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Effect of Polyacrylamide Application on Runoff, Erosion, and Soil Nutrient Loss Under Simulated Rainfall
Pedosphere 21(5): 628–638, 2011 ISSN 1002-0160/CN 32-1315/P c 2011 Soil Science Society of China Published by Elsevier B.V. and Science Press
Effect of Polyacrylamide Application on Runoff, Erosion, and Soil Nutrient Loss Under Simulated Rainfall∗1 WANG Ai-Ping1 , LI Fa-Hu2,3,∗2 and YANG Sheng-Min4 1
Inner Mongolia Coal Mine Design and Research Institute, No. 55 Xinhua Road, Hohhot 010010 (China) College of Water Conservancy and Civil Engineering, China Agricultural University, No. 17 Qinghua East Avenue, Haidian District, Beijing 100083 (China) 3 State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, No. 26 Xinong Road, Yangling 712100 (China) 4 Beijing Vocational College of Agriculture, No. 5 Daotiannanli, Changyang Town, Beijing 102442 (China) 2
(Received December 20, 2010; revised June 9, 2011)
ABSTRACT Soil erosion affects soil productivity and environmental quality. A laboratory research experiment under simulated heavy rainfall with tap water was conducted to investigate the effects of anionic polyacrylamide (PAM) application rates (0, 0.5, 1.0, and 2.0 g m−2 ) and molecular weights (12 and 18 Mg mol−1 ) on runoff, soil erosion, and soil nutrient loss at a slope of 5◦ . The results showed the two lower rates of PAM application decreased runoff while the highest rate increased runoff as compared with the control. Sediment concentration and soil mass loss increased significantly with the increasing − PAM application rate. Compared with the control, PAM application decreased K+ , NH+ 4 , and NO3 concentrations in + − + + sediment and K and NH4 concentrations in runoff, but significantly increased the mass losses of K , NH+ 4 , and NO3 over + −2 soil surface except for the NH4 at PAM application rate lower than 1.0 g m . PAM application decreased the proportion of K+ loss with runoff to its total mass loss over soil surface from 60.1% to 16.4%. However, it did not affect the NH+ 4 and NO− losses with runoff, and more than 86% of them were lost with runoff. A higher PAM molecular weight resulted in less 3 − soil erosion and K+ mass loss but had little effect on runoff and NH+ and NO losses. PAM application did not prevent 4 3 soil erosion and the mass losses of K+ and NO− 3 under experimental conditions. Key Words:
application rate, mass loss, molecular weight, nutrient concentration, sediment concentration
Citation: Wang, A. P., Li, F. H. and Yang, S. M. 2011. Effect of polyacrylamide application on runoff, erosion, and soil nutrient loss under simulated rainfall. Pedosphere. 21(5): 628–638.
INTRODUCTION Soil nutrients following rainfall can be lost by surface runoff and/or vertical percolating water in a dissolved form and with the eroded soil particles in an adsorbed form. Soil nutrient loss decreases soil fertility and soil productivity, and hence increases the cost of agricultural production. Meanwhile, it also results in water pollution especially for the surface water body when the lost nutrient elements enter into catchment areas. Soil erosion and nutrient loss are the main concerns for soil, agricultural, and environmental scientists all over the world. ∗1
Linear anion polyacrylamide (PAM) is a common soil structure ameliorant. PAM application can increase cohesion among soil particulates, improve soil microstructure and soil aggregate stability against water (Miller et al., 1998; Levy and Miller, 1999; Zhang, 2006; Mamedov et al., 2007), and consequently depress soil sealing formation and soil erosion (Ben-Hur et al., 1985; Bradford et al., 1987; Ben-Hur and Keren, 1997; Bjorneberg et al., 2003; Sojka et al., 2007). However, controversies exist among published literature about PAM effects on soil permeability (Falatah et al., 1999; Santos and Serralheiro, 2000; Peng et al., 2006; Li et al., 2010) and surface runoff (Shainberg et al., 1990;
Supported by the National Natural Science Foundation of China (No. 40635027) and the Fund of State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, China (No. 10501-169). ∗2 Corresponding author. E-mail: [email protected].
PAM EFFECT ON RUNOFF, EROSION AND NUTRIENT LOSS
Levy and Agassi, 1995; Sirjacobs et al., 2000; Sepaskhah and Bazrafshan-Jahromi, 2006). PAM properties and its application rate and application method, soil characteristics, and topography etc. all influence PAM efficiency on soil permeability and soil erosion (Zhang and Miller, 1996; Green et al., 2000; Lentz, 2003; Bhardwaj et al., 2009; Young et al., 2009). The length of polymer chain and the viscosity of PAM solution increase with the increase of PAM molecular weight (Green et al., 2000). PAM with a high molecular weight is more effective for clay flocculation, and its influence on infiltration is also greater than PAM with a low molecular weight (Levy and Agassi, 1995; Green et al., 2000). As compared with the control treatments, the application of PAM with a range of molecular weights from 6 to 18 Mg mol−1 increased soil permeability by 3 to 5 times (Green et al., 2000). Sojka et al. (1998) thought PAM use increased the infiltration of fine and medium texture soils, and its effect on the infiltration of coarse texture soil was small. The experimental results of Sirjacobs et al. (2000) showed PAM application decreased the infiltration rate of Alfisol but increased it in the Vertisol. Shainberg et al. (1990) reported PAM application with a rate greater than 20 kg ha−1 increased infiltration rate significantly. However, the experimental results of Ajwa and Trout (2006), Soupir et al. (2004), and Yu et al. (2003) demonstrated PAM application decreased soil infiltration and increased surface runoff in loam soils. Inconsistent experimental results are also present on soil erosion (Trout et al., 1995; Lentz and Sojka, 2000; Cochrane et al., 2005; Sepaskhah and BazrafshanJahromi, 2006; Lentz and Sojka, 2009). Research results indicated that PAM application decreased soil erosion in furrow irrigation significantly (Trout et al., 1995; Lentz and Sojka, 2000, 2009). Zhang and Miller (1996) reported a similar result about PAM application under simulated rainfall. However, the field experimental results of Levy et al. (1991) showed that PAM application resulted in a similar sediment concentration to the control treatment. It was also reported that the presence of PAM in the simulated rainfall or in inflow water acted to enhance soil loss when runoff was sufficient to transport sediment (Flanagan et al., 1997). Generally, the influence of PAM application on infiltration or runoff is less than that on soil erosion (Yu et al., 2003; Cochrane et al., 2005; Sepaskhah and Bazrafshan-Jahromi, 2006), and it is also more transient under sprinkler irrigation than under furrow irrigation (Sojka et al., 1998). PAM application influences surface runoff and soil
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erosion, so it will impose certain effects on soil nutrient loss (Kay-Shoemake et al., 1998; Lentz et al., 1998; Entry and Sojka, 2003; Flanagan and Canady, 2006b; Oliver and Kookana, 2006). By furrow irrigation experimentation, Oliver and Kookana (2006) thought anion PAM application did not significantly affect N and K+ concentrations in the runoff and the eroded soil particulates, but decreased their mass losses by 94% and 60%, respectively. The experimental results of Lentz et al. (1998, 2001) showed PAM use had little influence on NO− 3 concentration in runoff. Flanagan and Canady (2006b) reported that PAM applied in lagoon effluent decreased NH+ 4 mass loss from 34% to 92%. Soil erosion depends on the interaction of soil erosion resistance and water erosive force. The variations of both forces after PAM application will change their balance, and hence result in different outcomes. As an erosion inhibitor, more researches about PAM effects on soil erosion were reported, however less information was available on PAM influences on soil nutrient loss. It is not well understood how PAM application affects the loss of soil nutrients, and how the proportion of soil nutrient loss with runoff to its total mass loss over soil surface changes after PAM application under rainfall conditions. It is important to look for more effective control measures to improve utilization efficiency of soil fertilizer and protect the water environment, especially surface water bodies. Thus, the objectives of this study were to investigate the effects of PAM application rate and molecular weight on runoff, soil erosion, soil nutrient losses and their loss proportions with runoff over the soil surface. These effects were tested under a simulated rainfall with tap water so as to search for more suitable management schemes for sprinkler irrigation. MATERIALS AND METHODS The soil samples were collected from the East Campus of China Agricultural University (uncultivated land). Soil texture was loam soil with 471 g kg−1 sand, 422 g kg−1 silt, and 107 g kg−1 clay. Of the clay mineralogy, the main minerals were chlorite, illite, and montmorillonite measured by a D/Max-RC ray diffractometer (Rigaku Co., Japan). The cation exchange capacity (CEC), exchangeable sodium percentage (ESP), and organic matter content of the tested soil were 12.6 cmolc kg−1 , 2.03%, and 3.82 g kg−1 , respectively. The pH in the soil extract (water:soil = 2:1) was 7.91. Total soluble salt and chemical element concentrations in the tested soil are listed in Table I. After air-drying and passed through a 4-mm sieve, the soil sample was artificially fertilized with the che-
