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Nitrate vertical transport in the main paddy soils of Tai Lake region, China
Geoderma 142 (2007) 136 – 141 www.elsevier.com/locate/geoderma
Nitrate vertical transport in the main paddy soils of Tai Lake region, China ☆ Xiaomin Chen ⁎, Huashan Wu, Fei Wo College of Resources and Environmental Sciences, Nanjing Agricultural University, Nanjing 210095, The People's Republic of China Received 30 August 2006; received in revised form 17 March 2007; accepted 5 August 2007 Available online 5 September 2007
Abstract The influence of texture, bulk density, and organic matter content on the process of nitrate vertical transport in the three main paddy soils (Bai soil, Huangni soil and Wushan soil) of the Tai Lake region were studied in the soil columns. Breakthrough curves (BTC) were obtained separately for each of thirteen horizons. The results were as follows: vertical transport velocity of nitrate decreased, and the BTCs of nitrate were more dispersed, in each horizon from the surface layer to the bottom in every soil profile. Among the three soils, the average flux of Wushan soil was the least and its nitrate BTC was the most dispersed. Under saturated conditions, nitrate penetrated the soil column quickly. The transport of nitrate was affected by clay content. As the clay content increased, nitrate outflow was retarded, and the peak concentration was reduced. Nitrate BTCs rose and fell gently when the nitrate concentration was lower. All nitrate BTCs were asymmetric, and tailing was more obvious when clay content was high. Soil bulk density and the organic matter content also affected the vertical transport of nitrate. Low bulk density and high organic matter content were each associated with faster nitrate transport. An analytical solute transport model (CXTFIT 2.0) was used to estimate the nitrate dispersion coefficient and average pore-water velocity from the observed breakthrough curves. The results showed that the analytical solute transport model was suitable in fitting the observed nitrate transport in these soils. The dispersion coefficient was found to be a function of average pore-water velocity. © 2007 Elsevier B.V. All rights reserved. Keywords: Nitrate vertical transport; Influence factors; CXTIT model; Dispersion coefficient; Pore-water velocity; Solute transport; Tai Lake region
1. Introduction In rice production, farmers tend to apply great amounts of nitrogen fertilizer to certain high yielding fields, from 270 kg N ha− 1 in 1998 to 366 kg N ha− 1 in 2002 (Wang et al., 2003). This high application rate resulted in total N loading much greater than the N demand of rice (Zheng et al., 2001), and the utilization ratio of nitrogen was only 20.9 ∼ 34.3%, lower than the national average of 30 ∼ 41% (Zhu and Wen, 1992). Volatilization of nitrogen into the atmosphere and leaching into ground water not only wastes fertilizer nitrogen and energy resources but also increases the risk of ground water contamination. Nitrate (NO3−) is a familiar pollutant in groundwater. Nitrate can be converted into nitrite (NO2−N). Nitrate not used by the ☆ Foundation item: The National Natural Science Foundation of China (no. 40371055) and the Special Scientific Research Foundation of College Doctor Station, China (no. 20030307018). ⁎ Corresponding author. Tel./fax: +86 25 84396842. E-mail address: [email protected] (X. Chen).
0016-7061/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2007.08.004
crop can be leached below the root zone into groundwater or run off into surface water, which is a problem because NO3− concentrations often exceed acceptable contamination limits (Strebel et al., 1989; Fletcher, 1991). Large amounts of nitrate in drinking water are a cause of methemoglobinemia, a blood disorder primarily affecting infants under six months of age (Bengtsm and Annadotter, 1989). Excessive levels of nitrate in drinking water may induce cancer in human and animals (Zhu and Wen, 1992). Therefore, nitrate concentration is an important criterion of groundwater quality (Wang, 1997; William et al., 2004). The Tai Lake region is one of China's most intensive agricultural regions, with a long history of rice cultivation. Various paddy soils covering 90% of the region have been developed over several centuries. Excessive chemical fertilizer application in the region has led to an increase of NO3− in the groundwater, which in turn has led to eutrophication of Tai Lake. The concentration of NO3− detected in water bodies has been influenced by agricultural drainage. Since the population is increasing and farmland is limited in this region, an increase
X.M. Chen et al. / Geoderma 142 (2007) 136–141
137
Table 1 Selected characteristics of the soil used in the study Soil names
Soil horizon (cm)
Bulk density (g cm− 3)
Organic matter (g kg− 1)
Clay (g kg− 1) (b0.002 mm)
Silt ( g kg− 1) (0.02–0.002 mm)
Sand ( g kg− 1) (N0.02 mm)
Bai soil
0–12 12–20 20–30 30–55 55–100 0–15 15–25 25–65 65–100 0–15 15–28 28–42 42–70
1.33 1.40 1.60 1.48 1.49 1.12 1.23 1.42 1.39 1.04 1.28 1.43 1.19
25.72 26.39 14.36 5.34 7.6 36.15 17.19 12.58 8.06 40.72 29.06 19.23 24.66
186.1 192.6 249.3 276.7 418.4 310.7 299.2 445.7 349.4 326.1 361.9 401.2 405.2
472.2 545.0 262.8 508.3 145.6 390.7 399.7 350.4 349.5 446.5 452.5 506.1 582.1
341.7 262.4 487.9 215.0 436.0 298.6 301.1 203.9 271.1 227.4 185.6 92.7 12.7
Huangni soil
Wushan soil
