Soil Quality Improvement with Increasing Reclamation Years in the Yellow River Delta

: The saline soils in the Yellow River Delta are primarily a ﬀ ected by seawater


Introduction
Increased global population and subsequent food demands evidently accelerate arable land exploitation.Reclamation, thus, becomes a vital approach to maintaining the balance of arable land resources.However, soil salinity poses a significant challenge to the sustainable agricultural development of reclaimed lands.The primary salinity is attributed to climate conditions and sea-land position, and the secondary salinity is due to inappropriate, irrational practices [1,2].In China, various types of saline soils cover an area of 2.67 × 10 7 hm 2 , of which 6.7 × 10 6 hm 2 have been reclaimed, which stems from agricultural development [3].The Yellow River Delta, large areas of which are characterized as saline soils with low nutrient content and poor soil quality, has been significantly impacted by natural factors such as global climate change and seawater intrusion [4].In particular, the shallow water table and high mineralization lead to salt accumulation on the soil surface, resulting in a widespread distribution of saline soils [5,6].With the increased food demands, it is crucial to reclaim the saline soils of the Yellow River Delta in a rational manner to increase crop yields by promoting sustainable land use and managing the saline soil resources.
Reclamation, with soil quality changing throughout the process, also has a significant impact on the soil environment [7].Several studies examined the variations in soil properties during different reclamation periods and generally observed that soil organic matter and nitrogen content decreased significantly after the reclamation of saline soils [8].For black soils in Northeast China, it was observed that the initial 20 years of reclamation had much higher rates of decreasing soil nitrogen and organic matter content, after which the soil nitrogen and organic matter content decreased stably [9].To counteract the carbon and nutrient decline, enhanced fertilizer applications and strengthened nutrient management practices are needed.The impact of reclamation on the characteristics of soil salinity and nutrient levels and influencing factors across different topographic and geological zones have been extensively studied, which provides a theoretical foundation for implementing appropriate fertilization techniques in regional farmlands [10].For instance, in Northeast China, rice cultivation significantly lowered the organic carbon content in the 0-20 cm and 20-40 cm soil layers compared to the primarily saline soil after 85 years, with a decrease of 19.93% and 25.51%, respectively [11].In arid areas with saline mine spoils, the content of total nitrogen and organic carbon gradually upgraded with reclamation years, while organic carbon and total phosphorus decreased initially but eventually increased in carbonate rock areas [12].However, no regular effect was observed for long and short reclamation times on soil nutrient content.The results indicated that soil nutrient content was also significantly influenced by other factors [13].Furthermore, massive research has been conducted to explore the spatial characterization of arable land, as well as the relationship of the temporal and spatial variability of soil nutrients with environmental factors, including fertilization, topography, crop type, etc. [14].
Soil quality is reflected by soil characteristics such as physicochemical properties.It encompasses soil productivity, the soil environment, and the health of plants and animals and plays a crucial role in maintaining ecosystem sustainability [15].Owing to the multifaceted nature of soil quality, relying on a single indicator is insufficient to accurately assess changes in soil fertility.Therefore, it is essential to select appropriate evaluation methods for a precise evaluation of the soil nutrient level [16].Presently, there is no universally standardized method to access soil quality.Researchers worldwide have conducted extensive studies on soil quality using various approaches such as cluster analysis, principal component analysis, gray correlation analysis, and comprehensive index methods [17].Of the various methods, the Soil Quality Index (SQI) has gained widespread and successful applications in numerous studies conducted at different scales and locations.SQI integrates physical, chemical, and biological indicators, including soil organic carbon content, pH, nutrient contents, etc., allowing for a visually accurate assessment of soil quality [18].By incorporating measured soil indicators, SQI utilizes principal component analysis to define the weight of every indicator within a uniformity system after homogenization.Subsequently, the index is calculated by applying scoring equations to compare the final values and enable soil quality assessments.Remote sensing technology has been used by experts to evaluate the temporal and spatial dynamics of soil salinity and nutrients [13].However, most of the findings have been limited to the visual characteristics of multiple indicators.
The Yellow River Delta currently experiences land degradation, which leads to a decline in soil quality.The long-term reclamation of this area has resulted in variations in soil salinity and quality.However, there is a lack of overall analysis regarding soil quality, and very few studies have compared soil quality among different reclamation years.Therefore, it is crucial to study the changing properties of saline soils and investigate the evolution of soil quality during the reclamation process in order to guide subsequent development and ecological restoration of the Yellow River Delta [3,19].By focusing on food security and the sustainable use of arable land in the Yellow River Delta, this study investigated the spatial variation of typical reclaimed saline soils with different reclamation years.A soil quality evaluation system was established by selecting 13 soil properties and calculating the SQI using PCA.The objectives of this work were to (1) compare the significant characteristics of soil properties among different reclamation years, (2) analyze the impact of the reclamation year on soil salinity and soil nutrients, and (3) evaluate the soil quality of different reclamation years using SQI and provide guidance for the rational utilization of saline soils in the Yellow River Delta.By addressing these objectives, the study aimed to contribute to the effective utilization of reclaimed saline soils, ultimately supporting sustainable agricultural practices and land development in the Yellow River Delta.

