Saline–Alkaline Characteristics during Desalination Process and Nitrogen Input Regulation in Reclaimed Tidal Flat Soils

agriculture cultivation and useful for salinity and alkalinity judgement. Abstract: Coastal salt-affected soils account for a large area all around the world. Soil salinity and pH are two important parameters affecting soil quality. Investigating the correlation of electrical conductivity (EC) and pH at different soil depths in saline soil was useful for quickly assessing the saline–alkaline characteristics. During the natural desalination process in the ﬁeld area of reclaimed lands, the phenomena of pH increase and nitrogen accumulation may occur. A ﬁeld sampling experiment was conducted in slightly saline soil affected by natural desalination and newly reclaimed heavily saline soil. A series of soil–water ratio extracts consisting of 1:2.5, 1:5, 1:10, 1:20, and 1:40 was designed to measure the EC and pH for simulating the saline–alkaline characteristics during the soil desalination process. Meanwhile, for reasonable utilization of the naturally ameliorated slightly saline soil which consists of a high content of nitrogen, a plastic mulching (PM) accompanied with nitrogen (N) fertilizer addition experiment in maize cultivation plots was designed. Results showed that a signiﬁcant correlation of EC and/or pH existed in all ratios of soil extracts, and the slightly saline soil had a higher nitrogen content (1.06 g kg − 1 ). The EC was negatively correlated with pH at a depth of 0~100 cm in the coastal saline soil, which indicated the increase of pH value and alkalization during its natural desalination. Furthermore, PM treatments showed no signiﬁcant difference with N treatments in soil bulk density and soil water content in the slightly saline soil. The PM and N treatments obtained similar grain yield, which was between 6.2 and 6.5 t ha − 1 . The soil salinity decreased in all treatments and the harvest index was largest in PM treated plots. Our study was beneﬁcial for rapidly monitoring saline–alkaline characteristics and sustainable utilization of coastal saline soil resources. In addition, we should focus far more on pH improvement during the desalination process and rational utilization of chemical fertilizer for obtaining sustainable beneﬁts in the coastal saline soil.


Introduction
As an important land resource, coastal tidal flat soil occurs widely in the world [1].China has a vast area throughout the coastline, and considerable proportions of coastal saline tidelands have been reclaimed for agricultural land uses in the past thirty years [2].In the coastal area of Jiangsu Province, eastern China, a total land area of approximately 0.44 million hectares was reclaimed between the years 1951 to 2020 [3].However, high salt content limits the reclaimed land soil productivity seriously due to the effects of the high underground water level and seawater intrusion [4].In the time since the reclamation, the soil salinity potentially decreased, and the soil nutrition content increased after rain washing and natural plant growth.The total salt content is always used for describing soil salinity, but some studies reported that it cannot reflect effective field salt content with sound reliability, especially when exploring the correlation between soil salinity and plant growth [5,6].Electrical conductivity (EC) of soil-water extracts is commonly used to assess soil saline-alkaline characteristics, and, with sound comparability and repeatability, it is convenient to obtain [7].Many papers have reported the positive correlation of EC at different soil layers [8,9], and a negative correlation between EC and pH may have existed in coastal salt-affected soil at the same time [10,11].Soil pH, as an important parameter, is equally important as salinity in shaping bacterial communities in saline soils [12].Otherwise, coastal alkaline problems have been reported by Hu in the last century [13], and they have received little attention in recent times.The reduction of calcium ions during soil desalination may be the main factor that causes pH increase [14,15].Therefore, the pH is as important as the soil salinity in the coastal soil, and it needs more research in the coastal tidal flat area.
Although the pH increase during soil desalination has been reported in some studies [16,17], few care about the soil nitrogen accumulation after natural desalination in coastal saline soil.Nitrogen was mainly accumulated by plant residues and microbial nitrogen fixation [18,19].In order to improve the productivity of the coastal salty soil, nitrogen management methods need to be developed to ameliorate the soil situation [20].Among all remediation options, mulching is widely used to reduce salt damage during seedling establishment and yield [21,22].The widespread use of plastic mulching in agriculture is due to its easy processing, excellent chemical resistance, high durability, flexibility, and freedom from odor and toxicity as compared to other polymers [23].The plastic mulching practice could reduce evaporation, increase soil moisture, prevent soluble salt migration, improve soil temperature, and hence increase crop yield.Furthermore, nitrogen fertilizer, particularly urea, has been the most common agricultural practice used to improve soil fertility and crop production in the last three decades [24].However, excessive fertilizer application could cause eutrophication and pollution, which seriously affect the coastal ecosystem environment [25].Moreover, a study showed prominent nitrogen pollution in the intertidal sediments of Jiangsu Province, and about 70% of inorganic nitrogen comes from losses of agricultural nitrogen fertilizers [26].Furthermore, in a two-year field trial, we found that soil sediments from reclamation were rich in nitrogen, and N fertilizer addition accelerated soil nutrition consumption [3].So, taking measures to reduce nitrogen fertilizer addition may be a practical solution to achieving acceptable crop yield in short-term farming.
In this study, the correlation of EC and pH of soil extracts in various soil-water ratio solutions at different soil depths in a reclaimed coastal saline soil was calculated.The design aimed to verify the increase of alkalinity during natural desalination in coastal areas.Furthermore, a field experiment was conducted to investigate the application impact of plastic mulching and/or nitrogen fertilizer on maize growth and soil salt alleviation in nitrogen abundant slightly saline soil.The primary aim of the present study was to determine the suitable soil-water ratio of slightly and heavily saline soil and develop the correlation equation of pH and EC applied to the solution with different salt content.Then, we studied the effects of plastic mulching-reduced N on soil quality and crop growth in slightly saline soil.The study was beneficial for quick judgement of saline-alkaline characteristics and rational utilization of nitrogen fertilizer in coastal slightly and heavily saline soils.