in agricultural productivity is essential. To keep pace with population expansion, the application rate of chemical fertilizers, which are the major source of agricultural non-point pollution, has been increasing in recent years. To minimize the effect on Tai Lake, control of rainfall runoff and infiltrating water containing nutrients from farmland is needed. Therefore, understanding the process of nitrate vertical movement is necessary to assess the consequences of new agricultural management practices (Chen et al., 2003). Many factors affect soil nitrate movement, including the utilization ratio of fertilizer, fertilizer application and irrigation times, soil texture and structure in the soil profile and so on (Guo and Zhang, 2001). However, no reports are available on the influence of these factors on parameters of nitrate vertical transport in the major paddy soils of the Tai Lake region (Yuan and Wang, 2000). Analytical solutions of the governing steady-state solute transport models may be used in an inverse way to estimate nitrate transport parameters (Toride et al., 1995). Using inverse procedures, breakthrough curves from laboratory or field observations are matched to the analytical solutions. Computer codes such as CXTFIT 2.0 (Toride et al., 1995) are available to predict nitrate distributions in time and space for specified model parameters and to estimate nitrate parameters in an inverse way. Therefore, the objective of this work was to study the factors and parameters influencing vertical nitrate transport processes in the Tai Lake area and to provide useful scientific information for reasonable fertilizer application and environmental protection in sustainable agriculture.
Chuangshu city, Jiangsu Province. The two soil profiles are divided into four horizons, at depths 0–15, 15–25, 25–65, 65– 100 cm and 0–15, 15–28, 28–42, and 42–70 cm respectively. Some characteristics of these soils are given in Table 1. 2.2. Experimental methods Packed columns of each soil were installed in the laboratory apparatus for measuring nitrate transport, shown in Fig. 1. Soil samples from each horizontal layer were air-dried and crushed to pass a 20-mesh screen. In accordance with its fieldmeasured bulk density, each soil layer sample was packed into a PVC column for determination of nitrate vertical transport. All soil columns were 50 cm high and 20 cm in diameter. A quartz sand layer about 1 cm depth was placed on the top and bottom of the soil column to protect the packed soil column from disturbance by water flow. The columns were saturated with distilled water, then washed with distilled water using a syringe peristaltic pump for 2–3 days until solutes in the soil were completely leached out of the columns. After establishing steady-state saturated flow using distilled water, 100 mL of a solution containing 100 mg L− 1 nitrate was applied continuously to the soil columns. The nitrate solution was applied from 0 to 120 h or so, depending on soil texture. The cumulative outflow solution was
2. Materials and methods 2.1. Testing and basic properties of soils The three main paddy soils used in this study were collected from the Tai Lake region of China. Bai soil was in Yixing County, Jiangsu Province. This soil profile can be divided into five horizons occurring at depths of 0–12, 12–20, 20–30, 30– 55 and 55–100 cm. Huangni soil and Wushan soil are found in
Fig. 1. Schematics of the apparatus for vertical nitrate transport study.
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Table 2 Parameters of nitrate BTCs Soil types
Soil horizon (cm)
Flux (Q), (cm h− 1)
Relative concentration C/C0
Initial breakthrough time (h)
Terminative time, (h)
Total time, (h)
Time of curve climax, (h)
Bai soil
0–12 12–20 20–30 30–55 55–100 0–15 15–25 25–65 65–100 0–15 15–28 28–42 42–70
20.12 16.82 12.4 11.68 6.07 12.38 11.54 6.56 7.11 18.23 12.68 8.12 8.48
0.4826 0.4437 0.3744 0.3960 0.2364 0.3232 0.3001 0.2448 0.2568 0.4031 0.3454 0.2410 0.2503
8 10 14 14 22 10 12 22 20 12 16 20 18
30 38 60 58 142 54 74 144 128 42 62 122 110
22 28 46 44 120 44 62 122 108 30 46 102 92
18 24 34 32 74 30 40 74 66 24 36 64 58
Huangni soil
Wushan soil
collected every 2 h. After the nitrate solution had infiltrated, distilled water was applied again until all nitrates were completely leached out of the soil columns. Nitrate concentrations in outflow were analyzed using the Griess–Ilosvay method (Mulvaney, 1996). The observed breakthrough curves were fitted to the convection-dispersion equation available in CXTFIT 2.0 (Toride et al., 1995) to estimate nitrate transport parameters. 3. Results and discussion 3.1. Nitrate breakthrough curves Breakthrough curves (BTCs) show the relative solute concentration in the outflow from a column of soil or porous medium after a one step change in solute concentration has been applied to the inlet end of the column, plotted against the volume of outflow. The BTC can be used to elucidate the characteristics of solute transport in the soil. In Table 2, Flux (Q) was the volume of outflow collected from the soil column, divided by the area of the column bottom and the time interval of collection. C/C0 is the ratio of the concentration of nitrate in outflow to the nitrate concentration applied (100 mg L− 1). Initial breakthrough time was when NO3− could be measured in the outflow solution (initial breakthrough). Terminative time was when NO3− could not be detected in the outflow after
Fig. 2. Nitrate BTCs of layers in Bai soil.