Study Area
The Yellow River Delta is situated in the northeast of Shandong Province, China, which maintains a warm, temperate continental monsoon climate characterized by cold winters and hot summers.The average annual temperature is around 12.8 °C, and the average annual precipitation is 555.9 mm, while the average annual potential evaporation is 2049 mm [20].The region has a shallow water table depth with high mineralization that exceeds 5 g/L.The predominant soil textures are silt and fine clay, largely influenced by sediment deposits from the Yellow River.These soils are classified as coastal tidal soils [19].The native vegetation in the Yellow River Delta consists mainly of salt-tolerant species such as Suaeda heteroptera Kitog, Phragmites communis, Imperata cylindrica, etc. Artificial vegetation, including Fraxinus chinensis, Black Locust, Populus L. is also commonly found in this region [21].Over the past 50 years, extensive reclamation efforts have transformed saline lands into arable lands for cultivating crops such as wheat, corn, cotton, sorghum, rice, and others [22].Agricultural practices play a significant role.For example, irrigation from the Yellow River presses the surface salts to deeper layers, deep plowing blocks the soil capillary to transport water and salt in the soil, and planting saline-tolerant crops such as cotton and rice reduces the accumulation of salts in the surface soil [23].Fertilization practices also greatly affected the reclamation by introducing nitrogen, phosphorus, and potassium to the soil.

Soil Collection
According to the present land use and field surveys, soil samples were taken in Dongying City, Shandong Province, China, in October 2020.Soil samples were taken at 6 depth layers of 0-10 cm, 10-20 cm, 20-40 cm, 40-60 cm, 60-80 cm, and 80-100 cm in 15 sampling sites for different reclamation years (i.e., 0a, 1a, 5a, 10a, and 20a) in the Yellow River Delta (Figure 1).The soil of reclamation year 0a is primarily saline soil without anthropogenic influence, and the soils of reclamation year 1a to 20a are optimal for wheatmaize farming.All soil samples were kept in self-sealing bags and placed in a cold holding tank.For this study, soil water content (SWC), pH, electrical conductivity (EC), sand content, slit content, clay content, soil bulk density (SD), ammonium nitrogen (NH4 + -N), nitrate nitrogen (NO3 − -N), available nitrogen (AN), available phosphorous (AP), available potassium (AK), soil organic matter content (SOC), soil soluble cations (Na + , K + , Ca 2+ , Mg 2+ ), and anions (Cl − , SO4 2− , CO3 2− , HCO3 − ) were determined following the procedures described below.Specifically, SWC was measured by drying the soil samples in a baking oven at 105 °C for 24 h to stabilize the weight.SD was dug with a ring knife and measured by the drying approach.NH4 + -N and NO3 − -N were measured by potassium chloride immersion-UV spectrophotometry.After air-dried at natural temperature and sieved, pH and EC were determined by a water quality analyzing instrument (HACH, USA) with a soil-water ratio of 1:5.Sand, slit, and clay content were determined by the pipetting approach.AN, Ap, and AK were determined by the alkaline diffusion absorption approach, sodium bicarbonate immersion-molybdenum antimony anti-colorimetric approach, and ammonium acetate immersion-flame photometry, respectively.SOC was characterized by potassium dichromate dilution calorimetry.Soil soluble cations (Na + , K + , Ca 2+ , and Mg 2+ ) and anions (Cl − , SO4 2− , CO3 2− , and HCO3 − ) were determined by ion chromatography (ICP-OES and ICS-900) after filtration [24].The measurement instrumentation was calibrated following standard procedures, and the entire operation was carried out by professional lab technicians.Standard samples were placed in each batch for comparison of determined results, and repeat measurements were performed if abnormal values were found.All samples were determined in triple with reproducibility better than 5%, and then averaged results were reported to ensure quality control and assurance.
Soil electrical conductivity was converted to soil salt content, and then the values of the soil salt content were compared to classify soil salinity level according to the Chinese Coastal Saline Soil Classification.Specifically, soil salt content (SSC, g/kg) was cast from EC (dS/m) according to the empirical relationship built up by preceding research for the Yellow River Delta [25].SSC = 2.18 × EC + 0.727, R 2 = 0.9387 (1)