Site Description and Experimental Design
The study area was located in the Tiaozini reclamation area (32 • 50 6.88N, 120 • 56 35.56 E), Jianggang Township, Dontai City, Jiangsu province, China (Figure 1).The investigated site was reclaimed in October 2013 and is situated at the shore of the China Yellow Sea.The land had been prepared in 2015, with neat fields and complete irrigation and drainage system.The climate is subtropical and characterized by monsoons and oceans with high seasonal fluctuations in temperature and precipitation.The mean annual temperature is 14.6 • C. The long-term average annual evaporation and precipitation were 1417 mm and 1042 mm, respectively.The total sunshine time per year amounted to 2130 h, and annual frost-free days reached up to 220 d per year.The meteorological data were collected from the meteorological data center, China Meteorological Administration (website: data.cma.cn)(accessed on 1 January 2015).The soil was collected from fluvial and marine sediments, with silty loam in texture.The salt content was uneven in the soil, some areas were slightly saline and some points contained high salinity [4].The EC, pH, TOC, Bulk density, total N, available P, and total K and Cd in the slightly saline soil were 480 µS cm −1 , 9.24, 7.69 g kg −1 , 1.34 g cm −3 , 1.06 g kg −1 , 0.029 g kg −1 , 0.21 g kg −1 , and 0.094 mg kg −1 , respectively.In heavily saline soil, the correlated same parameters were 2500 µS cm −1 , 9.04, 1.02 g kg −1 , 1.38 g cm −3 , 0.29 g kg −1 , 0.008 g kg −1 , 0.06 g kg −1 , and 0.102094 mg kg −1 , respectively.The particle size composition was sand 3.48%, silt 75.76%, and clay 20.76% in slightly saline soil, and sand 3.41%, silt 76.80%, and clay 20.79 in heavily saline soil.The maize planting experiment was conducted in the slightly saline soil area, which had been sampled for EC and pH measurement.The slightly saline soil was abundant in nitrogen, with an average value of 1.06 g kg −1 (Table 1).Four treatments were designed in the plot, which contains plastic mulching (PM), nitrogen fertilizer (N), PM+N, and no nitrogen fertilizer planting (CK).There were three replicates in each treatment, and each experimental plot distribution appears to be completely random.The block size was 6 m×8 m, and with 30 cm away from each other.Maize (Suyu 80) was sown in June 2016 and harvested in October 2016.The plastic mulching was 0.025 mm in thickness and 2 m in width.The plant spacing and row spacing was 30 cm and 50 cm, respectively.A 5 cm radius hole was dug around the seedlings in the PM and PM+N treatments after the seedling emergence.Basal fertilizers of urea and calcium superphosphate were applied at 135 kg ha −1 N and 90 kg ha −1 P2O5, respectively, at seedling stage.The N fertilization was only applied in N and PM+N treatments.The basal fertilization was spread on the surface and blend with the soil of 0~10 cm.At the jointing period, 90 kg ha −1 N as urea was applied in the row as supplementary fertilizer to a depth of 10 cm and covered with in situ soil.

