Sewage Sludge Amendment Combined with Green Manuring to a Coastal Mudflat Salt-Soil in Eastern China : Effects on Soil Physicochemical Properties and Maize Yield

1College of Environmental Science and Engineering, Yangzhou University, Yangzhou 225127, China 2Jiangsu Collaborative Innovation Center for Solid Organic Waste Resource Utilization, Nanjing 210095, China 3College of Agriculture, Yangzhou University, Yangzhou 225127, China 4Department of Geological and Environmental Sciences, Appalachian State University, Boone, NC 28607, USA 5Jiangsu Cultivated Land Quality and Agro-Environment Protection Station, Nanjing 210036, China


Introduction
Soil salinity is a major obstacle in the potential utilization of land resources in salt-affected areas.Salt-affected soils are found in many parts of the world, and about 20% of the world's cultivated land is salt-affected [1].In the east of China, there were approximately 1.1∼1.2 million ha saltsoil reclaimed from coastal mudflats in the past five decades [2,3].It is estimated that additional 2000 km 2 mudflats along the Jiangsu coast will be reclaimed by 2020, and another reclamation of 4400 km 2 mudflats located in the Yellow River Delta is under evaluation [4].The newly reclaimed coastal mudflats, as typical salt-soil, have high salinity, high pH, and macronutrient deficiencies [5].High salinity of mudflat salt-soil is driven by the strong evaporation demand of the air, shallow salt solution rise to the soil surface through soil capillary in the dry season [6].In the area rich in freshwater resources, soil salinity reduction is often accomplished through freshwater irrigation [7].However, faced with the scarcity of freshwater resources, organic matter amendment is an effective alternative to decrease soil salinity by forming soil aggregates and breaking capillary rise in saltsoil reclaimed from coastal mudflats in the eastern area of China [8,9].
Salt-soil generally exhibits poor aggregates due to low organic matter content.Previous studies indicated that soil aggregates can be improved by the application of organic materials such as poultry manures, green manures, and compost [10][11][12][13].Barzegar et al. [14] found that the addition of plant residues to a salt-soil had a positive influence on the International Journal of Agronomy stability of soil aggregates and decreased the soil salinity and exchangeable sodium percentage (ESP).
Sewage sludge generated in municipal wastewater treatment plants has caused serious environmental pollution and ecological safety concern.Safe disposal of sewage sludge is a challenge in China.Approximately 78.4 billion metric tons of sewage sludge was derived from more than 2600 wastewater treatment plants in China in 2015 [15].Agricultural application of sewage sludge is an effective waste disposal and soil amendment technique that can improve the chemical, physical, and biological characteristics of soils [16][17][18][19].Sewage sludge is rich in organic matter and other nutrients and can be used for mudflat salt-soil amendment [9,20,21].
However, the quality and application rate of sewage sludge must be controlled, because it might bring various pollutants such as heavy metals, pathogens, and organic micropollutants into the environment and subsequently into the food chain [22,23].Thus, total amount of sewage sludge applied must be controlled in compliance with the China national standard for agricultural use of sewage sludge (GB/T 24600-2009) and on the premise of total amount of toxins elements from sewage sludge.Green manuring, on the other hands, plays an important role in soil quality and sustainability of agricultural systems and can be used as an alternative to sewage sludge to increase organic carbon to the soil [24].Soil water-stable aggregates stability and soil organic matter can be improved by green manuring [25].Green manuring was potential source for organic matter and nutrient [26,27] and may provide economic and biological advantages over the traditional approach of soil incorporation [28].Zubair et al. [29] found that green manuring in salt-affected soil increased soil organomineral interactions.Bruning et al. [30] also found the use of green manure can potentially increase crop productivity in a salt-affected soil and thus contribute to the sustainability of saline agricultural systems.Green manuring in mudflat salt-soil can be an effective way to minimize input of sewage sludge as the source of toxins elements.
The past research mainly focused on the application of sewage sludge or green manuring in farmlands [31][32][33][34].However, the application of sewage sludge combined with green manuring to amend mudflat salt-soils has received no attention.The impacts of applying sewage sludge alone to reclaimed mudflat soil on fertility development, growth of ryegrass, Sesbania, and maize and on heavy metals behaviors including their accumulation and fraction in soil and uptake by plants were investigated and evaluated in our previous studies [9,20,21,35].This study was one of pioneering studies to investigate and evaluate the sustaining effects of sewage sludge amendment combined with green manuring on the soil aggregates, organic carbon, salinity, pH, and N and P of mudflat salt-soil, as well as yield of maize (Zea mays L.) in mudflats salt-soil in a rain-fed agroecosystem.The experimental sewage sludge was collected from Sewage Treatment Plant of Rudong County in September, 2011.The quality of the sludge complied with the China national standard for agricultural use of sewage sludge (GB/T 24600-2009).Some chemical properties of mudflat saltaffected soil and sewage sludge were shown in Table 1.