Data Analysis
We applied a series of methods for data analysis.ArcGIS 10.2 was applied to locate the study region and sampling points.Excel 2019 was applied to manage the data.SPSS 23.0 was applied to implement Pearson correlation analysis to get the correlation of the soil properties of different reclamation years.Origin 2021 was applied for table and chart drawing.One-way ANOVA was applied to describe the difference of significance.

Soil Quality Assessment
Soil quality was estimated by Soil Quality Index (SQI), which was weighted additive when contrasted to other approaches.SQI was established by means of downscaling and quantifying the soil indicators by integrating every indicator in a unified system to calculate weight through principal component analysis (PCA) with ascending and descending membership functions [26].SQI thus provided an intuitive and accurate evaluation of soil quality by combining a variety of soil information.
(1) Calculate the affiliation value The affiliation value was measured by the affiliation function, which includes ascending and descending affiliation functions, relying on the positive and negative effects of soil indicators on soil quality.The ascending function, which refers to soil chemical and biological properties, is applied in the assessment of soil nutrient indicators, with more being better.The descending function, which refers to soil physical properties, is applied in the assessment of soil salt indicators, with less being better.
The ascending membership function formula is as follows: The descending membership function formula is as follows: where x is the average value of the soil indicator, F(x) is the affiliation degree, and a and b are the lower and upper thresholds of the determined indicators, respectively.
(2) Calculate the indicator weights The factor significance of every soil indicator was measured by PCA, and the weight of every soil indicator was the proportion of the factor significance of every soil indicator to the overall factor significance within a uniformity system after homogenization.
(3) Calculate the soil quality index After a comparison of the three approaches of SQI evaluation, the weighted additive SQI was chosen for soil quality estimation: where n directs the order of soil indicators, Wi directs the weight value of the i th soil indicator, and Ni directs the affiliation degree of the i th indicator.