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TABLE I Total soluble salt (TSS) and chemical element concentrations in the tested soil Total
Soluble K+
N
P
0.10
g kg−1 0.52 12.00
TSS 0.67
Ca2+ 60
Mg2+ 36
Na+ 53
micals (reagent pure grade) of KNO3 (2 g kg−1 soil) and KH2 PO4 (1 g kg−1 soil). According to the designed application rates of fertilizers, KNO3 and KH2 PO4 masses were calculated, weighed, and then dissolved in a given volume of distilled water. The prepared solutions of fertilizers were evenly sprayed on the soil sample. The fertilized soil was fully mixed, sealed, and stored for 12 h, then spread and air-dried. An experimental soil tray (60 cm × 30 cm × 15 cm) made of steel was fixed at a slope gradient of 5◦ (about 9%). A V-shaped runoff collection plate was connected to the end of the tray. A series of small holes were drilled on the bottom of the tray. Cleaned gravel with a thickness of 2 cm and a piece of coarse gauze were laid on the bottom of the tray to facilitate free drainage and avoid soil particles dropping from the holes. The prepared soil sample was evenly packed into the soil tray at a bulk density of 1.35 Mg m−3 . The soil thickness in the tray was 8 cm. The soil surface in the tray was at the same level as the bottom of the V-shaped runoff collection plate. The tested PAM was linear anion polymer (Hanlimiao Hi-Tech. Co. Ltd., Beijing). Its major active ingredient was acrylamide and sodium acrylate with a charge density of 30%. PAM masses of 90, 180, and 360 mg were weighed, dissolved in 1 L of distilled water, and stirred fully by hand for 30–60 min. The prepared PAM solutions were evenly sprayed on soil surfaces to get different PAM application rates. After the prepared soils were dried for about 1 h, a simulated rainfall process began. The artificial rainfall simulator consisted of a water supply system, a water-spraying system, and an effluent collector. The water supply system included two sets of vertical pipes with a height of 4.75 m, connected by horizontal pipes with a length of 0.9 m, which was fixed by tripods and ‘stay wires’. Two sprayingdownward, conical emitters (Spraco, Phoenix, USA) were connected with the end of the horizontal pipes. The distance between the two sprayers was 4 m. Under a water pressure of 75 kPa, the calibrated rainfall
Cl−
SO2− 4
HCO− 3
52
mg kg−1 86 183
N
P
K+
13.32
1.78
84.90
intensity of the simulator was 60 mm h−1 , and its rainfall uniformity coefficient and raindrop median diameter were 0.955 and 1.44 mm, respectively. The calculated kinetic energy of rainfall was 27.96 J m−2 mm−1 (Brown and Foster, 1987). Municipal tap water was applied to simulate rainwater in the experiment. The electrical conductivity of the tap water was about 0.5 − dS m−1 , and the K+ , NH+ 4 , and NO3 concentrations were 1.591, 0.245, and 9.349 mg L−1 , respectively. The rainfall duration was 80 min for all experimental treatments. After surface runoff occurred on the soil surface of experimental trays, runoff water from the outlet of the soil tray was continually collected at regular intervals with plastic buckets. The collection duration for each bucket was 5 min and the runoff volume collected in each bucket was measured. After settling, sediments in the buckets were dried in an oven at 105 ◦ C and weighed. The concentrations of suspended material in runoff were calculated according to the sediment mass and runoff volume collected in each bucket. After filtering through 0.45-μm filter paper, the − concentrations of K+ , NH+ 4 , and NO3 in runoff were determined with an ionic chromatographic instrument (ICS-1500, Dionex, USA). The concentrations of NH+ 4 and NO− 3 in sediment were measured with a flow analyzer (AA3, Bran-Luebbe Co., Norderstedt, Germany) after the sediment was eluted with 1 mol L−1 KCl solution at a water:soil ratio of 5:1. Potassium ionic concentration in the sediment was determined with a flame photometer (PF640, Shanghai Precision & Scientific Instrument Co. Ltd., Shanghai) after it was eluted with 1 mol L−1 NH4 OAC solution at a water:soil ratio of 10:1. All measurements for the chemical elements were carried out according to standard procedures. Experimental treatments consisted of three application rates of PAM (0.5, 1.0, and 2.0 g m−2 ) and two molecular weights of PAM (12 and 18 Mg mol−1 ), with a treatment without PAM application as the control. All experimental treatments were carried out in triplicate. In order to calculate the mass loss of nu-
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trient elements with runoff, the background values of tested nutrient elements in the simulated rainfall (tap water) were subtracted from their measured concentrations. The significance of difference among treatment means was tested by the analysis of variance (one-way ANOVA) using SPSS 16.0. When the F -value in the ANOVA was statistically significant, a least significant difference test was used for the separation of means at P = 0.05. RESULTS AND DISCUSSION Surface runoff and soil erosion During the experimental process, no visible rill was formed on the soil surface, and soil erosion predominantly was interrill. The variation of surface runoff rate with rainfall duration for various PAM application rates under the PAM molecular weight of 18 Mg mol−1 is shown in Fig. 1. The experimental data for the trea-
Fig. 1 Variations of surface runoff rate at a polyacrylamide (PAM) molecular weight of 18 Mg mol−1 and runoff volume at PAM molecular weights of 12 and 18 Mg mol−1 with PAM application rates of 0, 0.5, 1.0, and 2.0 g m−2 (AR0, AR0.5, AR1.0, and AR2.0, respectively). Bars with the same letter are not significantly different at P < 0.05.
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tment with a PAM molecular weight of 12 Mg mol−1 are not shown thereafter because of its similar variation tendency to that with a molecular weight of 18 Mg mol−1 . Runoff rate increased with rainfall duration and then it gradually approached a steady value after rainfall of about 40 min (Fig. 1). The steady runoff rate increased with the increased PAM application rate. However, it was smaller than that in the control at a PAM application rate of 1.0 g m−2 or less, which is consistent with many experimental results (Moore, 1981; Flanagan and Canady, 2006a), and was greater than that in the control at a greater application rate of PAM. The average steady runoff rates were 153.0, 148.5, 149.5, and 161.5 mL min−1 , respectively, for the PAM application rates of 0, 0.5, 1.0, and 2.0 g m−2 under two tested PAM molecular weights. As compared with the control, the steady runoff rate decreased by 2.94% for the PAM application rate of 0.5 g m−2 and 2.29% for the rate of 1.0 g m−2 , but it increased by 5.56% for the rate of 2.0 g m−2 . PAM molecular weight did not impose an obvious effect on runoff rate (data not shown) possibly because of the relatively fine texture of the tested soil (Levy and Agassi, 1995). Similar to the steady runoff rate, runoff volume also increased with the increase of PAM application rate (Fig. 1). The runoff volume was smaller at the PAM application rates of 0.5 and 1.0 g m−2 and greater at the application rate of 2.0 g m−2 than that in the control. Under rainfall or sprinkler irrigation conditions, raindrops ‘attack’ soil surface and results in soil compaction, aggregate breakup, soil particle displacement, and consequently seal formation on the soil surface. PAM application increases the stability of soil aggregates and depresses the occurrence of soil seal (Ajwa and Trout, 2006). Therefore, it improves soil permeability and decreases surface runoff (Moore, 1981; Bradford et al., 1987; Sepaskhah and BazrafshanJahromi, 2006). Our experimental results confirmed this conclusion under the PAM application rate of 1.0 g m−2 or less (Fig. 1). However, the viscosity of PAM water solution increases with the increased PAM concentration (Levy and Agassi, 1995; Li et al., 2010). The increased viscosity of soil solution at the PAM application rate of 2.0 g m−2 possibly resulted in the decrease of soil permeability and subsequently the increase of surface runoff (Fig. 1). This phenomenon was also observed by other investigators (Falatah et al., 1999; Soupir et al., 2004; Ajwa and Trout, 2006; Young et al., 2009).
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Sediment concentration in runoff generally was low and steady with rainfall duration under no PAM application (Fig. 2). However, PAM application significantly increased the sediment concentration after about 25 min of rainfall beginning. The greater the PAM application rate, the higher the sediment concentration in runoff (Fig. 2). Similarly, soil mass loss also increased significantly (P < 0.001) with the increase of PAM application rate for all experimental treatments (Fig. 2), which is consistent with the experimental results of Flanagan et al. (1997). When the PAM application rates were 0.5, 1.0, and 2.0 g m−2 , the average soil mass losses were 1.11, 2.81, and 5.07 times, respectively, greater than that in the control for both tested PAM molecular weights.
Fig. 2 Variations of sediment concentration at a polyacrylamide (PAM) molecular weight of 18 Mg mol−1 and soil mass loss at PAM molecular weights of 12 and 18 Mg mol−1 with PAM application rates of 0, 0.5, 1.0, and 2.0 g m−2 (AR0, AR0.5, AR1.0 and AR2.0, respectively). Bars with the same letter are not significantly different at P < 0.05.