distilled water was applied to leach the NO3− in the soil column. Total time is the whole process of nitrate transport. The fluxes in all three soil types trended from high in the top layer to low in the bottom layer. The peak NO3− relative concentration (C/C0) showed a similar trend, becoming smaller with depth in the soil profiles. These trends are in accord with the clay distribution in the soil profiles.The BTCs of NO3− in the three paddy soils are shown in Figs. 2–4. There are substantial differences among the BTCs of the three paddy soils in Figs. 2–4. The fluxes in Bai soil were larger than in the two other soils. Initial breakthrough time, terminative time and total time were much shorter, and the peak values of the BTCs were correspondingly high. In every soil profile, the flux of the surface soil layer was larger than those of other layers. Nitrate transport also changed with depth. Initial breakthrough time, terminative time and total time were short in soil top layers, and generally increased with depth. Similarly the peak values of BTCs in soil top layers decreased with depth in the other soil layers. 3.2. Nitrate transport parameters Estimated nitrate transport parameters [average pore-water velocity (V) and dispersion coefficient (D)] in three soil profiles are given in Table 3. A good fit between the experimental data and the analytical model, CXTFIT, was obtained, as shown by
Fig. 3. Nitrate BTCs of layers in Huangni soil.
X.M. Chen et al. / Geoderma 142 (2007) 136–141
Fig. 4. Nitrate BTCs of layers in Wushan soil profile.
Fig. 5. Correlations between the average pore-water velocities (v) and the dispersion coefficients (D) in the three soils.
the high R2 values. The significant (p ≤ 0.01) correlation between the fitted V and D values in the three soil profiles (Fig. 5) indicates that the relationship between them was consistent over all soils and depths. Pore-water velocities and dispersion coefficients decreased with soil depth in the three soil profiles. 3.3. Factors influencing nitrate transport 3.3.1. Soil clay content Both flux and peak nitrate concentration (maximal C/C0) in the three types of soils showed a highly significant negative correlation with clay content. The correlation formulas were y = − 0 .0 43 9 × + 25 .9 52 , r ⁎⁎ = 0 .8 10 0 ( n = 1 3) a nd y = − 0.0009 × + 0.6243, r⁎⁎ = 0.8963 (n = 13) respectively. Both initial breakthrough time and total time showed a highly significant positive correlation with clay content. The correlation formulas were y = 0.0503 × − 1.0933, r⁎⁎ = 0.8724 (n = 13) and y = 0.3729 × − 54.429, r ⁎⁎ = 0.8421 (n = 13) respectively (Figs. 6–9). This means that both flux and BTC peak values decreased as clay content increased, but both nitrate initial breakthrough time and total outflow time increased as clay content increased. BTCs were influenced mainly by clay content. This can be explained by clay particle surface interactions with water. Planar particles such
as clay have a high specific surface area, resulting in high capacity for water adsorption and retention. In sieved, packed soil columns, the velocity of water movement will be slow, initial breakthrough of nitrate will be delayed, and the total outflow time will be prolonged. Therefore, BTCs will rise and fall gently with lower peak values and larger areas under the curves. All of the BTCs were asymmetric and tailing was more obvious when the clay content was high (Figs. 2–4). This phenomenon can be explained by the classic one-dimensional convection-dispersion equation (CDE). Moisture in soil can be divided into two portions, mobile moisture and immobile moisture (analogous to preferential flow and matrix flow). Convective-dispersive movement of solute occurs mostly within mobile moisture, while dispersion due to solute exchange between mobile moisture and immobile moisture depends on diffusion. The dominance of convective effects relative to dispersion is related to the proportion of porosity occupied by immobile moisture, which increases with clay content. As clay content increases, mobile moisture must move more slowly through the reduced area for flow, BTC peak values are lower, and tailing is more obvious.