Overall Soil Characteristics for Different Reclamation Years
Overall soil physical and chemical characteristics of different reclamation years are shown in Table 1.SSC decreased significantly (p < 0.05) from 6.60 g/kg of reclamation year 0a to 1.63 g/kg for year 10a and increased slightly to 2.85 g/kg for year 20a.Soil salinity degree was reduced from saline soil of year 0a to slight salinity by year 10a and then increased to medium salinity by year 20a according to the Chinese Coastal Saline Soil Classification of non-salinity (SSC < 1 g/kg), slight salinity (1-2 g/kg), medium salinity (2-4 g/kg), heavy salinity (4-6 g/kg), and saline soil (>6 g/kg) [3].SWC was in the range of 25.79-29.70%for different reclamation years, with no significant difference.Soil pH of different reclamation years was weakly alkaline with a gradual decrease from 1a to 20a, which was apparently higher than that of the unreclaimed soils (p < 0.05).SD declined slightly from 1.48 g/cm 3 to 1.40 g/cm 3 with increasing reclamation years.The sand content decreased from 50.99% to 19.37% with increasing reclamation years, while the slit and clay content increased from 42.34% to 68.37% and 6.66% to 12.26%, respectively.NH4 + -N increased by 8.31 mg/kg from 0a to 5a and then decreased by 2.56 mg/kg at 20a.NO3 − -N of reclaimed soils was significantly less than that of the unreclaimed soils (p < 0.05), yet it continued to increase from 6.89 mg/kg at 1a to 15.85 mg/kg at 20a.AN significantly increased by 24.87 mg/kg after reclamation.AP significantly increased by 10.11 mg/kg after 10 years of reclamation and then slightly decreased to 8.64 mg/kg at 20a.AK decreased gradually with increasing reclamation years but increased at 20a.SOC increased from 4.71 g/kg to 11.47 g/kg with increasing reclamation years.The ion composition of different reclamation years and the clustering results are illustrated in the Piper diagram (Figure 2).In terms of cations, Na + and K + were the dominating ones in each soil layer for 0a and 1a, accounting for about 75% and 90%, respectively, while these two ions fluctuated at different depths, ranging from 45% to 85%, and mainly concentrated in the deep layers of 5a and 10a.The concentrations of Na + and K + in each layer became steady and accounted for about 70% of the cations for 20a.The overall proportions of Ca 2+ and Mg 2+ were mostly maintained below 30% with a slight dispersion, which did not change significantly with increasing reclamation years.In terms of anions, the proportion of SO4 2− in each soil layer of 0a was less than 5%, which gradually increased to 20%.The proportions of CO3 2− and HCO3 − in each layer were slightly higher than that of SO4 2− , ranging mostly from 10% to 30%.The proportion of Cl − exceeded 90% in each layer of 0a, which gradually decreased to 70-80% with increasing reclamation years.In particular, the change in ionic concentrations of 5a was more dramatic, with a greater deviation than in other years.

Vertical Distribution of Soil Properties of Different Reclamation Years
As shown in Figure 3, the SSC of 0a was significantly higher than that of the other reclamation years (p < 0.05), which gradually decreased with increasing reclamation years.The average of SSC in each layer of 0a, 1a, 5a, 10a, and 20a was 6.60 g/kg, 4.36 g/kg, 1.75 g/kg, 1.63 g/kg, and 2.85 g/kg, respectively.The SSC showed a peak of 9.68 g/kg in 0a, mainly concentrated in the shallow layer of 10-20 cm.The SSC peak gradually moved down, and the salinity gradually faded with downward depth.SSC peaked at 80-100 cm after long-term reclamation, which decreased to 5.93 g/kg, 2.34 g/kg, and 2.46 g/kg, corresponding to a decrease of 38.8%, 75.8%, and 74.6%, respectively.However, there was no dramatic difference in salt distribution between 5a and 10a.The SSC of 20a peaked at 60-80 cm, reaching 3.72 g/kg, which was 61.2% lower than that of 0a.Additionally, there was no apparent difference between the vertical distribution of 10a and 20a.SWC and SD of different reclamation years demonstrated a slightly increasing trend with downward depth, and the coefficient of variation at different depths was below 0.20 and 0.06, respectively, which did not demonstrate evident differences (p > 0.05).The soil pH at reclamation years of 0a and 10a showed a constantly increasing trend, which slightly decreased at 5a and 20a.Except for NO3 − -N, the NH4 + -N, AN, AP, AK, and SOC of the reclaimed soils were higher than those of the unreclaimed soil.It was interesting to observe that the depth of 40 cm was a turning point of the vertical distribution of the above parameters (Figure 4).The concentrations of NO3 − -N, AN, AP, and SOC above the 40 cm depth were significantly higher than those below this depth, demonstrating obvious surface aggregation.NH4 + -N and AK were stable with slight variations.Specifically, NH4 + -N increased slightly with downward depth, with 10a exhibiting slightly higher values than the others at layers above the 40 cm depth and 5a exhibiting significantly higher values than those of 10a and 20a below the 40 cm depth.NH4 + -N of 0a and 1a was low at each depth layer.Except for year 1a, AK slightly varied above the 40cm depth and then was maintained at around 100 mg/kg at 40 cm depth and downwards.NO3 − -N, AN, AP, and SOC of different reclamation years below 40 cm were, respectively, close (p > 0.05).In the layer of 40 cm, NO3 − -N of 0a was 2-6 times higher than that of the reclaimed soil, and the differences narrowed at depths below 40 cm.AN and SOC increased with the increase of reclamation years at depths above 40cm, which was obviously less for the year 10a than the others.The AP of 10a and 5a was significantly greater than 1a and 20a.