When PAM application rate was 1.0 g m−2 or less, PAM molecular weight affected sediment concentration (data not shown) and soil mass loss significantly
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(Fig. 2). A greater PAM molecular weight resulted in a smaller sediment concentration and less soil loss. However, PAM molecular weight did not impose a significant effect on soil erosion when the PAM application rate was 2.0 g m−2 , which is consistent with the experimental results of Mamedov et al. (2007). The decrease of erosion resistance of surface soil other than the increase of surface runoff possibly is the main reason for the increased soil erosion under the experimental conditions, because a greater dependence of soil mass loss on the average sediment concentration (R2 = 0.99) than on the runoff volume (R2 = 0.56) existed (data not shown). Five possible mechanisms are responsible for the increased soil loss after PAM use: 1) The physical process of soil interrill erosion is a function of detachment by raindrops and the shallow flow transport of sediment particles (Flanagan et al., 1997). Soil mass loss depends on the interactions of rainfall erosivity (raindrop impact), the kinetic energy of runoff, and soil erosion resistance. Soil sealing formation decreases soil permeability (Ramos et al., 2003), and hence increases runoff and soil detachment capacity (Ramos et al., 2000; Robinson and Woodun, 2008; Warrington et al., 2009). However, as compared with the splashed erosion, the erosion loss induced by runoff wash under laboratory conditions generally only accounts for a small proportion of total erosion loss (Flanagan et al., 1997; Van Dijk et al., 2003; Robinson and Woodun, 2008). Meanwhile, soil sealing formation also increases soil strength and subsequently soil erosion resistance due to the increase of bulk density of surface soil (Levy et al., 1991; Roth, 1997; Neave and Rayburg, 2007; Warrington et al., 2009; Ekwue and Harrilal, 2010). After PAM application, the formation and development of soil sealing are restricted, and the resistance of surface soil against erosion by raindrop splash decreases (Flanagan et al., 1997). When the greater soil aggregates that form under the aid of PAM (Zhang and Miller, 1996; Miller et al., 1998; Ross et al., 2003), as compared with the control, are initiated or activated by raindrop impact or runoff under a relatively steep slope, more soils will be lost. 2) PAM adsorption in soil is a kinetic process because it can be adsorbed by both the external and internal surfaces of soil aggregates (Bajpai and Bajpai, 1995; Miller et al., 1998; Levy and Miller, 1999; Taylor et al., 2002). The contact time of PAM molecules with soil aggregate affects the effectiveness of PAM function (Flanagan et al., 1997). A relatively short drying duration between
PAM EFFECT ON RUNOFF, EROSION AND NUTRIENT LOSS
PAM application and rainfall occurrence reduced PAM efficiency on restraining soil erosion in this research. 3) A greater viscosity at the water-soil boundary after PAM application enhances soil detachment and sediment transport through greater hydraulic drag force (Flanagan et al., 1997; Li et al., 2010). 4) A relatively high pH in soil solution in this study also reduced PAM efficiency on depressing soil erosion (Taylor et al., 2002; Deng et al., 2006). 5) Moreover, a relatively high electrolyte concentration in the simulated rainfall (tap water) led to shorter polymer chains and less efficiency in binding together soil particles that are far apart, and hence the efficiency of PAM on inhibiting soil loss was decreased (Yu et al., 2003; Mamedov et al., 2007). However, this effect of electrolyte on PAM’s length may be small (Levy and Ben-Hur, 1998). Concentrations of nutrient elements in runoff and their mass losses − The variations of K+ , NH+ 4 , and NO3 concentrations with rainfall duration are showed in Fig. 3 under the PAM application rates of 0, 0.5, 1.0, and 2.0 g m−2 and the molecular weight of 18 Mg mol−1 . The − K+ , NH+ 4 , and NO3 concentrations in runoff decreased monotonously with the increase of rainfall duration (Fig. 3), which is the result of dissolved nutrient element loss with runoff or infiltration. Compared with the control, PAM application decreased the concentra− tions of K+ and NH+ 4 but increased NO3 concentration in runoff (Fig. 3). The adsorption of the anionic functional group in PAM molecules for the cationic nutrient elements of K+ and NH+ 4 in soil solution and its repulse for the anionic element of NO− 3 possibly are responsible for this result (Entry and Sojka, 2003). PAM molecular weight did not have obvious effects on the
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− concentrations of K+ , NH+ 4 , and NO3 in runoff (data not shown). As compared with the control, the average mass loss of K+ with runoff (after subtracting the background value in the simulated rainfall) at the two tested PAM molecular weights decreased by 37.37%, 51.70%, and 34.91% at the PAM application rates of 0.5, 1.0, and 2.0 g m−2 , respectively (Fig. 4). Correspondingly, the average mass loss of NH+ 4 decreased by 9.45% and 10.82%, respectively, at the PAM application rates of 0.5 and 1.0 g m−2 , however, it increased by 4.49% at the PAM application rate of 2.0 g m−2 (Fig. 4). The average mass loss of NO− 3 with runoff at the two tested PAM molecular weights increased significantly (P < 0.05) with the increase of PAM application rate, by 23.82%, 87.40%, and 173.65% at the PAM application rates of 0.5, 1.0, and 2.0 g m−2 , respectively, as compared with the control (Fig. 4). Compared with the K+ , the slightly smaller variation ranges of NH+ 4 concentration in runoff (Fig. 3) and its mass loss with runoff (Fig. 4) after PAM application could not be interpreted. This is possibly related to the complex transformation cycle of nitrogen in soil.
Concentrations of nutrient elements in sediment and their mass losses − The concentrations of K+ , NH+ 4 , and NO3 in sediment decreased with rainfall duration (Fig. 5). Generally, the greater the PAM application rate, the lower their concentrations in sediment for all experimental treatments (Fig. 5). The eroded soil particulates usually are fine silt or clay with high nutrient concentrations due to their greater adsorption (Warrington et al., 2009). Compared with the control treatment, PAM application
− Fig. 3 Variations of K+ , NH+ 4 , and NO3 concentrations in runoff at polyacrylamide application rates of 0, 0.5, 1.0, and 2.0 −2 g m (AR0, AR0.5, AR1.0 and AR2.0, respectively) and a molecular weight of 18 Mg mol−1 . The dashed lines represent the corresponding ion concentrations in rainfall.
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− Fig. 4 Variations of K+ , NH+ 4 , and NO3 mass losses with surface runoff (after subtracting their background value in simulated rainfall) at polyacrylamide (PAM) application rates of 0, 0.5, 1.0, and 2.0 g m−2 (AR0, AR0.5, AR1.0 and AR2.0, respectively) and molecular weights of 12 and 18 Mg mol−1 . Bars with the same letter(s) within each molecular weight are not significantly different at P < 0.05.
− Fig. 5 Variations of K+ , NH+ 4 , and NO3 concentrations in sediment at polyacrylamide application rates of 0, 0.5, 1.0, and −2 2.0 g m (AR0, AR0.5, AR1.0 and AR2.0, respectively) and molecular weight of 18 Mg mol−1 .
results in the erosion of soil aggregate with greater size, in which nutrient concentrations generally are lower (Miller et al., 1998). This, together with the continual losses of nutrient elements with infiltration and runoff, is the reason that nutrient concentrations in sediment decreased with the increased PAM application rate (Fig. 5). − The mass losses of K+ , NH+ 4 , and NO3 with the eroded soil particulates generally increased with the increase of PAM application rate (Fig. 6). However, the − mass losses of NH+ 4 and NO3 with the eroded soil particulates were less than about 0.8 mg under the experimental conditions, which was much smaller than those lost with runoff (Fig. 4). A greater PAM molecular weight generally resulted − in smaller concentrations of K+ , NH+ 4 , and NO3 in sediment (data not shown) and their less mass losses with the eroded soil particulates (Fig. 6). The effects of
− tested PAM molecular weights on K+ , NH+ 4 , and NO3 concentrations in sediment and their mass losses were not significant at P < 0.05. The mass loss of nutrient elements with the eroded soil particulates depends on soil erosion rate and the concentrations of nutrient elements in sediment. PAM application decreased the concentrations of K+ , − NH+ 4 , and NO3 in sediment (Fig. 5), but significantly increased the soil erosion rate and soil mass loss (Fig. 2). The increased degree of the soil erosion rate (1.49 to 6.50 times) after PAM application was much greater than the decreased degrees of the nutrient element concentrations (3.33% to 17.50%, 14.32% to 70.87%, and 22.25% to 73.62%, respectively, for K+ , − NH+ 4 , and NO3 ) in sediment. Therefore, the mass losses of tested nutrient elements with the eroded soil particulates increased with the increase of PAM application rate (Fig. 6).
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− Fig. 6 Variations of K+ , NH+ 4 , and NO3 mass losses with eroded soil particulates at polyacrylamide (PAM) application −2 rates of 0, 0.5, 1.0, and 2.0 g m (AR0, AR0.5, AR1.0 and AR2.0, respectively) and molecular weights of 12 and 18 Mg mol−1 . Bars with the same letter are not significantly different at P < 0.05.
− Fig. 7 Variations of K+ , NH+ 4 , and NO3 mass losses over soil surface (after subtracting their background values in simulated rainfall) at polyacrylamide (PAM) application rates of 0, 0.5, 1.0, and 2.0 g m−2 (AR0, AR0.5, AR1.0 and AR2.0, respectively) and molecular weights of 12 and 18 Mg mol−1 . Bars with the same letter(s) are not significantly different at P < 0.05.