Table 3 Estimated nitrate transport parameters in three soil profiles Soil types
Soil horizon (cm)
V(cm h− 1)
D(cm2 h− 1)
R2
Bai soil
0–12 12–20 20–30 30–55 55–100 0–15 15–25 25–65 65–100 0–15 15–28 28–42 42–70
6.6651 4.7903 3.4772 3.2235 1.6883 3.6443 2.6921 1.5993 1.4638 4.4562 3.0383 1.8226 1.6663
1.9103 0.9741 0.7116 0.5723 0.4732 1.1532 0.5586 0.2953 0.2654 0.9482 0.5749 0.3656 0.3136
0.9215 0.9148 0.8937 0.8912 0.8843 0.9419 0.9137 0.9038 0.8842 0.9787 0.9543 0.9436 0.9148
Huangni soil
Wushan soil
139
Fig. 6. Relation between flux and clay content.
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Fig. 9. Relation between total time and clay content.
Fig. 7. Relation between peak value of curves and clay content.
3.3.2. Soil bulk density Soil porosity depends on soil bulk density. Nitrate outflow showed a significant negative correlation with bulk density. By regression analysis, a highly significant negative correlation (p b 0.01) was found between flux and soil bulk density. The regression formula was y = − 4305.7 × + 9294.7 (where y = flux, x = soil bulk density) [r⁎⁎ = 0.7446(n = 13)]; Nitrate initial breakthrough time had a significant positive correlation with soil bulk density, y = 0.0421 × − 0.0096, r = 0.8871⁎⁎ (n = 13). These observations showed that nitrate movement and initial breakthrough time were related to soil bulk density. However, peak C/C0 values and total outflow time were not significantly correlated with bulk density. This was because total time was most influenced by clay content.
adsorption and wet aggregate stability. When the soil is saturated, the increase in water-filled porosity (especially macropores stabilized by organic matter) will accelerate nitrate vertical transport. The effect of organic matter content on the shape of BTCs was similar to the influence of clay content. 4. Conclusions
3.3.3. Organic matter content The correlation between the organic matter content and flux is shown in Fig. 10. The correlation between organic matter content and water flux was highly significant [r = 0.8272⁎⁎ (n = 13)]. The flux increased as the organic matter content decreased during the process of nitrate vertical transport. Organic matter has two effects in soils. One is to improve the soil structure, which increases porosity and permeability. The other is to increase soil water
Nitrate is not absorbed by soil. Vertical transport of nitrate may be judged by nitrate breakthrough curves in experiment. The velocity of nitrate transport decreased from the surface layer to the bottom layer in every soil profile. Clay content was the major influence on the vertical transport of nitrate in these soils. All parameters of BTCs were significantly correlated with clay content. Both flux and peak concentration of nitrate decreased as clay content increased. Soil bulk density and organic matter content also affected the vertical transport of nitrate. When soil bulk density was low and organic matter content was high, nitrate breakthrough was fast and the outflow time was short. When the organic matter content was higher; the outflow time of nitrate became faster. Clear linear relationships between the nitrate dispersion coefficient and the pore-water velocity were identified. This confirms our hypothesis that vadose zone solute transport parameters in Tai Lake region soils largely conform to classical models of the flow regime in porous media. Further experimental studies at field scales will explore the relationships between nitrate transport parameters and water flow regimes imposed by
Fig. 8. Relation between starting time and clay content.
Fig. 10. Relation between flux and organic matter content.
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the surface boundary conditions, in order to quantify nitrate transport and reasonable nitrate fertilizer application rates under field conditions. Acknowledgements The authors thank Dr. Christopher Ogden (Cornell Medical College in Qatar) for his check of English and comments on this paper. We also wish to express our thanks to anonymous reviewers for providing useful comments to improve the paper. This study is jointly supported by funding from the National Natural Science Foundation of China (no. 40371055). References Bengtsm, Goran, Annadotter, Helence, 1989. Nitrate reduction in a ground water microcosm determined by 15N gas chromatograph mass spectrometry. Applied and Environmental Microbiology 55 (11), 2861–2870. Chen, X.M., Shen, Qi.R., Pan, G.X., Liu, Z.P., 2003. Characteristics of nitrate horizontal transport in a paddy field of the Tai Lake region, China. Chemosphere 50, 703–706. Fletcher, D.A., 1991. A national perspective. In: Follett, R.F. (Ed.), Managing Nitrogen for Groundwater Quality and Farm Profitability. SSSA, Madison, WI, pp. 9–17.