Principal Component Analysis
The selected soil indicators were analyzed by PCA, from which each principal component and its variance contribution rate, cumulative variance contribution rate, and the loading value of each physical and chemical indicator in each principal component were extracted within a uniformity system after homogenization (Table 2).Since the eigenvalues of the first four factors were greater than 1, these four factors were selected to characterize the results of this study.The cumulative variance contribution rate reached 80.87%, showing that the four principal components basically covered the main contents of the 13 indicators and could be used to reflect the variability of soil properties.It is generally accepted that the greater the factor loadings, the greater the weights of the variables in the corresponding principal components.The first principal component contributed 33.31% to the total variance, with sand, slit, and SOC contributing the most; the second principal component contributed 20.18%, with NO3 − -N, SWC, and pH contributing the most; the third principal component contributed 14.85% with SSC, NH4 + -N, and AK contributing the most; and the fourth principal component contributed 12.54%, with pH, SWC, and AP contributing the most.The first two principal components were extracted in order to view the data intuitively (Figure 5).Despite some overlap between the soils reclaimed for 10 and 20 years, soils of different reclamation years were mostly located in different areas of the coordinate system, suggesting the soil property characteristics were actually affected by different reclamation years, especially the soils of 5a and 20a.The line lengths of different indicators in Figure 5 reflected the degree of influence of different reclamation years on the soil, with longer line lengths indicating a greater influence of the indicator.It was revealed that sand, slit, and SOC were the three variables with the highest contribution to the first principal component, while NO3 − -N, SWC, and pH were the three variables with the highest contribution to the second principal component.This was consistent with the results of the ranking of the percentages of the four principal components (Table 2).The angle between the line segments showed the correlation between different indicators, and the acute angle showed a positive correlation, while the obtuse angle showed a negative correlation.A strong negative correlation was found between SSC, NH4 + -N, and AP.SOC was significantly positive with AN and slit and clay content and negative with sand content and SD.Additionally, the concentration variation of the four cations was strongly positively correlated with the change of SSC.The sand content, SSC, and NO3 − -N were all in the third quadrant and maintained a good positive correlation.The physical and chemical indicators of the unreclaimed soils at different depths were mostly distributed in this area, indicating that these three indicators had a greater influence on the native saline soils.The physicochemical indicators of different depths of the reclaimed soil were mostly distributed in the first and fourth quadrants, which were more influenced by SWC, pH, SLIT, and CLAY.

Soil Quality Index
The SQI of different reclamation years was determined according to the soil quality evaluation system, and the results were as follows: 0.307 for 0a, 0.339 for 1a, 0.320 for 5a, 0.318 for 10a, and 0.327 for 20a.The soil quality of the Yellow River Delta was at the medium level based on the six levels of soil quality adopted by the China second national soil survey.Box charts were drawn to characterize the dispersion of SQI for different reclamation years (Figure 6).In general, the soil quality of the reclaimed soils was higher than that of the unreclaimed soils.In particular, the SQI of 1a was significantly higher than others, and the soil quality continued to be improved with increasing reclamation years.