Measurements and Sampling
Prior to the initiation of the maize cultivation, the soil in the 0~20 cm was sampled in May 2016 for measuring the basic properties.Soil samplings at depths of 0~20 cm and Based on the previous field survey, we chose two sites for sampling.One plot was prepared for maize planting with a total salt content of less than 2.5 g kg −1 after natural desalination (slightly saline soil, SS), and the other plot was newly reclaimed and lying idle for its salt content higher than 6 g kg −1 (heavily saline soil, HS).The plants grown in slightly saline soil consists of reeds and Spartina alterniflora.A 5 m × 5 m square was designated randomly in the two plots.Five points were collected in each plot, with 1 m deep in every point and samples were taken at 20 cm intervals.The same layer soils were combined into one sample, and the total soil samples were five in each plot.The soils were taken to the laboratory, then air-dried and sieved to pass through a 2 mm mesh sieve.The treated soil samples were stored for making soil extracts with different soil-water ratios (w/w).For simulating the various saline degree under the soil desalination process, five kinds of soil-water ratios were designed as follows, 1:2.5, 1:5, 1:10, 1:20, and 1:40.The soil water mixture was made by adding 1, 2, 4, 8, and 16 g soil to the 40 mL deionized water in 50 mL capacity of centrifuge tubes.Then we shocked the tubes for 1 h and separated soil and water by the measures of rapid centrifugation and filtration.The leach solution was collected for electrical conductivity and pH measurement.The total organic carbon (TOC), bulk density (BD), total nitrogen, and other nitrogen elements of the 0~20 cm soil were tested according to our previous study [3] at the same time (Table 1).The maize planting experiment was conducted in the slightly saline soil area, which had been sampled for EC and pH measurement.The slightly saline soil was abundant in nitrogen, with an average value of 1.06 g kg −1 (Table 1).Four treatments were designed in the plot, which contains plastic mulching (PM), nitrogen fertilizer (N), PM+N, and no nitrogen fertilizer planting (CK).There were three replicates in each treatment, and each experimental plot distribution appears to be completely random.The block size was 6 m × 8 m, and with 30 cm away from each other.Maize (Suyu 80) was sown in June 2016 and harvested in October 2016.The plastic mulching was 0.025 mm in thickness and 2 m in width.The plant spacing and row spacing was 30 cm and 50 cm, respectively.A 5 cm radius hole was dug around the seedlings in the PM and PM+N treatments after the seedling emergence.Basal fertilizers of urea and calcium superphosphate were applied at 135 kg ha −1 N and 90 kg ha −1 P 2 O 5 , respectively, at seedling stage.The N fertilization was only applied in N and PM+N treatments.The basal fertilization was spread on the surface and blend with the soil of 0~10 cm.At the jointing period, 90 kg ha −1 N as urea was applied in the row as supplementary fertilizer to a depth of 10 cm and covered with in situ soil.

Measurements and Sampling
Prior to the initiation of the maize cultivation, the soil in the 0~20 cm was sampled in May 2016 for measuring the basic properties.Soil samplings at depths of 0~20 cm and 20~40 cm were obtained during the seedling stage (15 June), jointing stage (1 July), tasseling stage (20 August), and mature stage (24 September).Three samples were collected at the same treatment plot and combined as one sample.The soil was sieved through a 1 mm sieve after air drying for determining soil salinity (electrical conductivity, EC), and pH in 1:5 w/v ratio soil suspensions.Undisturbed soil cores were taken from the soil at depths of 10~15 cm by a cylinder of 100 cm 3 in volume to determine the bulk density (BD) and water-holding capacity at harvest stage.Maize plant heights were measured in each plot with five plants during the harvest stage.Maize grains were laid to dry, then we determined the grain weight and cob weight.The harvest index was calculated through the grain weight by whole biological weight.
For all soil samples, soil EC and pH were measured by conductivity and pH sensors (SevenExcellence Cond meter, Mettler Toledo, Zurich City, CH, USA).The grain and cob weight were determined on the electronic weighing scale (ME3002, Zurich City, CH, USA).