Mass losses of nutrient elements over soil surface and their loss proportions The mass losses of nutrient elements over soil surface (i.e., both with surface runoff and the eroded soil particulates) are shown in Fig. 7. Generally, PAM application increased the mass losses of K+ , NH+ 4 , and + over soil surface except for NH at the PAM apNO− 3 4 plication rate less than 1.0 g m−2 (Fig. 7). The two tested molecular weights of PAM did not have significant effects on the mass losses of all tested nutrient elements (Fig. 7). The mass loss of nutrient element over soil surface depends on runoff rate, soil erosion rate, and nutrient element concentrations in runoff and sediment. Although PAM application decreased the concentrations of nutrient elements (with the exception of
NO− 3 ) in runoff (Fig. 3) and sediment (Fig. 5), it significantly increased the soil erosion rate and erosion mass (Fig. 2). Hence, PAM application generally resulted in − the increased mass losses of K+ , NH+ 4 , and NO3 over soil surface (Fig. 7). The proportions of the mass losses of tested nutrient elements with runoff to their mass losses over soil surface (including all losses with runoff and the eroded soil particulates) are calculated and shown in Fig. 8 under various tested PAM application rates and molecular weights. Compared with the control, PAM application significantly decreased (P < 0.05) the percentage of K+ loss with runoff (Fig. 8). Its average percentage for the two tested molecular weights was 60.13%, 34.29%, 17.59%, and 16.36% at the PAM application rates of 0, 0.5, 1.0, and 2.0 g m−2 , respectively. PAM application also decreased the percentage of NH+ 4 loss
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− Fig. 8 Variations of the percentages of K+ , NH+ 4 , and NO3 mass losses with runoff (after subtracting their background values in simulated rainfall) to those over soil surface at polyacrylamide (PAM) application rates of 0, 0.5, 1.0, and 2.0 g m−2 (AR0, AR0.5, AR1.0 and AR2.0, respectively) and molecular weights of 12 and 18 Mg mol−1 . Bars with the same letter(s) are not significantly different at P < 0.05.
with runoff from 93.25% in the control to an averaged value of 87.62% after PAM application (Fig. 8). However, the PAM application rate did not show any obvious effect on the proportion of NH+ 4 loss with runoff (Fig. 8). PAM application did not significantly affect the proportion of NO− 3 loss with runoff to its mass loss over soil surface (Fig. 8). About 86.13% of the NO− 3 mass was lost with runoff, which is comparable with the experimental results of Soupir et al. (2004). This value, together with the proportion value of NH+ 4 mass loss with runoff (Fig. 8), indicates that soluble N is the main loss form of N from soil under a rainfall condition. The tested PAM molecular weight did not impose an obvious effect on the percentages of mass loss for the tested nutrient elements with runoff to those over soil surface (Fig. 8). CONCLUSIONS By the simulated rainfall experiment in the laboratory, we investigated the effects of PAM application rate and molecular weight on runoff, soil erosion, and soil nutrient loss under a relatively short drying time after PAM application. The results showed that the effect of PAM application on runoff depended on its application rate. PAM application increased infiltration rate and decreased runoff when the application rate was 1.0 g m−2 or less. However, opposite results were obtained when the PAM application rate was 2.0 g m−2 . Compared with the control, PAM application increased soil erosion rate and soil mass loss. The soil erosion rate and soil mass loss significantly increased
with the PAM application rate. PAM application de− creased the concentrations of K+ , NH+ 4 , and NO3 in + + sediment and K and NH4 in runoff, but increased + NO− 3 concentration in runoff. Except for the NH4 at PAM application rate of 1.0 g m−2 or less, PAM appli− cation increased the mass losses of K+ , NH+ 4 , and NO3 + over soil surface, and the mass losses of K and NO− 3 over soil surface increased significantly with the increase of PAM application rate. Under the experimental conditions, PAM application did not impose signi− ficant effects on the loss forms of NH+ 4 and NO3 , and more than 86% of them were lost with runoff. However, PAM application decreased K+ loss with runoff. The percentage of the K+ mass loss with runoff to that over soil surface decreased from 60.1% to 16.4% with the increase of PAM application rate from 0 to 2.0 g m−2 . A higher PAM molecular weight resulted in less soil erosion and K+ mass loss, but the effects of PAM molecular weight on runoff and the mass losses of NH+ 4 and NO− were not significant. 3 It can be concluded that PAM application does not prevent soil erosion and the mass losses of soil nutrient elements, especially K+ and NO− 3 , under the experimental conditions. The effects of soil erodibility variation after PAM application and interacting time between PAM molecules and soil particulates on soil loss need to be further investigated. ACKNOWLEDGEMENT The authors wish to thank Dr. F. Ernst from University of California in Riverside, USA for his technological supports in the study.
PAM EFFECT ON RUNOFF, EROSION AND NUTRIENT LOSS
REFERENCES Ajwa, H. A. and Trout, T. J. 2006. Polyacrylamide and water quality effects on infiltration in sandy loam soils. Soil Sci. Soc. Am. J. 70: 643–650. Bajpai, A. K. and Bajpai, S. K. 1995. Kinetics of polyacrylamide adsorption at the iron oxide-solution interface. Colloid. Surface. A. 101: 21–28. Ben-Hur, M. and Keren, R. 1997. Polymer effects on water infiltration and soil aggregation. Soil Sci. Soc. Am. J. 61: 565–570. Ben-Hur, M., Shainberg, I., Keren, R. and Gal, M. 1985. Effect of water quality and drying on soil crust properties. Soil Sci. Soc. Am. J. 49: 191–196. Bhardwaj, A. K., McLaughlin, R. A., Shainberg, I. and Levy, G. J. 2009. Hydraulic characteristics of depositional seals as affected by exchangeable cations, clay mineralogy, and polyacrylamide. Soil Sci. Soc. Am. J. 73: 910–918. Bjorneberg, D. L., Santos, F. L., Castanheira, N. S., Martins, O. C., Reis, J. L., Aase, J. K. and Sojka, R. E. 2003. Using polyacrylamide with sprinkler irrigation to improve infiltration. J. Soil Water Conserv. 58: 283–289. Bradford, J. M., Ferris, J. E. and Remley, P. A. 1987. Interrill soil erosion processes: I. Effect of surface sealing on infiltration, runoff, and soil splash detachment. Soil Sci. Soc. Am. J. 51: 1566–1577. Brown, L. C. and Foster, G. R. 1987. Storm erosivity using idealized intensity distributions. T. ASAE. 30: 379– 386. Cochrane, B. H. W., Reichert, J. M., Eltz, F. L. F. and Norton, L. D. 2005. Controlling soil erosion and runoff with polyacrylamide and phosphogypsum on subtropical soil. T. ASAE. 48: 149–154. Deng, Y., Dixon, J. B. and White, G. N. 2006. Adsorption of polyacrylamide on smectite, illite, and kaolinite. Soil Sci. Soc. Am. J. 70: 297–304. Ekwue, E. I. and Harrilal, A. 2010. Effect of soil type, peat, slope, compaction effort and their interactions on infiltration, runoff and raindrop erosion of some Trinidadian soils. Biosyst. Eng. 105: 112–118. Entry, J. A. and Sojka, R. E. 2003. The efficacy of polyacrylamide to reduce nutrient movement from an irrigated field. T. ASAE. 46: 75–83. Falatah, A. M., Al-Omran, A. M., Shalaby, A. A. and Mursi, M. M. 1999. Infiltration in a calcareous sandy soil as affected by water-soluble polymers. Arid Soil Res. Rehab. 13: 61–73. Flanagan, D. C. and Canady, N. H. 2006a. Use of polyacrylamide in simulated land application of lagoon effluent: Part I. Runoff and sediment loss. T. ASABE. 49: 1361–1369. Flanagan, D. C. and Canady, N. H. 2006b. Use of polyacrylamide in simulated land application of lagoon effluent: Part II. Nutrient loss. T. ASAE. 49: 1371–1381.
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Flanagan, D. C., Norton, L. D. and Shainberg, I. 1997. Effect of water chemistry and soil amendments on a silt loam soil: Part 2. Soil erosion. T. ASAE. 40: 1555– 1561. Green, V. S., Stott, D. E., Norton, L. D. and Graveel, J. G. 2000. Polyacrylamide molecular weight and charge effects on infiltration under simulated rainfall. Soil Sci. Soc. Am. J. 64: 1786–1791. Kay-Shoemake, J. L., Watwood, M. E., Lentz, R. D. and Sojka, R. E. 1998. Polyacrylamide as an organic nitrogen source for soil microorganisms with potential effects on inorganic soil nitrogen in agricultural soil. Soil Biol. Biochem. 30: 1045–1052. Lentz, R. D. 2003. Inhibiting water infiltration with polyacrylamide and surfactants: Applications for irrigated agriculture. J. Soil Water Conserv. 58: 290–300. Lentz, R. D. and Sojka, R. E. 2000. Applying polymers to irrigation water: Evaluating strategies for furrow erosion control. T. ASAE. 43: 1561–1568. Lentz, R. D. and Sojka, R. E. 2009. Long-term polyacrylamide formulation effects on soil erosion, water infiltration, and yields of furrow-irrigated crops. Agron. J. 101: 305–314. Lentz, R. D., Sojka, R. E. and Robbins, C. W. 1998. Reducing phosphorus losses from surface-irrigated fields: Emerging polyacrylamide technology. J. Environ. Qual. 27: 305–312. Lentz, R. D., Sojka, R. E., Robbins, C. W., Kincaid, D. C. and Westermann, D. T. 2001. Polyacrylamide for surface irrigation to increase nutrient-use efficiency and protect water quality. Commun. Soil Sci. Plant Anal. 32: 1203–1220. Levy, G. J. and Agassi, J. M. 1995. Polymer molecular weight and degree of drying effects on infiltration and erosion of three different soils. Aust. J. Soil Res. 33: 1007–1018. Levy, G. J. and Ben-Hur, M. 1998. Some uses of watersoluble polymers in soil. In Wallace, A. and Terry, E. R. (eds.) Handbook of Soil Conditioners: Substances that Enhance the Physical Properties of Soil. Marcel Dekker Inc., New York. pp. 399–428. Levy, G. J., Ben-Hur, M. and Agassi, M. 1991. The effect of polyacrylamide on runoff, erosion, and cotton yield from fields irrigated with moving sprinkler systems. Irrigation Sci. 12: 55–60. Levy, G. J. and Miller, W. P. 1999. Polyacrylamide adsorption and aggregate stability. Soil Till. Res. 51: 121–128. Li, F. H., Yang, S. M. and Peng, C. 2010. Effects of domestic sewage water and ameliorant effectiveness on soil hydraulic conductivity. Soil Sci. Soc. Am. J. 74: 461–468. Mamedov, A. I., Beckmann, S., Huang, C. and Levy, G. J. 2007. Aggregate stability as affected by polyacrylamide molecular weight, soil texture, and water quality. Soil Sci. Soc. Am. J. 71: 1909–1918.