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Guo, Li-ping, Zhang, Fu-suo, 2001. Effect of long-term fertilization on soil nitrate distribution (in Chinese). Journal of Environmental Sciences 13 (1), 58–63. Mulvaney, R.L., 1996. Nitrogen-Inorganic Forms, Methods of Soil Analysis (Part 3). Chemical Methods-SSSA Book series, vol.5, pp. 1123–1157. Strebel, O., Duynisveld, W.H.M., Bottcher, J., 1989. Nitrate pollution of ground water in Western Europe. Agriculture, Ecosystems and Environment 26, 189–214. Toride, N., Leij, F.J., van Genuchten, M.Th., 1995. The CXTFIT code for estimating transport parameters from laboratory or field tracer experiments. U.S. Salinity Laboratory Agricultural Research Service. Research Report, vol. 137. U.S. Department of Agriculture, Riverside, California. 121 pp. Wang, Chao, 1997. Experimental study of nitrogen transport and transformation in soils. Advances in Water Science 8 (2), 116–182 (in Chinese). Wang, De-jian, Lin, Jing-hui, Sun, Rui-juan, 2003. Optimum nitrogen rate for a high productive rice-wheat system and its impact on the ground water in the Taihu Lake area. Acta Pedologica Sinica 40 (3), 426–432 (in Chinese). William, H., Magill, Alison H., et al., 2004. Effects of chronic nitrogen amendment on dissolved organic matter and inorganic nitrogen in soil solution. Forest Ecology and Management 196, 29–41. Yuan, Xin-ming, Wang, Zhou-qiong, 2000. Nitrate nitrogen leaching and factors influencing it. Arid Zone Research 17 (4), 46–52 (in Chinese). Zheng, S.X., Jie, J., Xiong, J.Y., 2001. Study on role of controlled release fertilizer in increasing the efficiency of nitrogen utilization and rice yield. Nutrition and Fertilizer Science 7 (1), 11–16 (in Chinese). Zhu, Zao-liang, Wen, Qi-xiao, 1992. Nitrogen in Soils of China. Science and technology publishing house, Jiangsu, China, pp. 267–276 (in Chinese).
Nitrate vertical transport in the main paddy soils of Tai Lake region, China ☆ Xiaomin Chen ⁎, Huashan Wu, Fei Wo College of Resources and Environmental Sciences, Nanjing Agricultural University, Nanjing 210095, The People's Republic of China Received 30 August 2006; received in revised form 17 March 2007; accepted 5 August 2007 Available online 5 September 2007
Abstract The influence of texture, bulk density, and organic matter content on the process of nitrate vertical transport in the three main paddy soils (Bai soil, Huangni soil and Wushan soil) of the Tai Lake region were studied in the soil columns. Breakthrough curves (BTC) were obtained separately for each of thirteen horizons. The results were as follows: vertical transport velocity of nitrate decreased, and the BTCs of nitrate were more dispersed, in each horizon from the surface layer to the bottom in every soil profile. Among the three soils, the average flux of Wushan soil was the least and its nitrate BTC was the most dispersed. Under saturated conditions, nitrate penetrated the soil column quickly. The transport of nitrate was affected by clay content. As the clay content increased, nitrate outflow was retarded, and the peak concentration was reduced. Nitrate BTCs rose and fell gently when the nitrate concentration was lower. All nitrate BTCs were asymmetric, and tailing was more obvious when clay content was high. Soil bulk density and the organic matter content also affected the vertical transport of nitrate. Low bulk density and high organic matter content were each associated with faster nitrate transport. An analytical solute transport model (CXTFIT 2.0) was used to estimate the nitrate dispersion coefficient and average pore-water velocity from the observed breakthrough curves. The results showed that the analytical solute transport model was suitable in fitting the observed nitrate transport in these soils. The dispersion coefficient was found to be a function of average pore-water velocity. © 2007 Elsevier B.V. All rights reserved. Keywords: Nitrate vertical transport; Influence factors; CXTIT model; Dispersion coefficient; Pore-water velocity; Solute transport; Tai Lake region
1. Introduction In rice production, farmers tend to apply great amounts of nitrogen fertilizer to certain high yielding fields, from 270 kg N ha− 1 in 1998 to 366 kg N ha− 1 in 2002 (Wang et al., 2003). This high application rate resulted in total N loading much greater than the N demand of rice (Zheng et al., 2001), and the utilization ratio of nitrogen was only 20.9 ∼ 34.3%, lower than the national average of 30 ∼ 41% (Zhu and Wen, 1992). Volatilization of nitrogen into the atmosphere and leaching into ground water not only wastes fertilizer nitrogen and energy resources but also increases the risk of ground water contamination. Nitrate (NO3−) is a familiar pollutant in groundwater. Nitrate can be converted into nitrite (NO2−N). Nitrate not used by the ☆ Foundation item: The National Natural Science Foundation of China (no. 40371055) and the Special Scientific Research Foundation of College Doctor Station, China (no. 20030307018). ⁎ Corresponding author. Tel./fax: +86 25 84396842. E-mail address: [email protected] (X. Chen).