Statistical Analyses
All data were expressed as means and standard deviations (SD).We adopted SigmaPlot 12.5 (Systat Software Inc., San Jose, CA, USA) for graphing.Statistical analyses were carried out using SPSS 19.0 (SPSS Inc., Chicago, IL, USA).The significance for differences between the treatment means was calculated by one-way analysis of variance (ANOVA) at p < 0.05.

Correlation Analyses of EC, pH of Different Soil-Water Ratio Solutions
As shown in Table 1, there was a significant correlation between the EC of different soil-water ratio solutions of the heavily saline soil (n = 75).The correlation coefficient changed between 0.992 and 1.The highest correlation effect occurred in the 1:10 soil-water ratio solution, which was all higher than 0.995.In the slightly saline soil (n = 75), only 1:5 and 1:10 ratio soil solutions showed some significant correlations with other ratio extracts.The 1:10 ratio soil water combination presented a higher correlation coefficient than 1:5 ratio solution.The most relevant ratio in slightly saline soil was the 1:5 and 1:10, up to 0.985 at the 0.01 level.The 1:10 ratio of soil-water extracts was an optimal selection for soil EC testing.The significant correlation between different concentrations of soil extracts indicated that the main ions during the soil desalination were likely changed little.
The pH value showed a significant correlation between all soil-water ratio extracts in the heavily saline soil (Table 1).The correlation efficient was among 0.944 to 0.990.The highest efficient value was in the 1:5 ratio.The significant correlation (0.01 level) was only shown in 1:10 and 1:5, 1:10 and 1:2.5, 1:5 and 1:2.5, 1:5 and 1:40, and the correlation coefficient was 0.968, 0.980, 0.964, and 0.976, respectively.The 1:5 soil-water extracts show a better correlation effect in the slightly and heavily saline soil.The soil solution in different soil-water ratios represents the various concentrations of salt solutions.

Correlation Analyses of EC and pH of Different Soil-Water Ratio Solutions
The negative correlation between EC and pH was shown in Table 2 and Figure 2.Only in heavily saline soil (n = 75) was a significant correlation was observed; the correlation coefficient was between −0.946 and −0.987.In slightly saline soil (n = 75), the significant correlation occurred solely at the depth of 0~40 cm, with the coefficient −0.899 at 0~20 cm depth and −0.958 at 20~40 cm soil layer.The lowest coefficient was found at 80~100 cm depth, which was −0.091.Furthermore, the equation y = 0.02x 2 − 0.334x + 10.083 (y, pH; x, EC) was obtained for pH calculation in various soil-water ratio solutions through the data calculation in this study.The soil alkaline increase during soil desalination can be proved by the negative correlation between EC and pH.

Soil Saline-Alkaline Variation Characteristics
The variation tendency of EC was the same as that of pH in all treatments.Soil salt content decreased with the growth of maize at the 0~20 cm depth, and there was no significant difference between all four treatments during the cultivation experiment (Figure 3).At the depth of 20~40 cm, the EC value increased to the highest at the seedling stage in PM treatment then reduced gradually.No significant difference was found in all plots.The EC of 0~20 cm was approximately 180 μS cm −1 at the harvest stage, which is lower than that of 20~40 cm, 215 μS cm −1 .The pH increased up to the largest value in the tasseling stage, then decreased after harvest.The mean value of pH in the 0~20 cm layer was 9.12, which was lower than the 9.38 in the 20~40 cm layer.The pH increased with the EC decrease before the tasseling period, when the EC was higher than 200 μS cm −1 .The pH increased during the soil desalination process.However, in the harvest, period the pH decreased compared with the tasseling stage.The results showed that the pH increased during the soil desalination process and then increased when the soil salt content decreased to a certain extent.The pH during the change of soil salt is always greater than 8.8 at depths of 0~20 cm and 9.0 at the 20~40 cm depths.The soil showed an alkaline characteristic during the maize cultivation.The variation tendency of EC was the same as that of pH in all treatments.Soil salt content decreased with the growth of maize at the 0~20 cm depth, and there was no significant difference between all four treatments during the cultivation experiment (Figure 3).At the depth of 20~40 cm, the EC value increased to the highest at the seedling stage in PM treatment then reduced gradually.No significant difference was found in all plots.The EC of 0~20 cm was approximately 180 µS cm −1 at the harvest stage, which is lower than that of 20~40 cm, 215 µS cm −1 .The pH increased up to the largest value in the tasseling stage, then decreased after harvest.The mean value of pH in the 0~20 cm layer was 9.12, which was lower than the 9.38 in the 20~40 cm layer.The pH increased with the EC decrease before the tasseling period, when the EC was higher than 200 µS cm −1 .The pH increased during the soil desalination process.However, in the harvest, period the pH decreased compared with the tasseling stage.The results showed that the pH increased during the soil desalination process and then increased when the soil salt content decreased to a certain extent.The pH during the change of soil salt is always greater than 8.8 at depths of 0~20 cm and 9.0 at the 20~40 cm depths.The soil showed an alkaline characteristic during the maize cultivation.

