Improving aggregate stability and hydraulic properties of Sandy loam soil by applying polyacrylamide polymer

Highlights

• PAM improved soil physical properties.

• Low molecular weight PAM was more effective than high molecular weight in improving aggregate stability.

• PAM concentration plays a role under the low molecular weight PAM.

• PAM concentration was not significant in improving saturated hydraulic conductivity.

Abstract

Improving soil properties, especially in arid and semiarid regions, is an urgent need for sustainable food production. This study aims to evaluate the effect of applying two types of anionic polyacrylamide polymers (PAMs) with different molecular weights on: (1) soil aggregate stability, (2) infiltration rate and (3) saturated hydraulic conductivity of sandy loam soil and introduce a novel modelling approach to predict the effect of PAM addition on the mentioned soil properties. Polymers were applied at five different concentrations; 0, 100, 250, 500 and 1000 mg L⁻¹. Direct positive relations between the concentrations of PAM (low and high molecular weight) and saturated hydraulic conductivity and infiltration rate were observed. Nevertheless, the relations were more pronounced in case of low molecular weight PAM. The difference between the effect of the two PAMs on infiltration rate was statistically significant at α = 0.05. On the other hand, the differences were not significant except for the 1000 mg L⁻¹ concentration in case of saturated hydraulic conductivity. Meanwhile, concentration has an effect on increasing soil aggregate stability only in case of low molecular weight PAM. At the concentration 1000 mg L^-1 of low molecular weight PAM, aggregate stability and saturated hydraulic conductivity increased 3-fold while infiltration rate increased by more than 7-fold compared to the control. It is hypothesized that the change in the mentioned properties is correlated to the amount of PAM adsorbed by the soil and hence they can be used as surrogate parameters of adsorption. Modified versions of the Langmuir and Freundlich isotherm equations were used to model the change in aggregate stability and saturated hydraulic conductivity in response to the PAM concentration. The pseudo-first and second kinetics models were applied to predict the change in infiltration rate. The models showed excellent fit to the experimental data, thus supporting our hypothesis. The results suggest that low molecular weight PAM is more efficient in improving the physical properties of sandy loam soil. The modelling approach presented in this work may be extended to other types of soil. Other isotherm models may be used to predict the response of soil to PAM concentration where the Langmuir or Freundlich isotherms do not apply. This modelling approach provides land managers with a decision support tool to optimize PAM application.

Introduction

One of the most critical challenges facing the world in general and particularly in developing countries in the twenty-first century is the rapid increase in the population in conjunction with land degradation and the decline in arable land (Berek, 2014). Soil degradation undermines land productivity, accelerates soil erosion and emission of greenhouse gases through deforestation and intensive use of marginal and fragile lands (Lal and Stewart, 1990). Mediterranean soils are vulnerable to weathering and degradation due to the generally low organic matter content (Killi et al., 2014). Moreover, the majority of the agricultural soils are light-textured and do not retain water against drainage. Therefore, land managers need to conserve soil and water resources to achieve maximum yield and quality by reducing the percolation and retain water for plant uptake. There is abundant evidence in the soil science literature to suggest that polymers can enhance the properties of soils (Berek, 2014).

For more than 40 years organic polymers such as polyvinyl alcohols (PVAs) and polyacrylamides (PAMs) have been used as soil amendment to improve soil quality including soil physical and chemical properties (Abu-Hamdeh et al., 2018). Polyacrylamide is a synthetic water-soluble polymer with different molecular weight (MW), formed from acrylamide subunits (Moran, 2007). Polyacrylamides have the general chemical formula (−CH2CHCONH2−)n. They can be synthesized commercially in linear or cross-linked structure with MWs ranging from thousands to millions of Daltons. It is used in numerous applications such as food industry, well drilling, and wastewater treatment as a flocculation agent. Although PAM degradation could release acrylamide subunits, most of the applied PAM would remain in the soil. Also, PAM molecules are too large, due to its molecular weight, to penetrate any cell membranes (Xiong et al., 2018). Moreover, Labahn et al. (2010) demonstrated that acrylamide subunit could be degraded by naturally occurring microbial populations within the soils to non-toxic products over periods of days to months.