Experimental Design.
The field experiment was conducted in randomized complete block design (RCB) with each plot of 4 m length and 4 m width.The organic carbon content in farm soil in this region was about 0.5%-3.0% of total soil weight.Therefore, 30, 75, 150, and 300 t ha −1 (metric tons per hectare) (about 0.5%, 0.9%, 1.6%, and 2.9% organic carbon of soil weight in the 0-20 cm soil layer, resp.)sewage sludge amendment (SSA) rates on a dried weight basis by one-time application were used.The soil without sludge amendment was the control soil.The plots were repeated three times.On experimental site, the sewage sludge was mixed uniformly with the soil of 20 cm layer by a rototiller in late October 2011.Ryegrass (Lolium perenne L.) and Sesbania (Sesbania cannabina) were adopted as green manures due to their high-quality and salt-tolerance and successively planted and tilled from October 2011 to September 2012.The rain-fed ryegrass-maize rotation was carried out from October 2012 to September 2014.Maize (Zea mays L.) as a typical dryland and salt-susceptible crop was cultivated as a test crop.Maize was sowed two seeds per hole with average row spacing of 0.50 m and plant spacing of 0.25 m in early July 2013 and 2014 and harvested in late September 2013 and 2014.Thinning of maize seedlings was performed 20 days after sowing (DAS), leaving one seedling per hole.Weeds were hand-hoed two times at 20 and 48 DAS, respectively.During the experimental period, all plants were rain-fed, and no extra artificial irrigation was carried out.

Soil Analysis.
Soil samples from each plot at depths of 0-20 and 20-40 cm were collected in quadruplicate in late September 2013 and 2014 at maize harvest stage.The soil samples were immediately transported to the laboratory.Soil bulk density was measured by the core method.Soil salinity was measured by the gravimetric method [36].The size composition of soil water-stable aggregates was analyzed by wet-sieving method using soil aggregate analyzer (DIK-2001, Daiki, Japan).The K 2 Cr 2 O 7 method was used for analysis of soil organic carbon.The pH of soil was measured in suspension of 1 : 5 (weight/volume) by pH meter (Model IQ150, Spectrum, USA).Alkaline N and available P were determined by alkaline hydrolysis diffusion and sodium bicarbonate (NaHCO 3 ) extraction method, respectively.The cation exchange capacity (CEC) of soil was measured by using the ammonium acetate (NH 4 OAc) method after successively leached in sodium acetate (NaOAc) solution and alcohol.Exchangeable sodium content was analyzed to calculate exchangeable sodium percentage (ESP) and was extracted with ammonium acetate-ammonium hydroxide (NH 4 OAc-NH 4 OH) (pH 9) and measured by a flame spectrophotometer [36].The all methods and procedures for soil chemical properties involved in this study were described in detail by the Soil and Agro-Chemistry Analysis [36].