638 Miller, W. P., Willis, R. L. and Levy, G. J. 1998. Aggregate stabilization in kaolinitic soils by low rates of anionic polyacrylamide. Soil Use Manage. 14: 101–105. Moore, I. D. 1981. Effect of surface sealing on infiltration. T. ASAE. 24: 1546–1552. Neave, M. and Rayburg, S. 2007. A field investigation into the effects of progressive rainfall-induced soil seal and crust development on runoff and erosion rates: The impact of surface cover. Geomorphology. 87: 378–390. Oliver, D. P. and Kookana, R. S. 2006. Minimising off-site movement of contaminants in furrow irrigation using polyacrylamide (PAM). II. Phosphorus, nitrogen, carbon, and sediment. Aust. J. Soil Res. 44: 561–567. Peng, C., Li, F. H. and Pan, X. Y. 2006. Effects of polyacrylamide application on hydraulic conductivity of sodic and nonsodic soils. Acta Pedol. Sin. (in Chinese). 143: 835–842. Ramos, M. C., Nacci, S. and Pla, I. 2000. Soil sealing and its influence on erosion rates for some soils in the Mediterranean area. Soil Sci. 165: 398–403. Ramos, M. C., Nacci, S. and Pla, I. 2003. Effect of raindrop impact and its relationship with aggregate stability to different disaggregation forces. Catena. 53: 365–376. Robinson, D. A. and Woodun, J. K. 2008. An experimental study of crust development on chalk downland soils and their impact on runoff and erosion. Eur. J. Soil Sci. 59: 784–798. Ross, C. W., Sojka, R. E. and Foerster, J. A. 2003. Scanning electron micrographs of polyacrylamide-treated soil in irrigation furrows. J. Soil Water Conserv. 58: 327–331. Roth, C. H. 1997. Bulk density of surface crusts: Depth functions and relationships to texture. Catena. 29: 223–237. Santos, F. L. and Serralheiro, R. P. 2000. Improving infiltration of irrigated Mediterranean soils with polyacrylamide. J. Agr. Eng. Res. 76: 83–90. Sepaskhah, A. R. and Bazrafshan-Jahromi, A. R. 2006. Controlling runoff and erosion in sloping land with polyacrylamide under a rainfall simulator. Biosyst. Eng. 93: 469–474. Shainberg, I., Warrington, D. and Rengasamy, P. 1990. Water quality and PAM interactions in reducing surface sealing. Soil Sci. 149: 301–307. Sirjacobs, D., Shainberg, I., Rapp, I. and Levy, G. J. 2000. Polyacrylamide, sediments, and interrupted flow effects
A. P. WANG et al. on rill erosion and intake rate. Soil Sci. Soc. Am. J. 64: 1487–1495. Sojka, R. E., Bjorneberg, D. L., Entry, J. A., Lentz, R. D. and Orts, W. J. 2007. Polyacrylamide in agriculture and environmental land management. Adv. Agron. 92: 75–162. Sojka, R. E., Lentz, R. D., Ross, C. W., Trout, T. J., Bjorneberg, D. L. and Aase, J. K. 1998. Polyacrylamide effects on infiltration in irrigated agriculture. J. Soil Water Conserv. 53: 325–331. Soupir, M. L., Mostaghimi, S., Masters, A., Flahive, K. A., Vaughan, D. H., Mendez, A. and McClellan, P. W. 2004. Effectiveness of polyacrylamide (PAM) in improving runoff water quality from construction sites. J. Am. Water Resour. As. 40: 53–66. Taylor, M. L., Morris, G. E., Self, P. G. and Smart, R. S. C. 2002. Kinetics of adsorption of high molecular weight anionic polyacrylamide onto kaolinite: The flocculation process. J. Colloid Interf. Sci. 250: 28–36. Trout, T. J., Sojka, R. E. and Lentz, R. D. 1995. Polyacrylamide effect on furrow erosion and infiltration. T. ASAE. 38: 761–765. Van Dijk, A. I. J. M., Bruijnzeel, L. A. and Eisma, E. H. 2003. A methodology to study rain splash and wash processes under natural rainfall. Hydrol. Process. 17: 153–167. Warrington, D. N., Mamedov, A. I., Bhardwaj, A. K. and Levy, G. J. 2009. Primary particle size distribution of eroded material affected by degree of aggregate slaking and seal development. Eur. J. Soil Sci. 60: 84–93. Young, M. H., Moran, E. A., Yu, Z. B., Zhu, J. T. and Smith, D. M. 2009. Reducing saturated hydraulic conductivity of sandy soils with polyacrylamide. Soil Sci. Soc. Am. J. 73: 13–20. Yu, J., Lei, T. W., Shainberg, I., Mamedov, A. I. and Levy, G. J. 2003. Infiltration and erosion in soils treated with dry PAM and gypsum. Soil Sci. Soc. Am. J. 67: 630– 636. Zhang, X. C. 2006. Erosion and sedimentation control: Amendment techniques. In Lal, R. (ed.) Encyclopedia of Soil Science. 2nd Edition. Taylor & Francis, London, UK. pp. 544–547. Zhang, X. C. and Miller, W. P. 1996. Polyacrylamide effect on infiltration and erosion in furrows. Soil Sci. Soc. Am. J. 60: 866–872.
Effect of Polyacrylamide Application on Runoff, Erosion, and Soil Nutrient Loss Under Simulated Rainfall∗1 WANG Ai-Ping1 , LI Fa-Hu2,3,∗2 and YANG Sheng-Min4 1
Inner Mongolia Coal Mine Design and Research Institute, No. 55 Xinhua Road, Hohhot 010010 (China) College of Water Conservancy and Civil Engineering, China Agricultural University, No. 17 Qinghua East Avenue, Haidian District, Beijing 100083 (China) 3 State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, No. 26 Xinong Road, Yangling 712100 (China) 4 Beijing Vocational College of Agriculture, No. 5 Daotiannanli, Changyang Town, Beijing 102442 (China) 2
(Received December 20, 2010; revised June 9, 2011)
ABSTRACT Soil erosion affects soil productivity and environmental quality. A laboratory research experiment under simulated heavy rainfall with tap water was conducted to investigate the effects of anionic polyacrylamide (PAM) application rates (0, 0.5, 1.0, and 2.0 g m−2 ) and molecular weights (12 and 18 Mg mol−1 ) on runoff, soil erosion, and soil nutrient loss at a slope of 5◦ . The results showed the two lower rates of PAM application decreased runoff while the highest rate increased runoff as compared with the control. Sediment concentration and soil mass loss increased significantly with the increasing − PAM application rate. Compared with the control, PAM application decreased K+ , NH+ 4 , and NO3 concentrations in + − + + sediment and K and NH4 concentrations in runoff, but significantly increased the mass losses of K , NH+ 4 , and NO3 over + −2 soil surface except for the NH4 at PAM application rate lower than 1.0 g m . PAM application decreased the proportion of K+ loss with runoff to its total mass loss over soil surface from 60.1% to 16.4%. However, it did not affect the NH+ 4 and NO− losses with runoff, and more than 86% of them were lost with runoff. A higher PAM molecular weight resulted in less 3 − soil erosion and K+ mass loss but had little effect on runoff and NH+ and NO losses. PAM application did not prevent 4 3 soil erosion and the mass losses of K+ and NO− 3 under experimental conditions. Key Words:
application rate, mass loss, molecular weight, nutrient concentration, sediment concentration
Citation: Wang, A. P., Li, F. H. and Yang, S. M. 2011. Effect of polyacrylamide application on runoff, erosion, and soil nutrient loss under simulated rainfall. Pedosphere. 21(5): 628–638.
INTRODUCTION Soil nutrients following rainfall can be lost by surface runoff and/or vertical percolating water in a dissolved form and with the eroded soil particles in an adsorbed form. Soil nutrient loss decreases soil fertility and soil productivity, and hence increases the cost of agricultural production. Meanwhile, it also results in water pollution especially for the surface water body when the lost nutrient elements enter into catchment areas. Soil erosion and nutrient loss are the main concerns for soil, agricultural, and environmental scientists all over the world. ∗1
Linear anion polyacrylamide (PAM) is a common soil structure ameliorant. PAM application can increase cohesion among soil particulates, improve soil microstructure and soil aggregate stability against water (Miller et al., 1998; Levy and Miller, 1999; Zhang, 2006; Mamedov et al., 2007), and consequently depress soil sealing formation and soil erosion (Ben-Hur et al., 1985; Bradford et al., 1987; Ben-Hur and Keren, 1997; Bjorneberg et al., 2003; Sojka et al., 2007). However, controversies exist among published literature about PAM effects on soil permeability (Falatah et al., 1999; Santos and Serralheiro, 2000; Peng et al., 2006; Li et al., 2010) and surface runoff (Shainberg et al., 1990;
Supported by the National Natural Science Foundation of China (No. 40635027) and the Fund of State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, China (No. 10501-169). ∗2 Corresponding author. E-mail: [email protected].