Soil amendments including polyacrylamides improve soil aggregate formation by cohesion of adjacent particles (Albalasmeh and Ghezzehei, 2014). The efficiency of these amendments depends on many factors including MW, charge density, application rate and application method of the polymer as well as the soil type (Green et al., 2004; Mamedov et al., 2010, 2007).

Asghari et al. (2009) reported a significant increase in soil aggregate stability when PAM was added to a sandy loam soil compared to other conditioners and control treatments. Moreover, Melo et al. (2014) found that aqueous PAM solution of 100 mg kg⁻¹ was the most efficient in increasing aggregate stability. Similarly, Sojka and Lentz (1997) reported that using 10 ppm of PAM increased wet aggregate stability significantly over two consecutive years. Mamedov et al. (2007) studied the effect of two anionic PAM molecular weights on soil aggregate stability. They concluded that the addition of PAM enhances the stability of soil aggregates but moderate MW PAM produced more stable aggregates than high MW PAM. Also, Green et al. (2004) concluded that PAM with molecular weight ranges from 6 × 10⁶ to 18 × 10⁶ Daltons increased the aggregate stability for three soils of different mineralogy and texture.

Tümsavas and Kara (2011) and Mamedov et al. (2009) investigated the effect of applying dry PAM at different application rates on soil infiltration rate for different soil textures. They found that the effect of PAM is dependent on soil texture. However, Levy and Agassi (1995) showed that PAM of high molecular weight is more effective than low molecular weight PAM in maintaining high infiltration rate in coarse- textured soil. However, both molecular weights were better than the control treatment. Li et al. (2011) showed that the infiltration rate of silt loam soil increased as PAM concentration increased from 0.4 to 1.4 g kg⁻¹ reaching three times of the untreated soil at the PAM application rate of 1.3 g kg⁻¹.

Young et al. (2009) studied the effect of PAM on saturated hydraulic conductivity in unlined canals in sandy soils. They concluded that PAM reduced saturated hydraulic conductivity (40–98 %) in sand but the reductions were less in loamy sand (0–56 %). A study by Malik and Letey (1992) investigated the effect of adding PAM solutions of various concentrations (25−400 mg/L) on the hydraulic conductivity of fine and coarse sands. They found that the sand particle size and PAM concentrations play an important role in the hydraulic conductivity reduction. The hydraulic conductivity decreased as PAM concentrations increased in both sizes but the reduction in coarse sand (85 %) was lower than fine sand (94 %). In contrast, Santos and Serralheiro (2000) found that 10 g m⁻³ application rate of PAM increased the saturated hydraulic conductivity in furrow experiment of a Mediterranean, loamy sand soil by 168 %. In a field study, Zahow and Amrhein (1992) showed that 50 mg kg⁻¹ application rate of PAM in the presence of gypsum increased the hydraulic conductivity in saline sodic soil from 0.0 to 0.28 mm hr⁻¹.

The literature so far has explored the effect of PAM on aggregate stability, hydraulic conductivity, infiltration in different soils through experimental work (Abu-Hamdeh et al., 2018; Mamedov et al., 2010, 2009; Safari et al., 2015; Shainberg et al., 2011; Tadayonnejad et al., 2017; Tümsavas and Kara, 2011). However, it is important to be able to predict the effect of the addition of a certain amount of PAM on these soil properties to optimize the application rates. Lu et al. (2002) studied the adsorption of PAM by different soils and concluded that the adsorption was well described by the Langmuir isotherm. Nevertheless, to the best of our knowledge, no work has attempted to provide models to link the PAM concentration to the expected change in the soil physical parameters. Therefore, our current work aims to develop a novel modeling approach to this problem.