Soil Salinity and Reclamation Years
The native saline soils were mostly located in the northern and eastern coastlands in the Yellow River Delta, where the groundwater table was shallow and the soils contained soluble salts dominated by chlorides because of long-term seawater intrusion [3,27].Soil salt content (SSC) decreased notably by 33.9% for reclamation year 1a, after which the decline rate became moderate (Table 1).Previous studies of the study area indicated that the native saline soils were treated in a traditional way through flood irrigation and subsurface pipe drainage to leach the salts, causing the soil salts to be continuously drained and pressed downward [28,29].At the same time, the crop uptake and continuous field management of fertilization and irrigation eliminated the surface soil compaction, reduced the SD, and increased the porosity and water conductivity, resulting in salt leaching in the surface soil [30,31].With the reclamation year increasing, the soil salinity degree became slight, and the soil internal structure gradually stabilized, showing that SSC in the vertical profile decreased slightly, and the variability of vertical distribution was small (Figure 3) [32,33].
Due to the land-sea interface location of the Yellow River Delta, the content of Na + and Cl − accounted for 81.5% of the total cations (Figure 2).Because of the high Na + content, Ca 2+ and Mg 2+ were replaced and leached.Several studies found that mobile soil colloids facilitated soil reclamation by eliminating adsorbed Na + from the soil [34,35], which was significantly influenced by the soil pH.However, due to the large difference between the background Na + , Ca 2+ , and Mg 2+ (Na + /Ca 2+ = 11.89,Na + /Mg 2+ = 7.31), a large amount of Na + was leached out at the initial stage.With increasing reclamation years, the content of Ca 2+ and Mg 2+ tended to be steady, while Na + continued to be leached out.Native saline soils were subject to seawater intrusion and contained high levels of Cl − , which decreased slightly with increasing reclamation years because of crop uptake during the growth.The contents of other anions in the soil were relatively small, with relatively minor variation.
With the natural conservation and artificial reclamation of the Yellow River Delta, the vegetation and soil condition have been greatly improved, resulting in a series of feedback effects.Specifically, crops took up soil salts during growth, reducing the SSC to some extent.With increasing reclamation years, root growth improved the soil structure and increased soil porosity.The sand content decreased significantly from 50.99% to 19.37%, while the slit and clay content increased significantly from 42.34% to 68.37% and 6.66% to 12.26%, respectively.The above change in soil composition reduced soil bulk density from 1.48 g/cm 3 to 1.40 g/cm 3 .The lateral growth of crop roots interrupted the soil capillary channels, thus inhibiting the upward movement of soil salts in deep layers [36,37].
Soil pH was weakly alkaline as a result of seawater intrusion containing high salt, and fertilizer application made the pH of reclaimed soil higher than that of native saline soil, while the soil pH gradually decreased by 0.45 from 1a to 20a after reclamation.With crop growth and root development, the respiration of roots increased, which released more CO2, as shown in previous studies [38].The microbial decomposition of plant and animal residues gradually added organics into the soil, during which H + was released, causing the decline of the pH [39,40].In addition, crop growth reduced the wind speed on the ground and lowered the surface temperature.Subsequently, the soil water evaporation declined, and the salt accumulation weakened, slowing down the process of soil salinization [41,42].