Soil Bulk Density and Water Content
Compared to CK treatment, PM and/or N treatment significantly affected bulk density (BD) (Figure 4).The BD was 1.28 g cm −3 , and in the other three treatments was almost 1.33 g cm −3 .Soil water content was lowest in PM+N treatment, and it had a significant difference compared to N treatment.There was no difference between the PM/CK and other treatments.The soil water contents were all higher than 20 % at the harvest stage.

Agronomic Characteristics of Maize
A significant difference was found between PM and N treatments in the height of maize, but no difference existed in grain yield (Figure 5).PM and/or N treatments

Soil Bulk Density and Water Content
Compared to CK treatment, PM and/or N treatment significantly affected bulk density (BD) (Figure 4).The BD was 1.28 g cm −3 , and in the other three treatments was almost 1.33 g cm −3 .Soil water content was lowest in PM+N treatment, and it had a significant difference compared to N treatment.There was no difference between the PM/CK and other treatments.The soil water contents were all higher than 20 % at the harvest stage.

Soil Bulk Density and Water Content
Compared to CK treatment, PM and/or N treatment significantly affected bulk density (BD) (Figure 4).The BD was 1.28 g cm −3 , and in the other three treatments was almost 1.33 g cm −3 .Soil water content was lowest in PM+N treatment, and it had a significant difference compared to N treatment.There was no difference between the PM/CK and other treatments.The soil water contents were all higher than 20 % at the harvest stage.

Agronomic Characteristics of Maize
A significant difference was found between PM and N treatments in the height of maize, but no difference existed in grain yield (Figure 5).PM and/or N treatments increased grain yield, and they were all higher than CK.The yield in the N treatment was

Agronomic Characteristics of Maize
A significant difference was found between PM and N treatments in the height of maize, but no difference existed in grain yield (Figure 5).PM and/or N treatments increased grain yield, and they were all higher than CK.The yield in the N treatment was the highest, followed by PM+N, PM, and CK.The yield was 6536, 6349, 6231, and 5805 kg ha −1 , respectively.There was no significant difference in maize cob weight between all treatments (Table 3).The thousand-grain weight in PM treatment was higher than other treatments, with a value of 373.5 g.N treatments decreased the thousand-grain weight of grain.The grain yield was almost 3.25-3.80times higher than the cob weight.The largest harvest index was found in PM treatment, and it had a significant difference with N treatments.

Discussion
In the world, 23% of the arable lands are affected by salinity and a further 10% are saline-sodic soils [27].The coastal saline lands are mainly affected by sodium chloride which originates from seawater [28].Monitoring soil saline-alkaline characteristics and ameliorating soil salinity is beneficial for the rational utilization of coastal soil resources.To investigate the safe and environmental measures for getting more productivity in coastal saline areas, we need to pay more attention on the crop planting in coastal saline soils.

Saline-Alkaline Characteristics during the Soil Desalination Process
The correlation analysis of EC and pH showed that the higher the EC value, the lower pH (Table 2).Soil pH is undoubtedly one of the key parameters influencing soil biogeochemistry [3].Although the EC of a 1:5 soil extract has been used to describe soil salinity in many research institutes all around the world [29,30], the international criteria for measuring the EC of soil extract has not been established [31].In addition, the current saturated paste process takes a lot of time and is mainly based on subjective experience [32].In our study, the correlation coefficient of EC in 1:10 soil-water ratio extracts was the highest There was no significant difference in maize cob weight between all treatments (Table 3).The thousand-grain weight in PM treatment was higher than other treatments, with a value of 373.5 g.N treatments decreased the thousand-grain weight of grain.The grain yield was almost 3.25-3.80times higher than the cob weight.The largest harvest index was found in PM treatment, and it had a significant difference with N treatments.