Plant Analysis.
Ten maize plants were sampled randomly from each plot for estimation of aerial part growth at their respective harvest seasons in late September 2013 and 2014.Maize grains were threshed using a miniature thresher for each replica, and yields of maize were determined by weighing the grains after dried at 105 ∘ C for 15 minutes and oven-dried at 80 ∘ C until constant weight was achieved.

Statistical Analysis.
Analysis of variance (ANOVA) for the data obtained was conducted using SPSS 13.0 software (SPSS Inc., USA) with the model of RCB design.The differences between the treatments were detected by the least significant difference (LSD) method at the 0.05 level.

Size Composition of Soil Water-Stable Aggregates and
Bulk Density.Table 2 presents the test results of the waterstable aggregates size composition in the soil samples collected in September 2013 and 2014.Under the influence of different sewage sludge amendment combined with green manuring, the percentage of aggregates of different size classes significantly varied in the topsoil (0-20 cm) and subsurface (20-40 cm) layers.The percentage of >0.25 mm and 0.106-0.25 mm water-stable aggregates in the topsoil increased with increasing SSA rates, whereas no significant changes ( > 0.05) were found in the percentage of waterstable aggregates of the subsurface soil.The percentage of >0.25 mm water-stable aggregates in the topsoil increased by 7.5-25.1%,29.7-96.1%,105.5-121.6%,and 143.5-247.7% at 30, 75, 150, and 300 t ha −1 SSA rates in 2013 and 2014, respectively, compared to the control soil.There were sharp increases in the percentage of >0.25 mm water-stable aggregates in the topsoil at all SSA rates in September 2014, compared to those in 2013.
The sewage sludge amendment combined with green manuring significantly decreased bulk density in the salt-soil, and soil bulk density decreased with increasing SSA rates (Figure 1).The similar trend was found in 2013 and 2014.The soil bulk density decreased by 7.4%, 12.2%, 12.3%, and 18.5% at 30, 75, 150, and 300 t ha −1 SSA rates in 2014, respectively, compared to the control soil.3, the SSA combined with green manuring significantly decreased salinity of the topsoil, whereas no significant changes ( > 0.05) were found in salinity of the subsurface soil.The salinity of topsoil decreased by 26.5-37.9%,29.9-59.8%,44.7-66.6%, and 56.7-73.1% at 30, 75, 150, and 300 t ha −1 SSA rates in 2013 and 2014, respectively, compared to the control soil.

Soil OC, CEC, and Nutrient Content.
As shown in Figure 3(a), the sewage sludge amendment combined with  The sewage sludge amendment combined with green manuring significantly increased total N and P, alkaline N, and available P in mudflat salt-soil (Figure 4).The concentrations of total N and P, alkaline N, and available P in mudflat salt-soil increased with increasing SSA rates, and the similar trend was found in 2013 and 2014.The total N and P increased by 18.0%, 129.0%, 193.0%, and 311.0% and 62.7%, 71.2%, 232.2%, and 291.7% at 30, 75, 150, and 300 t ha −1 SSA rates in 2014, respectively, compared to the control soil.The increments of alkaline N and available P in mudflat salt-soil in 2014 were 209.8%, 203.4%, 402.8%, and 505.8% and 184.9%, 190.1%, 239.3%, and 309.6% at 30, 75, 150, and 300 t ha −1 SSA rates, respectively, compared to the control soil.

Maize Yields.
The sewage sludge amendment combined with green manuring significantly increased yields of maize grown in mudflat salt-soil (Figure 5).The maize yield increased by 225.0%, 471.1%, 546.1%, and 648.7% at 30, 75, 150, and 300 t ha to the control soil.The similar trend was found in 2013, and the yields of maize at ≥ 75 t ha −1 SSA rates were significantly higher than that in the control soil.