0016-7061/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2007.08.004
crop can be leached below the root zone into groundwater or run off into surface water, which is a problem because NO3− concentrations often exceed acceptable contamination limits (Strebel et al., 1989; Fletcher, 1991). Large amounts of nitrate in drinking water are a cause of methemoglobinemia, a blood disorder primarily affecting infants under six months of age (Bengtsm and Annadotter, 1989). Excessive levels of nitrate in drinking water may induce cancer in human and animals (Zhu and Wen, 1992). Therefore, nitrate concentration is an important criterion of groundwater quality (Wang, 1997; William et al., 2004). The Tai Lake region is one of China's most intensive agricultural regions, with a long history of rice cultivation. Various paddy soils covering 90% of the region have been developed over several centuries. Excessive chemical fertilizer application in the region has led to an increase of NO3− in the groundwater, which in turn has led to eutrophication of Tai Lake. The concentration of NO3− detected in water bodies has been influenced by agricultural drainage. Since the population is increasing and farmland is limited in this region, an increase
X.M. Chen et al. / Geoderma 142 (2007) 136–141
137
Table 1 Selected characteristics of the soil used in the study Soil names
Soil horizon (cm)
Bulk density (g cm− 3)
Organic matter (g kg− 1)
Clay (g kg− 1) (b0.002 mm)
Silt ( g kg− 1) (0.02–0.002 mm)
Sand ( g kg− 1) (N0.02 mm)
Bai soil
0–12 12–20 20–30 30–55 55–100 0–15 15–25 25–65 65–100 0–15 15–28 28–42 42–70
1.33 1.40 1.60 1.48 1.49 1.12 1.23 1.42 1.39 1.04 1.28 1.43 1.19
25.72 26.39 14.36 5.34 7.6 36.15 17.19 12.58 8.06 40.72 29.06 19.23 24.66
186.1 192.6 249.3 276.7 418.4 310.7 299.2 445.7 349.4 326.1 361.9 401.2 405.2
472.2 545.0 262.8 508.3 145.6 390.7 399.7 350.4 349.5 446.5 452.5 506.1 582.1
341.7 262.4 487.9 215.0 436.0 298.6 301.1 203.9 271.1 227.4 185.6 92.7 12.7
Huangni soil
Wushan soil
in agricultural productivity is essential. To keep pace with population expansion, the application rate of chemical fertilizers, which are the major source of agricultural non-point pollution, has been increasing in recent years. To minimize the effect on Tai Lake, control of rainfall runoff and infiltrating water containing nutrients from farmland is needed. Therefore, understanding the process of nitrate vertical movement is necessary to assess the consequences of new agricultural management practices (Chen et al., 2003). Many factors affect soil nitrate movement, including the utilization ratio of fertilizer, fertilizer application and irrigation times, soil texture and structure in the soil profile and so on (Guo and Zhang, 2001). However, no reports are available on the influence of these factors on parameters of nitrate vertical transport in the major paddy soils of the Tai Lake region (Yuan and Wang, 2000). Analytical solutions of the governing steady-state solute transport models may be used in an inverse way to estimate nitrate transport parameters (Toride et al., 1995). Using inverse procedures, breakthrough curves from laboratory or field observations are matched to the analytical solutions. Computer codes such as CXTFIT 2.0 (Toride et al., 1995) are available to predict nitrate distributions in time and space for specified model parameters and to estimate nitrate parameters in an inverse way. Therefore, the objective of this work was to study the factors and parameters influencing vertical nitrate transport processes in the Tai Lake area and to provide useful scientific information for reasonable fertilizer application and environmental protection in sustainable agriculture.
Chuangshu city, Jiangsu Province. The two soil profiles are divided into four horizons, at depths 0–15, 15–25, 25–65, 65– 100 cm and 0–15, 15–28, 28–42, and 42–70 cm respectively. Some characteristics of these soils are given in Table 1. 2.2. Experimental methods Packed columns of each soil were installed in the laboratory apparatus for measuring nitrate transport, shown in Fig. 1. Soil samples from each horizontal layer were air-dried and crushed to pass a 20-mesh screen. In accordance with its fieldmeasured bulk density, each soil layer sample was packed into a PVC column for determination of nitrate vertical transport. All soil columns were 50 cm high and 20 cm in diameter. A quartz sand layer about 1 cm depth was placed on the top and bottom of the soil column to protect the packed soil column from disturbance by water flow. The columns were saturated with distilled water, then washed with distilled water using a syringe peristaltic pump for 2–3 days until solutes in the soil were completely leached out of the columns. After establishing steady-state saturated flow using distilled water, 100 mL of a solution containing 100 mg L− 1 nitrate was applied continuously to the soil columns. The nitrate solution was applied from 0 to 120 h or so, depending on soil texture. The cumulative outflow solution was
2. Materials and methods 2.1. Testing and basic properties of soils The three main paddy soils used in this study were collected from the Tai Lake region of China. Bai soil was in Yixing County, Jiangsu Province. This soil profile can be divided into five horizons occurring at depths of 0–12, 12–20, 20–30, 30– 55 and 55–100 cm. Huangni soil and Wushan soil are found in
Fig. 1. Schematics of the apparatus for vertical nitrate transport study.