Discussion
In the world, 23% of the arable lands are affected by salinity and a further 10% are saline-sodic soils [27].The coastal saline lands are mainly affected by sodium chloride which originates from seawater [28].Monitoring soil saline-alkaline characteristics and ameliorating soil salinity is beneficial for the rational utilization of coastal soil resources.To investigate the safe and environmental measures for getting more productivity in coastal saline areas, we need to pay more attention on the crop planting in coastal saline soils.

Saline-Alkaline Characteristics during the Soil Desalination Process
The correlation analysis of EC and pH showed that the higher the EC value, the lower pH (Table 2).Soil pH is undoubtedly one of the key parameters influencing soil biogeochemistry [3].Although the EC of a 1:5 soil extract has been used to describe soil salinity in many research institutes all around the world [29,30], the international criteria for measuring the EC of soil extract has not been established [31].In addition, the current saturated paste process takes a lot of time and is mainly based on subjective experience [32].In our study, the correlation coefficient of EC in 1:10 soil-water ratio extracts was the highest (Table 1).Therefore, we suggested that in coastal saline soil, a 1:10 soil-water ratio solution may be the optimal ratio for EC testing.With the soil salt decrease, the sodium chloride accounted for was lower in the soil solution, and the carbonate content increased [33].As pH testing time is also longer than EC measurement, calculating pH through EC value means saving time.However, the EC and pH were all measured at a stable temperature (25 • C) in this experiment, and many studies have reported a linear positive correlation between EC and temperature [34][35][36].Otherwise, if the cultivation continued for a long time, the salt ion composition would change [37,38].So, the equation obtained in our study may apply to short-term saline soil in which sodium chloride is the main salt.Furthermore, the insignificant correlation between EC and pH at the 40~100 cm depths in slightly saline soil may be caused by the reclamation method which disturbed the primary soil [39].During natural desalination, the pH also increases, accompanied by the salt decrease.Although pH was mainly correlated with carbonate content in slightly saline soil, it was hard to test, and the soil situation changed in the soil-water solution which was different from the in situ soil.The negative correlation of pH and EC in our experiment, which consists of various concentrations of soluble salt, represented the pH decrease during the soil desalination.In other unpublished research, we calculated the correlation coefficient of EC, pH, and other main parameters after ten years of cultivation in the coastal saline land which was 1.5 km away from the area of this study.The K + , Na + , Ca 2+ , Mg 2+ , Cl − , SO 4 2− , and CO 3 2− + HCO 3 − in the normal cultivation plot were 17.2, 370, 51.5, 27.7, 781.6, 33.5 and 21.4 mg kg −1 , respectively.In addition, the pH values in 0-40 cm soil were all higher than 8.5 after four planting seasons in three years of cultivation [1].That experiment also discovered that the pH had a stronger correlation with EC than other parameters (Table 4).So, obtaining accurate EC and pH values is crucial for healthy plant growth in coastal saline soil, as it will affect the nutrient availability and growth activity of plants [40].For accelerating the time to obtain the saline-alkaline situation of the coastal saline soil, measuring soil pH and salt content in in situ conditions needs to be undertaken in further research.