Discussion
The sewage sludge amendment combined with green manuring improved physical properties of topsoil in mudflat salt-soil by increasing the percentage of >0.25 mm and 0.106-0.25 mm water-stable aggregates and by decreasing bulk density in 0-20 cm of the salt-soil layer.The improvement in these physical properties was due to increase in organic carbon content of salt-soil.Soil aggregates are formed from the combination of soil organic matter and soil mineral particles [37].A significant positive correlation was found between soil organic carbon and the percentage of >0.25 mm and 0.106-0.25 mm water-stable aggregates ( < 0.01) (Figure 6(a)).Release of carbohydrates and organic carbon from biodegradation of OM-rich sludge-amended soil may increase aggregate stability percentage, as it can complex soil organic matter and soil mineral particles to soil aggregate [38,39].The result in the reduction in bulk density by the amendments was not only due to the addition of less dense organic material, but also due to the increase in soil aggregate stability [40] (Figure 6(b)).Previous studies have shown that the application of sewage sludge increased organic carbon in saline and salinized soil [41,42], subsequently increased aggregate stability in saline-sodic and salinized soil Values of  are the  test in ANOVA; * *  < 0.01.[42,43], and decreased bulk density in salt-soil and salinized soil [9,42].
Topsoil salinity in mudflat salt-soil decreased as a result of increasing soil aggregate stability and decreasing bulk density.Increase in noncapillary porosity by the increment of soil aggregates stability percentage [8] breaks capillary rise [6] that translocates saline groundwater to topsoil.Guo and Liu [5] also reported that the high salinity in salt-soil is caused by capillary rise that brings saline groundwater to topsoil.In this study, the SSA significantly decreased soil salinity in 0-20 cm layer, whereas there were no significant changes ( > 0.05) in 20-40 cm layer.On the contrary, water-stable aggregates percentage significantly increased in 0-20 cm layer, whereas there are no significant changes ( > 0.05) in 20-40 cm layer.Statistical analysis showed that a significant negative correlation was found between soil salinity and soil aggregate stability percentage in topsoil and subsurface soil ( < 0.05) (Figure 6(c)).The increase in aggregate stability percentage in topsoil acted as a barrier for upward movement of saline groundwater.Thus, soil salinity in 0-20 cm layer was lower than in 20-40 cm layer.
The ESP values in mudflat salt-soil decreased with increasing SSA rates.Similar effect was also observed by García-Orenes et al. [42] after the biosolids application to a salinized soil.The decrease in ESP values might be associated with an increase in infiltration rate that enhances soil washing and thus sodium leachability [44].Moreover, sodium as a micronutrient could be taken up into plant tissue, specially after the three consecutive seasons of green manuring.In addition, a slight dilution effect can be caused by high doses of sewage sludge application and green manure incorporation.In the present study, statistical analysis also showed a significant negative correlation between soil ESP values and soil aggregate stability percentage in topsoil ( < 0.01) (Figure 6(d)).The similar negative effects of ESP on soil aggregation were found by other studies [45][46][47].This might be attributed to that sodium causes swelling and dispersion of soil mineral particles and slaking of unstable aggregates [45,48].In addition, high ESP value can reduce microbial biomass and activity, which may also contribute to the decrease in aggregation [49].
The pH in mudflat salt-soil decreased with increasing SSA rates, which was similar to our previous studies [9,20,21].Other authors have also found lowering of pH with SSA [50][51][52].Intense mineralization of the labile OM pool of sludge treated-soils [53] lowers pH by releasing humic acid from biodegradation of OM-rich sewage sludge and green manure [50].In addition, a slight dilution effect could be caused by lower-pH sewage sludge application, especially in treatments with high doses of application.
The sewage sludge amendment combined with green manuring improved salt-soil fertility by increasing CEC and concentration of N and P in mudflat salt-soil.Our previous studies also supported the results [9,20,21].In farmland, soil fertility improvement by SSA has also been reported widely [50,51,54].Soil N and P content increased in the mudflat saltsoil amended with sewage sludge due to higher levels of these nutrients in the sewage sludge.Moreover, green manuring, such as Sesbania as a legume, may supply N through the N fixation process, thereby increasing N for soil fertility supply [55,56].The increase in CEC of mudflat soil is probably due to increasing soil OM content [57,58].