PAM EFFECT ON RUNOFF, EROSION AND NUTRIENT LOSS
Levy and Agassi, 1995; Sirjacobs et al., 2000; Sepaskhah and Bazrafshan-Jahromi, 2006). PAM properties and its application rate and application method, soil characteristics, and topography etc. all influence PAM efficiency on soil permeability and soil erosion (Zhang and Miller, 1996; Green et al., 2000; Lentz, 2003; Bhardwaj et al., 2009; Young et al., 2009). The length of polymer chain and the viscosity of PAM solution increase with the increase of PAM molecular weight (Green et al., 2000). PAM with a high molecular weight is more effective for clay flocculation, and its influence on infiltration is also greater than PAM with a low molecular weight (Levy and Agassi, 1995; Green et al., 2000). As compared with the control treatments, the application of PAM with a range of molecular weights from 6 to 18 Mg mol−1 increased soil permeability by 3 to 5 times (Green et al., 2000). Sojka et al. (1998) thought PAM use increased the infiltration of fine and medium texture soils, and its effect on the infiltration of coarse texture soil was small. The experimental results of Sirjacobs et al. (2000) showed PAM application decreased the infiltration rate of Alfisol but increased it in the Vertisol. Shainberg et al. (1990) reported PAM application with a rate greater than 20 kg ha−1 increased infiltration rate significantly. However, the experimental results of Ajwa and Trout (2006), Soupir et al. (2004), and Yu et al. (2003) demonstrated PAM application decreased soil infiltration and increased surface runoff in loam soils. Inconsistent experimental results are also present on soil erosion (Trout et al., 1995; Lentz and Sojka, 2000; Cochrane et al., 2005; Sepaskhah and BazrafshanJahromi, 2006; Lentz and Sojka, 2009). Research results indicated that PAM application decreased soil erosion in furrow irrigation significantly (Trout et al., 1995; Lentz and Sojka, 2000, 2009). Zhang and Miller (1996) reported a similar result about PAM application under simulated rainfall. However, the field experimental results of Levy et al. (1991) showed that PAM application resulted in a similar sediment concentration to the control treatment. It was also reported that the presence of PAM in the simulated rainfall or in inflow water acted to enhance soil loss when runoff was sufficient to transport sediment (Flanagan et al., 1997). Generally, the influence of PAM application on infiltration or runoff is less than that on soil erosion (Yu et al., 2003; Cochrane et al., 2005; Sepaskhah and Bazrafshan-Jahromi, 2006), and it is also more transient under sprinkler irrigation than under furrow irrigation (Sojka et al., 1998). PAM application influences surface runoff and soil
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erosion, so it will impose certain effects on soil nutrient loss (Kay-Shoemake et al., 1998; Lentz et al., 1998; Entry and Sojka, 2003; Flanagan and Canady, 2006b; Oliver and Kookana, 2006). By furrow irrigation experimentation, Oliver and Kookana (2006) thought anion PAM application did not significantly affect N and K+ concentrations in the runoff and the eroded soil particulates, but decreased their mass losses by 94% and 60%, respectively. The experimental results of Lentz et al. (1998, 2001) showed PAM use had little influence on NO− 3 concentration in runoff. Flanagan and Canady (2006b) reported that PAM applied in lagoon effluent decreased NH+ 4 mass loss from 34% to 92%. Soil erosion depends on the interaction of soil erosion resistance and water erosive force. The variations of both forces after PAM application will change their balance, and hence result in different outcomes. As an erosion inhibitor, more researches about PAM effects on soil erosion were reported, however less information was available on PAM influences on soil nutrient loss. It is not well understood how PAM application affects the loss of soil nutrients, and how the proportion of soil nutrient loss with runoff to its total mass loss over soil surface changes after PAM application under rainfall conditions. It is important to look for more effective control measures to improve utilization efficiency of soil fertilizer and protect the water environment, especially surface water bodies. Thus, the objectives of this study were to investigate the effects of PAM application rate and molecular weight on runoff, soil erosion, soil nutrient losses and their loss proportions with runoff over the soil surface. These effects were tested under a simulated rainfall with tap water so as to search for more suitable management schemes for sprinkler irrigation. MATERIALS AND METHODS The soil samples were collected from the East Campus of China Agricultural University (uncultivated land). Soil texture was loam soil with 471 g kg−1 sand, 422 g kg−1 silt, and 107 g kg−1 clay. Of the clay mineralogy, the main minerals were chlorite, illite, and montmorillonite measured by a D/Max-RC ray diffractometer (Rigaku Co., Japan). The cation exchange capacity (CEC), exchangeable sodium percentage (ESP), and organic matter content of the tested soil were 12.6 cmolc kg−1 , 2.03%, and 3.82 g kg−1 , respectively. The pH in the soil extract (water:soil = 2:1) was 7.91. Total soluble salt and chemical element concentrations in the tested soil are listed in Table I. After air-drying and passed through a 4-mm sieve, the soil sample was artificially fertilized with the che-
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A. P. WANG et al.
TABLE I Total soluble salt (TSS) and chemical element concentrations in the tested soil Total
Soluble K+
N
P
0.10
g kg−1 0.52 12.00
TSS 0.67
Ca2+ 60
Mg2+ 36
Na+ 53
micals (reagent pure grade) of KNO3 (2 g kg−1 soil) and KH2 PO4 (1 g kg−1 soil). According to the designed application rates of fertilizers, KNO3 and KH2 PO4 masses were calculated, weighed, and then dissolved in a given volume of distilled water. The prepared solutions of fertilizers were evenly sprayed on the soil sample. The fertilized soil was fully mixed, sealed, and stored for 12 h, then spread and air-dried. An experimental soil tray (60 cm × 30 cm × 15 cm) made of steel was fixed at a slope gradient of 5◦ (about 9%). A V-shaped runoff collection plate was connected to the end of the tray. A series of small holes were drilled on the bottom of the tray. Cleaned gravel with a thickness of 2 cm and a piece of coarse gauze were laid on the bottom of the tray to facilitate free drainage and avoid soil particles dropping from the holes. The prepared soil sample was evenly packed into the soil tray at a bulk density of 1.35 Mg m−3 . The soil thickness in the tray was 8 cm. The soil surface in the tray was at the same level as the bottom of the V-shaped runoff collection plate. The tested PAM was linear anion polymer (Hanlimiao Hi-Tech. Co. Ltd., Beijing). Its major active ingredient was acrylamide and sodium acrylate with a charge density of 30%. PAM masses of 90, 180, and 360 mg were weighed, dissolved in 1 L of distilled water, and stirred fully by hand for 30–60 min. The prepared PAM solutions were evenly sprayed on soil surfaces to get different PAM application rates. After the prepared soils were dried for about 1 h, a simulated rainfall process began. The artificial rainfall simulator consisted of a water supply system, a water-spraying system, and an effluent collector. The water supply system included two sets of vertical pipes with a height of 4.75 m, connected by horizontal pipes with a length of 0.9 m, which was fixed by tripods and ‘stay wires’. Two sprayingdownward, conical emitters (Spraco, Phoenix, USA) were connected with the end of the horizontal pipes. The distance between the two sprayers was 4 m. Under a water pressure of 75 kPa, the calibrated rainfall
Cl−
SO2− 4
HCO− 3
52
mg kg−1 86 183
N
P
K+
13.32
1.78
84.90
intensity of the simulator was 60 mm h−1 , and its rainfall uniformity coefficient and raindrop median diameter were 0.955 and 1.44 mm, respectively. The calculated kinetic energy of rainfall was 27.96 J m−2 mm−1 (Brown and Foster, 1987). Municipal tap water was applied to simulate rainwater in the experiment. The electrical conductivity of the tap water was about 0.5 − dS m−1 , and the K+ , NH+ 4 , and NO3 concentrations were 1.591, 0.245, and 9.349 mg L−1 , respectively. The rainfall duration was 80 min for all experimental treatments. After surface runoff occurred on the soil surface of experimental trays, runoff water from the outlet of the soil tray was continually collected at regular intervals with plastic buckets. The collection duration for each bucket was 5 min and the runoff volume collected in each bucket was measured. After settling, sediments in the buckets were dried in an oven at 105 ◦ C and weighed. The concentrations of suspended material in runoff were calculated according to the sediment mass and runoff volume collected in each bucket. After filtering through 0.45-μm filter paper, the − concentrations of K+ , NH+ 4 , and NO3 in runoff were determined with an ionic chromatographic instrument (ICS-1500, Dionex, USA). The concentrations of NH+ 4 and NO− 3 in sediment were measured with a flow analyzer (AA3, Bran-Luebbe Co., Norderstedt, Germany) after the sediment was eluted with 1 mol L−1 KCl solution at a water:soil ratio of 5:1. Potassium ionic concentration in the sediment was determined with a flame photometer (PF640, Shanghai Precision & Scientific Instrument Co. Ltd., Shanghai) after it was eluted with 1 mol L−1 NH4 OAC solution at a water:soil ratio of 10:1. All measurements for the chemical elements were carried out according to standard procedures. Experimental treatments consisted of three application rates of PAM (0.5, 1.0, and 2.0 g m−2 ) and two molecular weights of PAM (12 and 18 Mg mol−1 ), with a treatment without PAM application as the control. All experimental treatments were carried out in triplicate. In order to calculate the mass loss of nu-
PAM EFFECT ON RUNOFF, EROSION AND NUTRIENT LOSS
trient elements with runoff, the background values of tested nutrient elements in the simulated rainfall (tap water) were subtracted from their measured concentrations. The significance of difference among treatment means was tested by the analysis of variance (one-way ANOVA) using SPSS 16.0. When the F -value in the ANOVA was statistically significant, a least significant difference test was used for the separation of means at P = 0.05. RESULTS AND DISCUSSION Surface runoff and soil erosion During the experimental process, no visible rill was formed on the soil surface, and soil erosion predominantly was interrill. The variation of surface runoff rate with rainfall duration for various PAM application rates under the PAM molecular weight of 18 Mg mol−1 is shown in Fig. 1. The experimental data for the trea-
Fig. 1 Variations of surface runoff rate at a polyacrylamide (PAM) molecular weight of 18 Mg mol−1 and runoff volume at PAM molecular weights of 12 and 18 Mg mol−1 with PAM application rates of 0, 0.5, 1.0, and 2.0 g m−2 (AR0, AR0.5, AR1.0, and AR2.0, respectively). Bars with the same letter are not significantly different at P < 0.05.