Nitrogen Input Regulation in Desalinated Slightly Saline Soil
The slightly saline lands account for many areas in China, reasonably using this kind of resources is beneficial for increasing farmlands and providing more food [41].Compared to the arid area, a coastal region not only has sufficient sunshine duration but also large amounts of precipitation (Figure 6), which is beneficial for salt leaching and crop growth [42,43].During the cultivation period, rainfall mainly occurred from June to September, approximately accounting for 80% of the annual rainfall.In addition, the nitrogen in slightly saline soil was obviously higher than that of heavily saline soil.The water and hot resources in the coastal area are richer than in arid area, which makes it easier to accumulate nutrition in natural desalination situations; there are also many halophytes grown in in situ soil [44,45].The salinity of the coastal ecosystem may slow down the halophyte residue decomposition processes in soil and thereby may increase the nutrition elements sequestration potential.As there was no significant difference in all treatments during maize planting (Figure 3), the decrease of soil salt was significantly affected by rainfall leach and the soil desalinized in the root zone.The EC of 20~40 cm was higher than that of 0~20 cm, and the pH at depths of 0~20 cm was lower than those of 0~20 cm, which indicated the negative correlation between EC and pH (Table 2).
growth [42,43].During the cultivation period, rainfall mainly occurred from June to September, approximately accounting for 80% of the annual rainfall.In addition, the nitrogen in slightly saline soil was obviously higher than that of heavily saline soil.The water and hot resources in the coastal area are richer than in arid area, which makes it easier to accumulate nutrition in natural desalination situations; there are also many halophytes grown in in situ soil [44,45].The salinity of the coastal ecosystem may slow down the halophyte residue decomposition processes in soil and thereby may increase the nutrition elements sequestration potential.As there was no significant difference in all treatments during maize planting (Figure 3), the decrease of soil salt was significantly affected by rainfall leach and the soil desalinized in the root zone.The EC of 20~40 cm was higher than that of 0~20 cm, and the pH at depths of 0~20 cm was lower than those of 0~20 cm, which indicated the negative correlation between EC and pH (Table 2).The bulk density increased with PM and/or N treatments after maize harvest (Figure 3), and this was contrary to some field experiments [46,47].Similar results had been reported in our previous studies [9,48]; this phenomenon was caused by a large amount of rainfall during the maize growth.Our experiment suggested that mulching treatment could produce a high grain yield compared to N treatment in the first planting season on the newly reclaimed coastal saline soil.It is obviously more beneficial to conduct mulching than N application in the coastal soil, for it can reduce pollution and prevent the eutrophication of water [49].Though the N treatment, we obtained the highest grain yield, its thousand-grain weight value was the lowest, which indicates that the quality of grain may not be as good as other PM treatments.The harvest index is defined as the pounds of grain divided by the total pounds of above ground biomass (straw plus grain).As a general rule, in a normal year (i.e., without weather extremes), the harvest index will be 0.50.While this index is a good rule of thumb and can be used to estimate straw production in various fields [50], the field experiment showed that PM treatment obtained the highest harvest index of 0.51, which was higher than 0.5.Despite PM+N treatment producing the most grain yield, it increased the environmental pollution risk and stimulated soil nutrient consumption [51].Therefore, PM treatment to reduce N was useful for crop planting in coastal slightly saline soil, but its long time effects need further study.The bulk density increased with PM and/or N treatments after maize harvest (Figure 3), and this was contrary to some field experiments [46,47].Similar results had been reported in our previous studies [9,48]; this phenomenon was caused by a large amount of rainfall during the maize growth.Our experiment suggested that mulching treatment could produce a high grain yield compared to N treatment in the first planting season on the newly reclaimed coastal saline soil.It is obviously more beneficial to conduct mulching than N application in the coastal soil, for it can reduce pollution and prevent the eutrophication of water [49].Though the N treatment, we obtained the highest grain yield, its thousandgrain weight value was the lowest, which indicates that the quality of grain may not be as good as other PM treatments.The harvest index is defined as the pounds of grain divided by the total pounds of above ground biomass (straw plus grain).As a general rule, in a normal year (i.e., without weather extremes), the harvest index will be 0.50.While this index is a good rule of thumb and can be used to estimate straw production in various fields [50], the field experiment showed that PM treatment obtained the highest harvest index of 0.51, which was higher than 0.5.Despite PM+N treatment producing the most grain yield, it increased the environmental pollution risk and stimulated soil nutrient consumption [51].Therefore, PM treatment to reduce N was useful for crop planting in coastal slightly saline soil, but its long time effects need further study.

Coastal Salt-Affected Soils Desalination and Crop Cultivation
Reclamation of saline soils is mainly dependent on the provision of effective drainage and the availability of good-quality irrigation water so that salts can be leached from the soil.The effects of seawater intrusion, solute components accumulation in the coastal area, and soil salinity hinder the sustainable development of agriculture.The higher salt may bring about damage to the coastal biological environment; the salt improved the cadmium and lead movement ability from soil to plants [52,53].The removal of soil salt was mainly by water, and rain was the economical and sufficient method.However, the rainfall in the coastal area was also too much to control.There may be a need for waterlogging drainages during the rainy season to rationally use rain for leaching the salt from the soil.In the natural situation, rain was useful for salt amelioration.Otherwise, the crops planted in the summer demand more attention be paid to precipitation during the seedling stage [54].As shown in Figure 3, the maize planted in April for saving water resources, the soil salt content can be leached to a suitable range which is not harmful to crop growth.The pH increased with the decrease of soil salinity, and it would affect the mineral nutrient availability [55].The crop productivity was not only related to soil salinity and pH but was also affected by soil nutrition.So, the nutritional elements should be retained during the desalting process, and water usage needs to be controlled.

