PVA and CMC liquid was dried by film casting. It flowed into a glass and was put on a bench for 3 days at room temperature, after which the process was repeated. The Nicolet 6700 FTIR, produced by the Thermo Fisher company, was used for observing the features of the hydromulching, which included structural analysis and measuring the absorption spectrum of the hydromulching.
Soil-hydromulching microscopic moisture morphometric determination
Three kinds of hydromulching were dissolved in water at a concentration of 1:20, using the performance of hm2 of liquid. After thorough stirring, the solution was sprayed on the surface of soil. The same amount of water was sprinkled as a control agent. After four months of spraying, the soil samples were sampled twice a month. A quantum TM 250 ESEM was applied to determine and analyze the soil micromorphology.
The FTIR results are shown in Figure 1, which shows that the PVA chain was flexible, while the CMC chain contained a large number of ring structures. Although there were obvious differences in the macromolecular chain structure between the two groups, all had hydroxyl groups, and the intermolecular hydrogen bonds between them could enhance the compatibility of PVA and CMC molecules.
In the pure PVA mulching film, the telescopic vibration peak was 3,251 cm−1 −OH; and 2,906 cm−1 was the telescopic vibration −CH. The 1,085 cm−1 was the absorption peak of the symmetric and asymmetric C-O bond. In pure cellulose membrane, stretching vibration appeared at 3,356 cm−1 −OH; 2,941 cm−1 was −CH stretching vibration peak, while substance was the symmetric and asymmetric vibration absorption peak of COONa group C = O. When CMC and PVA were mixed, the −OH on the PVA chain was bound to the ionic carboxylic acid. With Na + , −OH, there was the function to provide electrons. Therefore, C = O's infrared absorption peak moved to a high number of waves. In the liquid mulch, with the −OH peak of the position changed, it appeared at 3,303 cm−1, 3,265 cm−1, and 3200 cm−1. CMC and PVA also produced intermolecular hydrogen bonds. The intermolecular formation of intermolecular hydrogen bonds strengthened their compatibility, and made the sprayed matrix form dense network structure, which was better for water and soil moisture retention.
Microscopic soil moisture mechanism of hydromulching
Figure 2 compares the soil surface construction between CK and hydromulching, which showed that they had completely different structural features. The upper left image revealed that the CK soil surface was compact, the porosity of the soil particles was small, the larger pores were filled with small particles, and the soil crust had formed. The surface of soil with hydromulching presented a different picture. The upper right image displays the soil surface when completely covered by hydromulching, in which the hydromulching completely wrapped into the surface of the soil particles and became interconnected to form a sealed soil membrane structure. The bottom left image shows the soil surface with incomplete coverage; this soil had more porosity, with less effect from moisture. The lower right image shows the effect of degradation after four months, in which the hydromulching covering the soil surface degrades completely. The only remaining function of the hydromulching was to connect the soil particles, and the moist conditions were basically lost.
Effects of three types of hydromulching coverage
Figure 3 shows the effects of hydromulching coverage on the soil surface micromorphology after half a month. The upper left image shows preparation 1's effect. The hydromulching completely covered the soil surface, creating a better sealing effect. The upper right image shows preparation 2's incomplete coverage, with greater porosity which made the hydromulching less effective. The lower left picture shows that preparation 3 created more filamentous material. It connected the different particles, but the sealing effect was not ideal compared to the alternatives.
The reasons for these results are as follows:
The differences among soil surface micromorphology are that, as the upper left picture shows, the compact structure of the soil particles makes the pores less conducive to hydromulching coverage.
The differences in the hydromulchings' compositions are that the high viscosity of CMC gives it poor fluidity in contrast to PVA. Therefore, preparation 1 created the best covering effect because of its high PVA content and so was the best, while preparations 2 and 3 performed poorly.
Hydromulching composition and particle binding mode
Figure 4 shows the connection of the hydromulching preparations with soil particles. The hydrophilic groups in the hydromulching may form hydrogen bonds with many substances as well as adsorption bonds between particles. Typically, one molecule can simultaneously bond with several other particles. As Figure 4 shows, the surface of the large particles played a role in the soil adsorption of the fine particles. The more common floc linked between the particles formed a grid-like structure with the pores in the soil. The sticky wadding material was similar to the molecular structure, which could form and connect to two or more soil particles to form a stable structure. The dissolved filamentous material played the key role in the structure of the soil surface. It exerted a strong adhesive force. Due to its role as a chemical bond in the molecular structure, the soil particles could no longer make point-to-point contact with each other; however, the trellis was connected such that the structure of the soil is more stable and the soil porosity causes less destruction. When hydromulching covered soil particles, its filamentous ingredients bonded them to improve the soil's physical and chemical properties, promoted the soil membrane structure formation through organic matter accumulation, and enhanced surface stability.