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Table 2 Parameters of nitrate BTCs Soil types
Soil horizon (cm)
Flux (Q), (cm h− 1)
Relative concentration C/C0
Initial breakthrough time (h)
Terminative time, (h)
Total time, (h)
Time of curve climax, (h)
Bai soil
0–12 12–20 20–30 30–55 55–100 0–15 15–25 25–65 65–100 0–15 15–28 28–42 42–70
20.12 16.82 12.4 11.68 6.07 12.38 11.54 6.56 7.11 18.23 12.68 8.12 8.48
0.4826 0.4437 0.3744 0.3960 0.2364 0.3232 0.3001 0.2448 0.2568 0.4031 0.3454 0.2410 0.2503
8 10 14 14 22 10 12 22 20 12 16 20 18
30 38 60 58 142 54 74 144 128 42 62 122 110
22 28 46 44 120 44 62 122 108 30 46 102 92
18 24 34 32 74 30 40 74 66 24 36 64 58
Huangni soil
Wushan soil
collected every 2 h. After the nitrate solution had infiltrated, distilled water was applied again until all nitrates were completely leached out of the soil columns. Nitrate concentrations in outflow were analyzed using the Griess–Ilosvay method (Mulvaney, 1996). The observed breakthrough curves were fitted to the convection-dispersion equation available in CXTFIT 2.0 (Toride et al., 1995) to estimate nitrate transport parameters. 3. Results and discussion 3.1. Nitrate breakthrough curves Breakthrough curves (BTCs) show the relative solute concentration in the outflow from a column of soil or porous medium after a one step change in solute concentration has been applied to the inlet end of the column, plotted against the volume of outflow. The BTC can be used to elucidate the characteristics of solute transport in the soil. In Table 2, Flux (Q) was the volume of outflow collected from the soil column, divided by the area of the column bottom and the time interval of collection. C/C0 is the ratio of the concentration of nitrate in outflow to the nitrate concentration applied (100 mg L− 1). Initial breakthrough time was when NO3− could be measured in the outflow solution (initial breakthrough). Terminative time was when NO3− could not be detected in the outflow after
Fig. 2. Nitrate BTCs of layers in Bai soil.
distilled water was applied to leach the NO3− in the soil column. Total time is the whole process of nitrate transport. The fluxes in all three soil types trended from high in the top layer to low in the bottom layer. The peak NO3− relative concentration (C/C0) showed a similar trend, becoming smaller with depth in the soil profiles. These trends are in accord with the clay distribution in the soil profiles.The BTCs of NO3− in the three paddy soils are shown in Figs. 2–4. There are substantial differences among the BTCs of the three paddy soils in Figs. 2–4. The fluxes in Bai soil were larger than in the two other soils. Initial breakthrough time, terminative time and total time were much shorter, and the peak values of the BTCs were correspondingly high. In every soil profile, the flux of the surface soil layer was larger than those of other layers. Nitrate transport also changed with depth. Initial breakthrough time, terminative time and total time were short in soil top layers, and generally increased with depth. Similarly the peak values of BTCs in soil top layers decreased with depth in the other soil layers. 3.2. Nitrate transport parameters Estimated nitrate transport parameters [average pore-water velocity (V) and dispersion coefficient (D)] in three soil profiles are given in Table 3. A good fit between the experimental data and the analytical model, CXTFIT, was obtained, as shown by
Fig. 3. Nitrate BTCs of layers in Huangni soil.
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Fig. 4. Nitrate BTCs of layers in Wushan soil profile.
Fig. 5. Correlations between the average pore-water velocities (v) and the dispersion coefficients (D) in the three soils.