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tment with a PAM molecular weight of 12 Mg mol−1 are not shown thereafter because of its similar variation tendency to that with a molecular weight of 18 Mg mol−1 . Runoff rate increased with rainfall duration and then it gradually approached a steady value after rainfall of about 40 min (Fig. 1). The steady runoff rate increased with the increased PAM application rate. However, it was smaller than that in the control at a PAM application rate of 1.0 g m−2 or less, which is consistent with many experimental results (Moore, 1981; Flanagan and Canady, 2006a), and was greater than that in the control at a greater application rate of PAM. The average steady runoff rates were 153.0, 148.5, 149.5, and 161.5 mL min−1 , respectively, for the PAM application rates of 0, 0.5, 1.0, and 2.0 g m−2 under two tested PAM molecular weights. As compared with the control, the steady runoff rate decreased by 2.94% for the PAM application rate of 0.5 g m−2 and 2.29% for the rate of 1.0 g m−2 , but it increased by 5.56% for the rate of 2.0 g m−2 . PAM molecular weight did not impose an obvious effect on runoff rate (data not shown) possibly because of the relatively fine texture of the tested soil (Levy and Agassi, 1995). Similar to the steady runoff rate, runoff volume also increased with the increase of PAM application rate (Fig. 1). The runoff volume was smaller at the PAM application rates of 0.5 and 1.0 g m−2 and greater at the application rate of 2.0 g m−2 than that in the control. Under rainfall or sprinkler irrigation conditions, raindrops ‘attack’ soil surface and results in soil compaction, aggregate breakup, soil particle displacement, and consequently seal formation on the soil surface. PAM application increases the stability of soil aggregates and depresses the occurrence of soil seal (Ajwa and Trout, 2006). Therefore, it improves soil permeability and decreases surface runoff (Moore, 1981; Bradford et al., 1987; Sepaskhah and BazrafshanJahromi, 2006). Our experimental results confirmed this conclusion under the PAM application rate of 1.0 g m−2 or less (Fig. 1). However, the viscosity of PAM water solution increases with the increased PAM concentration (Levy and Agassi, 1995; Li et al., 2010). The increased viscosity of soil solution at the PAM application rate of 2.0 g m−2 possibly resulted in the decrease of soil permeability and subsequently the increase of surface runoff (Fig. 1). This phenomenon was also observed by other investigators (Falatah et al., 1999; Soupir et al., 2004; Ajwa and Trout, 2006; Young et al., 2009).
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Sediment concentration in runoff generally was low and steady with rainfall duration under no PAM application (Fig. 2). However, PAM application significantly increased the sediment concentration after about 25 min of rainfall beginning. The greater the PAM application rate, the higher the sediment concentration in runoff (Fig. 2). Similarly, soil mass loss also increased significantly (P < 0.001) with the increase of PAM application rate for all experimental treatments (Fig. 2), which is consistent with the experimental results of Flanagan et al. (1997). When the PAM application rates were 0.5, 1.0, and 2.0 g m−2 , the average soil mass losses were 1.11, 2.81, and 5.07 times, respectively, greater than that in the control for both tested PAM molecular weights.
Fig. 2 Variations of sediment concentration at a polyacrylamide (PAM) molecular weight of 18 Mg mol−1 and soil mass loss at PAM molecular weights of 12 and 18 Mg mol−1 with PAM application rates of 0, 0.5, 1.0, and 2.0 g m−2 (AR0, AR0.5, AR1.0 and AR2.0, respectively). Bars with the same letter are not significantly different at P < 0.05.
When PAM application rate was 1.0 g m−2 or less, PAM molecular weight affected sediment concentration (data not shown) and soil mass loss significantly
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(Fig. 2). A greater PAM molecular weight resulted in a smaller sediment concentration and less soil loss. However, PAM molecular weight did not impose a significant effect on soil erosion when the PAM application rate was 2.0 g m−2 , which is consistent with the experimental results of Mamedov et al. (2007). The decrease of erosion resistance of surface soil other than the increase of surface runoff possibly is the main reason for the increased soil erosion under the experimental conditions, because a greater dependence of soil mass loss on the average sediment concentration (R2 = 0.99) than on the runoff volume (R2 = 0.56) existed (data not shown). Five possible mechanisms are responsible for the increased soil loss after PAM use: 1) The physical process of soil interrill erosion is a function of detachment by raindrops and the shallow flow transport of sediment particles (Flanagan et al., 1997). Soil mass loss depends on the interactions of rainfall erosivity (raindrop impact), the kinetic energy of runoff, and soil erosion resistance. Soil sealing formation decreases soil permeability (Ramos et al., 2003), and hence increases runoff and soil detachment capacity (Ramos et al., 2000; Robinson and Woodun, 2008; Warrington et al., 2009). However, as compared with the splashed erosion, the erosion loss induced by runoff wash under laboratory conditions generally only accounts for a small proportion of total erosion loss (Flanagan et al., 1997; Van Dijk et al., 2003; Robinson and Woodun, 2008). Meanwhile, soil sealing formation also increases soil strength and subsequently soil erosion resistance due to the increase of bulk density of surface soil (Levy et al., 1991; Roth, 1997; Neave and Rayburg, 2007; Warrington et al., 2009; Ekwue and Harrilal, 2010). After PAM application, the formation and development of soil sealing are restricted, and the resistance of surface soil against erosion by raindrop splash decreases (Flanagan et al., 1997). When the greater soil aggregates that form under the aid of PAM (Zhang and Miller, 1996; Miller et al., 1998; Ross et al., 2003), as compared with the control, are initiated or activated by raindrop impact or runoff under a relatively steep slope, more soils will be lost. 2) PAM adsorption in soil is a kinetic process because it can be adsorbed by both the external and internal surfaces of soil aggregates (Bajpai and Bajpai, 1995; Miller et al., 1998; Levy and Miller, 1999; Taylor et al., 2002). The contact time of PAM molecules with soil aggregate affects the effectiveness of PAM function (Flanagan et al., 1997). A relatively short drying duration between
PAM EFFECT ON RUNOFF, EROSION AND NUTRIENT LOSS
PAM application and rainfall occurrence reduced PAM efficiency on restraining soil erosion in this research. 3) A greater viscosity at the water-soil boundary after PAM application enhances soil detachment and sediment transport through greater hydraulic drag force (Flanagan et al., 1997; Li et al., 2010). 4) A relatively high pH in soil solution in this study also reduced PAM efficiency on depressing soil erosion (Taylor et al., 2002; Deng et al., 2006). 5) Moreover, a relatively high electrolyte concentration in the simulated rainfall (tap water) led to shorter polymer chains and less efficiency in binding together soil particles that are far apart, and hence the efficiency of PAM on inhibiting soil loss was decreased (Yu et al., 2003; Mamedov et al., 2007). However, this effect of electrolyte on PAM’s length may be small (Levy and Ben-Hur, 1998). Concentrations of nutrient elements in runoff and their mass losses − The variations of K+ , NH+ 4 , and NO3 concentrations with rainfall duration are showed in Fig. 3 under the PAM application rates of 0, 0.5, 1.0, and 2.0 g m−2 and the molecular weight of 18 Mg mol−1 . The − K+ , NH+ 4 , and NO3 concentrations in runoff decreased monotonously with the increase of rainfall duration (Fig. 3), which is the result of dissolved nutrient element loss with runoff or infiltration. Compared with the control, PAM application decreased the concentra− tions of K+ and NH+ 4 but increased NO3 concentration in runoff (Fig. 3). The adsorption of the anionic functional group in PAM molecules for the cationic nutrient elements of K+ and NH+ 4 in soil solution and its repulse for the anionic element of NO− 3 possibly are responsible for this result (Entry and Sojka, 2003). PAM molecular weight did not have obvious effects on the
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− concentrations of K+ , NH+ 4 , and NO3 in runoff (data not shown). As compared with the control, the average mass loss of K+ with runoff (after subtracting the background value in the simulated rainfall) at the two tested PAM molecular weights decreased by 37.37%, 51.70%, and 34.91% at the PAM application rates of 0.5, 1.0, and 2.0 g m−2 , respectively (Fig. 4). Correspondingly, the average mass loss of NH+ 4 decreased by 9.45% and 10.82%, respectively, at the PAM application rates of 0.5 and 1.0 g m−2 , however, it increased by 4.49% at the PAM application rate of 2.0 g m−2 (Fig. 4). The average mass loss of NO− 3 with runoff at the two tested PAM molecular weights increased significantly (P < 0.05) with the increase of PAM application rate, by 23.82%, 87.40%, and 173.65% at the PAM application rates of 0.5, 1.0, and 2.0 g m−2 , respectively, as compared with the control (Fig. 4). Compared with the K+ , the slightly smaller variation ranges of NH+ 4 concentration in runoff (Fig. 3) and its mass loss with runoff (Fig. 4) after PAM application could not be interpreted. This is possibly related to the complex transformation cycle of nitrogen in soil.