DISCUSSION AND CONCLUSION
Analysis of soil microstructure showed that correctly spraying hydromulching so that it covered the surface bonds' soil particles effectively reduced evaporation. The ratio of different components within the hydromulching created different effects. For example, the PVA content of preparation 1 gave the best effect, because compared with CMC, PVA has low viscosity, good fluidity, and a short degradation cycle. On the other hand, preparation 3 had a higher CMC proportion, giving it a longer degradation cycle and making it possible to effectively prolong the service life of the liquid film.
Soil represents an extremely complex system of porous solid soil particles and intergranular porosity. The liquid film injected into the soil can move through pores between the soil particles; after contacting the porous soil particles, the film-forming properties of the hydromulching closes the voids between soil particles. The greater the viscosity of the liquid, the greater its diffusion resistance. Therefore, the longitudinal diffusion of low-viscosity liquid under gravity has a clearly greater trend than that of high-viscosity liquid. When a solution of a linear polymer material was added, longitudinal diffusion distance decreases, so that the moisture content in the soil rises within a certain range. The linear polymer structure had good continuity and film-forming properties. Infiltration of the polymer hydrosol in the soil particles formed a large number of tiny pores in the membrane, so that the soil's aggregate structure changed in order to reduce moisture diffusion and the enrichment zone of the moisture, thereby improving water use efficiency.
The improved material mechanisms of pure clay minerals were convenient in order to study the role of polymers such as montmorillonite in soil structure. Many factors affected the performance properties of the polymers, including molecular weight, the effective group type, and the polymer configuration. Research had confirmed that the higher the molecular weight, the more able the polymer was to bind the soil particles into a single molecular chain, creating a better soil stabilization effect (Davies 1975; Wallace et al. 1986; Barvenik 1994; Long et al. 2002). However, from a cost-effectiveness perspective, different types of polymers should have a suitable molecular weight range. The effect of the polymer's group on its performance was more complex, and a variety of polymers contain large active groups that can be combined with soil particles, such as carboxyl groups, hydroxyl groups, amino groups, sulfonic acid groups, and quaternary ammonium groups. These groups can be divided according to ion types into cationic, non-ionic, and anionic. It was generally believed that non-ionic groups combine with clay particles by hydrogen bonds' forces, while cationic groups produced negatively charged clay particles, and electrostatic attraction combined anionic groups through mutual bonding of shared clay particles of polyvalent metal cations (Cao 1996; Chen et al. 2001).
A great deal of research has studied soil surface structures through the mini-state characteristics of porosity, infiltration, crust strength, amount of splash erosion, bulk density changes, soil crust definition, classification, analysis of the impact of factors, crust formation, and soil erosion, but in-depth study on the structure of the soil surface is still needed. The structure of the soil surface creates thin sampling difficulties, and its formation is a dynamic process influenced by many factors; this makes quantitative study of its physical and chemical properties as well as its formation process more difficult, such that it is still stuck in the research phase of qualitative description. Therefore, in-depth quantitative study is required to assess the morphological characteristics of the soil surface structure and to judge the standard of its formation, the dynamics of the formation process, and the quantitative determination of soil erosion. However, many studies have focused on artificial simulated rainfall rather than natural rainfall conditions, so that the results obtained in the laboratory demonstrate certain differences in the formation of the soil surface structure under natural rainfall conditions. It is difficult to effectively control the slope of farmland's soil surface structural formation in order to provide strong theoretical support. In addition, many types of soil require very meaningful in-depth study on soil surface structural differences.
This research was supported by the projects in the National Science & Technology Program during the Twelfth Five-year plan period (2013BAD05B07, 2013BAD07B03), National Public Welfare Industry (Agriculture) Special (201303125), Key Researches in National Science & Invention Program during the Thirteenth Five-year plan period (2017YFD0300704) and Guidance plan of Liaoning Natural Science Foundation (20170520423). The authors Gu Jian and Ma Ningning contributed equally to this work.