the high R2 values. The significant (p ≤ 0.01) correlation between the fitted V and D values in the three soil profiles (Fig. 5) indicates that the relationship between them was consistent over all soils and depths. Pore-water velocities and dispersion coefficients decreased with soil depth in the three soil profiles. 3.3. Factors influencing nitrate transport 3.3.1. Soil clay content Both flux and peak nitrate concentration (maximal C/C0) in the three types of soils showed a highly significant negative correlation with clay content. The correlation formulas were y = − 0 .0 43 9 × + 25 .9 52 , r ⁎⁎ = 0 .8 10 0 ( n = 1 3) a nd y = − 0.0009 × + 0.6243, r⁎⁎ = 0.8963 (n = 13) respectively. Both initial breakthrough time and total time showed a highly significant positive correlation with clay content. The correlation formulas were y = 0.0503 × − 1.0933, r⁎⁎ = 0.8724 (n = 13) and y = 0.3729 × − 54.429, r ⁎⁎ = 0.8421 (n = 13) respectively (Figs. 6–9). This means that both flux and BTC peak values decreased as clay content increased, but both nitrate initial breakthrough time and total outflow time increased as clay content increased. BTCs were influenced mainly by clay content. This can be explained by clay particle surface interactions with water. Planar particles such
as clay have a high specific surface area, resulting in high capacity for water adsorption and retention. In sieved, packed soil columns, the velocity of water movement will be slow, initial breakthrough of nitrate will be delayed, and the total outflow time will be prolonged. Therefore, BTCs will rise and fall gently with lower peak values and larger areas under the curves. All of the BTCs were asymmetric and tailing was more obvious when the clay content was high (Figs. 2–4). This phenomenon can be explained by the classic one-dimensional convection-dispersion equation (CDE). Moisture in soil can be divided into two portions, mobile moisture and immobile moisture (analogous to preferential flow and matrix flow). Convective-dispersive movement of solute occurs mostly within mobile moisture, while dispersion due to solute exchange between mobile moisture and immobile moisture depends on diffusion. The dominance of convective effects relative to dispersion is related to the proportion of porosity occupied by immobile moisture, which increases with clay content. As clay content increases, mobile moisture must move more slowly through the reduced area for flow, BTC peak values are lower, and tailing is more obvious.
Table 3 Estimated nitrate transport parameters in three soil profiles Soil types
Soil horizon (cm)
V(cm h− 1)
D(cm2 h− 1)
R2
Bai soil
0–12 12–20 20–30 30–55 55–100 0–15 15–25 25–65 65–100 0–15 15–28 28–42 42–70
6.6651 4.7903 3.4772 3.2235 1.6883 3.6443 2.6921 1.5993 1.4638 4.4562 3.0383 1.8226 1.6663
1.9103 0.9741 0.7116 0.5723 0.4732 1.1532 0.5586 0.2953 0.2654 0.9482 0.5749 0.3656 0.3136
0.9215 0.9148 0.8937 0.8912 0.8843 0.9419 0.9137 0.9038 0.8842 0.9787 0.9543 0.9436 0.9148
Huangni soil
Wushan soil
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Fig. 6. Relation between flux and clay content.
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Fig. 9. Relation between total time and clay content.
Fig. 7. Relation between peak value of curves and clay content.
3.3.2. Soil bulk density Soil porosity depends on soil bulk density. Nitrate outflow showed a significant negative correlation with bulk density. By regression analysis, a highly significant negative correlation (p b 0.01) was found between flux and soil bulk density. The regression formula was y = − 4305.7 × + 9294.7 (where y = flux, x = soil bulk density) [r⁎⁎ = 0.7446(n = 13)]; Nitrate initial breakthrough time had a significant positive correlation with soil bulk density, y = 0.0421 × − 0.0096, r = 0.8871⁎⁎ (n = 13). These observations showed that nitrate movement and initial breakthrough time were related to soil bulk density. However, peak C/C0 values and total outflow time were not significantly correlated with bulk density. This was because total time was most influenced by clay content.
adsorption and wet aggregate stability. When the soil is saturated, the increase in water-filled porosity (especially macropores stabilized by organic matter) will accelerate nitrate vertical transport. The effect of organic matter content on the shape of BTCs was similar to the influence of clay content. 4. Conclusions
3.3.3. Organic matter content The correlation between the organic matter content and flux is shown in Fig. 10. The correlation between organic matter content and water flux was highly significant [r = 0.8272⁎⁎ (n = 13)]. The flux increased as the organic matter content decreased during the process of nitrate vertical transport. Organic matter has two effects in soils. One is to improve the soil structure, which increases porosity and permeability. The other is to increase soil water
Nitrate is not absorbed by soil. Vertical transport of nitrate may be judged by nitrate breakthrough curves in experiment. The velocity of nitrate transport decreased from the surface layer to the bottom layer in every soil profile. Clay content was the major influence on the vertical transport of nitrate in these soils. All parameters of BTCs were significantly correlated with clay content. Both flux and peak concentration of nitrate decreased as clay content increased. Soil bulk density and organic matter content also affected the vertical transport of nitrate. When soil bulk density was low and organic matter content was high, nitrate breakthrough was fast and the outflow time was short. When the organic matter content was higher; the outflow time of nitrate became faster. Clear linear relationships between the nitrate dispersion coefficient and the pore-water velocity were identified. This confirms our hypothesis that vadose zone solute transport parameters in Tai Lake region soils largely conform to classical models of the flow regime in porous media. Further experimental studies at field scales will explore the relationships between nitrate transport parameters and water flow regimes imposed by
Fig. 8. Relation between starting time and clay content.
Fig. 10. Relation between flux and organic matter content.
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