Conclusions
The sewage sludge amendment combined with green manuring increased organic carbon in mudflat salt-soil, which improved the physical properties of mudflat salt-soil by increasing aggregate stability of the 0-20 cm soil layer and decreasing topsoil bulk density.The increase in water-stable aggregate percentage in topsoil acted as a barrier for upward movement of saline groundwater.As a result, soil salinity of the topsoil decreased.The ESP values in mudflat saltsoil decreased with increasing sewage sludge amendment rates.The percentage of water-stable aggregate in mudflat salt-soil showed significantly negative correlation with soil salinity and ESP in mudflat salt-soil ( < 0.01).In addition, the sewage sludge amendment combined with green manuring decreased soil pH and improved soil fertility by International Journal of Agronomy  increasing CEC and concentration of N and P in mudflat saltsoil.The improved physicochemical properties subsequently enhanced the yields of maize (Zea mays L.) in the mudflat salt-soil.

Figure 1 :
Figure 1: Effects of sewage sludge amendment (SSA) combined with green manuring on bulk density of coastal mudflat salt-soil.Vertical bars indicate standard deviations of the means.Columns with different letters show significant difference between sewage sludge amendment rates at  < 0.05 by LSD's multiple range test.Values of  are the  test in ANOVA; * *  < 0.01; ns  > 0.05.

Figure 2 :Figure 3 :
Figure 2: Effects of sewage sludge amendment (SSA) combined with green manuring on pH (a) and ESP (b) of coastal mudflat salt-soil.Vertical bars indicate standard deviations of the means.Columns with different letters show significant difference between sewage sludge amendment rates at  < 0.05 by LSD's multiple range test.Values of  are the  test in ANOVA; * *  < 0.01.pH, hydrogen ion concentration; ESP, exchangeable sodium percentage.

Figure 4 :
Figure 4: Effects of sewage sludge amendment (SSA) combined with green manuring on the concentration of total N (a), total P (b), alkaline N (c), and available P (d) of coastal mudflat salt-soil.Vertical bars indicate standard deviations of the means.Columns with different letters show significant difference between sewage sludge amendment rates at  < 0.05 by LSD's multiple range test.Values of  are the  test in ANOVA; * *  < 0.01.N, nitrogen; P, phosphorus.

Figure 5 :
Figure 5: Effects of sewage sludge amendment (SSA) combined with green manuring on yield of maize grown in the mudflat salt-soil.Vertical bars indicate standard deviations of the means.Columns with different letters show significant difference between sewage sludge amendment rates at  < 0.05 by LSD's multiple range test.Values of  are the  test in ANOVA; * *  < 0.01.

Table 1 :
The experiment site was located in the Senmao Farm (E 121 ∘ 23  23  , N 32 ∘ 20  03  ) of the Rudong County, Jiangsu Province, China.The land, as reclaimed from coastal mudflats in 2007, has Basic chemical properties of the coastal mudflat salt-soil and sewage sludge used in this study.

Table 2 :
Effects of sewage sludge amendment (SSA) combined with green manuring on the size composition of soil water-stable aggregates at 0-20 cm and 20-40 cm depths of coastal mudflat salt-soil.
nsValues are mean ± SD of three replicates.Different letters in each column indicate significant difference at  < 0.05 by LSD's multiple range test.Values of  are the  test in ANOVA; *  < 0.05; * *  < 0.01; ns  > 0.05.

Table 3 :
Effects of sewage sludge amendment (SSA) combined with green manuring on salinity of coastal mudflat salt-soil.SD of three replicates.Different letters in each column indicate significant difference at  < 0.05 by LSD's multiple range test.Values of  are the  test in ANOVA; *  < 0.05; * *  < 0.01; ns  > 0.05.