Conclusions
The results of this study showed that the EC and pH had a significant negative correlation with various soil-water ratio extracts at depths of 0~100 cm in heavily and slightly saline soil.In the nitrogen abundant slightly saline soil, the PM treatment was a better choice for reducing nitrogen addition and improving crop productivity.The nitrogen accumulated in slightly saline soil under natural situations, which makes mulching to alleviate soil salinity and improve crop production possible.More attention should be paid to the pH increase during soil desalination, and nitrogen nutrition needs more reasonable management as well.

Figure 1 .
Figure 1.Geographical location of Dongtai city and the experimental site (a), and the experimental design and scenes (b) in the study field.

Figure 1 .
Figure 1.Geographical location of Dongtai city and the experimental site (a), and the experimental design and scenes (b) in the study field.

Figure 2 .
Figure 2. EC and pH in heavily saline soil (a) and slightly saline soil (b) at the various soil depths in 0~100 cm.

Figure 2 .
Figure 2. EC and pH in heavily saline soil (a) and slightly saline soil (b) at the various soil depths in 0~100 cm.

Figure 3 .
Figure 3. EC and pH of soil extract at 0~40 cm depth during different growth stage of maize.(a) electrical conductivity; (b) pH.Note: N, PM+N, PM, CK indicate nitrogen fertilizer, polythene film mulching + nitrogen fertilizer, polythene film mulching and no nitrogen fertilizer application, the same bellow.

Figure 4 .
Figure 4. Bulk density and water content of 0~20 cm soil during ripping period of maize.Note: Different letters above the blocks indicate significant differences (p < 0.05) between the treatments.Symbols and bars represent the mean ± SD (n = 3).

Figure 3 .
Figure 3. EC and pH of soil extract at 0~40 cm depth during different growth stage of maize.(a) electrical conductivity; (b) pH.Note: N, PM+N, PM, CK indicate nitrogen fertilizer, polythene film mulching + nitrogen fertilizer, polythene film mulching and no nitrogen fertilizer application, the same bellow.

Sustainability 2023 , 14 Figure 3 .
Figure 3. EC and pH of soil extract at 0~40 cm depth during different growth stage of maize.(a) electrical conductivity; (b) pH.Note: N, PM+N, PM, CK indicate nitrogen fertilizer, polythene film mulching + nitrogen fertilizer, polythene film mulching and no nitrogen fertilizer application, the same bellow.

Figure 4 .
Figure 4. Bulk density and water content of 0~20 cm soil during ripping period of maize.Note: Different letters above the blocks indicate significant differences (p < 0.05) between the treatments.Symbols and bars represent the mean ± SD (n = 3).

Figure 4 .
Figure 4. Bulk density and water content of 0~20 cm soil during ripping period of maize.Note: Different letters above the blocks indicate significant differences (p < 0.05) between the treatments.Symbols and bars represent the mean ± SD (n = 3).

Figure 5 .
Figure 5. Plant height and grain weight of maize.Note: Different letters above the blocks indicate significant differences (p < 0.05) between the treatments.Symbols and bars represent the mean ± SD (n = 3).

Figure 5 .
Figure 5. Plant height and grain weight of maize.Note: Different letters above the blocks indicate significant differences (p < 0.05) between the treatments.Symbols and bars represent the mean ± SD (n = 3).

Figure 6 .
Figure 6.Precipitation and sunshine duration of each month in 2016.

Figure 6 .
Figure 6.Precipitation and sunshine duration of each month in 2016.

Table 1 .
Correlation analysis of EC, pH with different soil-water ratios at the same soil depth.
Note: *, significance at the 0.05 level; **, significance at the 0.01 level.The data in the table represents the correlation coefficient of the EC of the solution of different soil-water ratios.

Table 2 .
The correlation coefficient of EC and pH in different soil-water ratios extract and at various soil depth.

Table 2 .
The correlation coefficient of EC and pH in different soil-water ratios extract and at various soil depth.

Table 3 .
Components of maize harvest plant.
Note: Values are shown as mean ± SD.Different letters in same column indicates significant differences between the treatments (p < 0.05).

Table 3 .
Components of maize harvest plant.
Note: Values are shown as mean ± SD.Different letters in same column indicates significant differences between the treatments (p < 0.05).

Table 4 .
Correlation analysis of soil obstacle properties and crop production in coastal saline soil after ten years cultivation.