Concentrations of nutrient elements in sediment and their mass losses − The concentrations of K+ , NH+ 4 , and NO3 in sediment decreased with rainfall duration (Fig. 5). Generally, the greater the PAM application rate, the lower their concentrations in sediment for all experimental treatments (Fig. 5). The eroded soil particulates usually are fine silt or clay with high nutrient concentrations due to their greater adsorption (Warrington et al., 2009). Compared with the control treatment, PAM application
− Fig. 3 Variations of K+ , NH+ 4 , and NO3 concentrations in runoff at polyacrylamide application rates of 0, 0.5, 1.0, and 2.0 −2 g m (AR0, AR0.5, AR1.0 and AR2.0, respectively) and a molecular weight of 18 Mg mol−1 . The dashed lines represent the corresponding ion concentrations in rainfall.
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− Fig. 4 Variations of K+ , NH+ 4 , and NO3 mass losses with surface runoff (after subtracting their background value in simulated rainfall) at polyacrylamide (PAM) application rates of 0, 0.5, 1.0, and 2.0 g m−2 (AR0, AR0.5, AR1.0 and AR2.0, respectively) and molecular weights of 12 and 18 Mg mol−1 . Bars with the same letter(s) within each molecular weight are not significantly different at P < 0.05.
− Fig. 5 Variations of K+ , NH+ 4 , and NO3 concentrations in sediment at polyacrylamide application rates of 0, 0.5, 1.0, and −2 2.0 g m (AR0, AR0.5, AR1.0 and AR2.0, respectively) and molecular weight of 18 Mg mol−1 .
results in the erosion of soil aggregate with greater size, in which nutrient concentrations generally are lower (Miller et al., 1998). This, together with the continual losses of nutrient elements with infiltration and runoff, is the reason that nutrient concentrations in sediment decreased with the increased PAM application rate (Fig. 5). − The mass losses of K+ , NH+ 4 , and NO3 with the eroded soil particulates generally increased with the increase of PAM application rate (Fig. 6). However, the − mass losses of NH+ 4 and NO3 with the eroded soil particulates were less than about 0.8 mg under the experimental conditions, which was much smaller than those lost with runoff (Fig. 4). A greater PAM molecular weight generally resulted − in smaller concentrations of K+ , NH+ 4 , and NO3 in sediment (data not shown) and their less mass losses with the eroded soil particulates (Fig. 6). The effects of
− tested PAM molecular weights on K+ , NH+ 4 , and NO3 concentrations in sediment and their mass losses were not significant at P < 0.05. The mass loss of nutrient elements with the eroded soil particulates depends on soil erosion rate and the concentrations of nutrient elements in sediment. PAM application decreased the concentrations of K+ , − NH+ 4 , and NO3 in sediment (Fig. 5), but significantly increased the soil erosion rate and soil mass loss (Fig. 2). The increased degree of the soil erosion rate (1.49 to 6.50 times) after PAM application was much greater than the decreased degrees of the nutrient element concentrations (3.33% to 17.50%, 14.32% to 70.87%, and 22.25% to 73.62%, respectively, for K+ , − NH+ 4 , and NO3 ) in sediment. Therefore, the mass losses of tested nutrient elements with the eroded soil particulates increased with the increase of PAM application rate (Fig. 6).
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− Fig. 6 Variations of K+ , NH+ 4 , and NO3 mass losses with eroded soil particulates at polyacrylamide (PAM) application −2 rates of 0, 0.5, 1.0, and 2.0 g m (AR0, AR0.5, AR1.0 and AR2.0, respectively) and molecular weights of 12 and 18 Mg mol−1 . Bars with the same letter are not significantly different at P < 0.05.
− Fig. 7 Variations of K+ , NH+ 4 , and NO3 mass losses over soil surface (after subtracting their background values in simulated rainfall) at polyacrylamide (PAM) application rates of 0, 0.5, 1.0, and 2.0 g m−2 (AR0, AR0.5, AR1.0 and AR2.0, respectively) and molecular weights of 12 and 18 Mg mol−1 . Bars with the same letter(s) are not significantly different at P < 0.05.
Mass losses of nutrient elements over soil surface and their loss proportions The mass losses of nutrient elements over soil surface (i.e., both with surface runoff and the eroded soil particulates) are shown in Fig. 7. Generally, PAM application increased the mass losses of K+ , NH+ 4 , and + over soil surface except for NH at the PAM apNO− 3 4 plication rate less than 1.0 g m−2 (Fig. 7). The two tested molecular weights of PAM did not have significant effects on the mass losses of all tested nutrient elements (Fig. 7). The mass loss of nutrient element over soil surface depends on runoff rate, soil erosion rate, and nutrient element concentrations in runoff and sediment. Although PAM application decreased the concentrations of nutrient elements (with the exception of
NO− 3 ) in runoff (Fig. 3) and sediment (Fig. 5), it significantly increased the soil erosion rate and erosion mass (Fig. 2). Hence, PAM application generally resulted in − the increased mass losses of K+ , NH+ 4 , and NO3 over soil surface (Fig. 7). The proportions of the mass losses of tested nutrient elements with runoff to their mass losses over soil surface (including all losses with runoff and the eroded soil particulates) are calculated and shown in Fig. 8 under various tested PAM application rates and molecular weights. Compared with the control, PAM application significantly decreased (P < 0.05) the percentage of K+ loss with runoff (Fig. 8). Its average percentage for the two tested molecular weights was 60.13%, 34.29%, 17.59%, and 16.36% at the PAM application rates of 0, 0.5, 1.0, and 2.0 g m−2 , respectively. PAM application also decreased the percentage of NH+ 4 loss
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− Fig. 8 Variations of the percentages of K+ , NH+ 4 , and NO3 mass losses with runoff (after subtracting their background values in simulated rainfall) to those over soil surface at polyacrylamide (PAM) application rates of 0, 0.5, 1.0, and 2.0 g m−2 (AR0, AR0.5, AR1.0 and AR2.0, respectively) and molecular weights of 12 and 18 Mg mol−1 . Bars with the same letter(s) are not significantly different at P < 0.05.
with runoff from 93.25% in the control to an averaged value of 87.62% after PAM application (Fig. 8). However, the PAM application rate did not show any obvious effect on the proportion of NH+ 4 loss with runoff (Fig. 8). PAM application did not significantly affect the proportion of NO− 3 loss with runoff to its mass loss over soil surface (Fig. 8). About 86.13% of the NO− 3 mass was lost with runoff, which is comparable with the experimental results of Soupir et al. (2004). This value, together with the proportion value of NH+ 4 mass loss with runoff (Fig. 8), indicates that soluble N is the main loss form of N from soil under a rainfall condition. The tested PAM molecular weight did not impose an obvious effect on the percentages of mass loss for the tested nutrient elements with runoff to those over soil surface (Fig. 8). CONCLUSIONS By the simulated rainfall experiment in the laboratory, we investigated the effects of PAM application rate and molecular weight on runoff, soil erosion, and soil nutrient loss under a relatively short drying time after PAM application. The results showed that the effect of PAM application on runoff depended on its application rate. PAM application increased infiltration rate and decreased runoff when the application rate was 1.0 g m−2 or less. However, opposite results were obtained when the PAM application rate was 2.0 g m−2 . Compared with the control, PAM application increased soil erosion rate and soil mass loss. The soil erosion rate and soil mass loss significantly increased
with the PAM application rate. PAM application de− creased the concentrations of K+ , NH+ 4 , and NO3 in + + sediment and K and NH4 in runoff, but increased + NO− 3 concentration in runoff. Except for the NH4 at PAM application rate of 1.0 g m−2 or less, PAM appli− cation increased the mass losses of K+ , NH+ 4 , and NO3 + over soil surface, and the mass losses of K and NO− 3 over soil surface increased significantly with the increase of PAM application rate. Under the experimental conditions, PAM application did not impose signi− ficant effects on the loss forms of NH+ 4 and NO3 , and more than 86% of them were lost with runoff. However, PAM application decreased K+ loss with runoff. The percentage of the K+ mass loss with runoff to that over soil surface decreased from 60.1% to 16.4% with the increase of PAM application rate from 0 to 2.0 g m−2 . A higher PAM molecular weight resulted in less soil erosion and K+ mass loss, but the effects of PAM molecular weight on runoff and the mass losses of NH+ 4 and NO− were not significant. 3 It can be concluded that PAM application does not prevent soil erosion and the mass losses of soil nutrient elements, especially K+ and NO− 3 , under the experimental conditions. The effects of soil erodibility variation after PAM application and interacting time between PAM molecules and soil particulates on soil loss need to be further investigated. ACKNOWLEDGEMENT The authors wish to thank Dr. F. Ernst from University of California in Riverside, USA for his technological supports in the study.
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