Soil Nutrients and Reclamation Years
The soil nutrients showed obvious variations after the saline soils were reclaimed.Supplementation of chemical and organic fertilizers also upgraded the soil nutrient content.Since different types of crops required different amounts of nutrients at different growth stages during different reclamation years [43,44], soil nutrients, respectively, showed a certain pattern with increasing reclamation years, and the variations of nitrate nitrogen (NO3 − -N), available nitrogen (AN), available phosphorous (AP), and soil organic matter content (SOC) were significantly greater in the shallow surface layers than those of the deeper layers (Figure 4).Soil NH4 + -N and NO3 − -N were the main inorganic nitrogen sources absorbed by the roots to supply nitrogen for crop utilization [45].During the reclamation process, anthropogenic fertilization supplemented the nitrogen sources, increasing both NH4 + -N and NO3 − -N in the soil.The alkaline soil of the study area inhibited the process of nitrogen mineralization, while the crop growth required a low content of NH4 + -N, which led to NH4 + -N slight accumulation without significant difference after reclamation.Since crop growth mainly absorbed NO3 − -N, and NO3 − -N was readily mobilized with water, this resulted in the soil NO3 − -N residual values of different reclamation years being significantly lower than the background ones of native saline soils (Figure 4).
Reclamation mainly had a significant effect on AN at a depth of 0-40 cm, and the AN content rose with the increasing reclamation years (Figure 4).Field management practices would accumulate AN at the distribution range of root.Applied nitrogen fertilizers were leached to deeper soil layers through the irrigation process [46,47].Therefore, practical approaches should be taken to enhance the efficiency of fertilization for the farmlands that have been reclaimed for a long time.At the same time, accumulation of AN might be possible in the surface layers owing to the decomposition of crop residues and microbial activity.
The total content of available nutrients in the study area followed the following order: AN > AK > AP (Table 1).A fixed proportion of chemical fertilizers (N > K > P) was applied to farmlands to enhance the productivity of crops during the long-term reclamation.However, crops mainly absorbed AN, with a low use of the efficiency of AP and AK, leading to large quantities of AK and AP being left in the soil [48][49][50].The Yellow River Delta was significantly influenced by seawater immersion, and the high K + content caused the accumulation of AK in each layer of the native saline soils.AP input mainly came from fertilizer applications, which were relatively small.Therefore, the content of AK was obviously higher than that of AP.
SOC supplemented crop nutrients and promoted the formation of soil agglomerate structure to improve soil structure [51,52].SOC accumulated in the soil surface, resulting from the root system, mainly existed in the surface layer of 0-40 cm, and continuous crop growth and microbial decomposition played an important role.The SOC of the parent material was generally low, below 40 cm in the study region, and less susceptible to external influences.SOC increased with increasing reclamation years, especially when the content of 20a was significantly high, owing to an accumulation of excess fertilizer residues and microbial decomposition over decades (Figure 4).
In particular, soil nutrients at reclamation year 10a were slightly lower than those of other years because the sampling time coincided with crop harvesting in autumn.Crops left the soil, and the straws had not been returned to the soil, causing the cycle of original soil nutrients to be broken.Furthermore, the lack of vegetation cover on the surface soil also changed the soil quality condition.

Soil Quality and Reclamation Years
A prerequisite to ensuring a comprehensive and accurate soil quality evaluation is the selection of the correct evaluation indicators that effectively reflect the soil's physical and chemical property characteristics.The soil quality of the reclaimed soils was higher than that of the unreclaimed saline soil after crop growth and field management.In particular, the reclamation of year 1a apparently enhanced the soil quality from 0.307 to 0.339 (Figure 6) because of the dramatic changes in soil conditions.
Soil salt content and soil salinity degree showed an overall decreasing trend with increasing reclamation years, resulting from salt drainage by irrigation and crop growth [53,54].With the native saline soils being reclaimed and rational utilization, crop roots penetrated the soil, improving the soil structure and increasing the soil porosity and water-holding capacity, which obviously upgraded the soil condition [37].The change in land use formed a new cycle pattern of soil nutrients due to anthropogenic effects.Since a large amount of straw was returned to the field after crop harvest, carbon, nitrogen, and other nutrients were accumulated with increasing reclamation years [47].The mechanical action of the root system was enhanced, changing the soil's physical properties and directly or indirectly affecting the activity of soil microorganisms.Specifically, root secretions, root apoplast, and above-ground apoplast changed soil microhabitats for the reclaimed soils, leading to soil quality enhancement [48].
Soil quality changed rapidly in the early stage of reclamation and slowed down in the later stage.Soil salts were pressed into the deep layers through irrigation, and a large amount of compound fertilizers was applied to enhance soil fertility [44], which reduced soil salinity and improved soil nutrients.It should be noted that long-term monoculture farming may result in large-scale secondary salinization, decreasing soil quality gradually.In view of the unreasonable field management practices and soil conditions, this study provides theoretical support for sustainable management of the Yellow River Delta.For example, adjusting the fertilizer use based on the crop growth needs enables plants to fully absorb and use the soil nutrients in the soil.Moreover, crop succession planting can improve the conditions of soil nutrients and microbial communities to sustain soil quality.

Conclusions
The enhancement of the saline soil quality over the reclamation years of the Yellow River Delta was investigated, which was found to be a function of long-term crop growth and field management, such as fertilization and irrigation.For reclamation up to 20 years, SSC decreased significantly (p < 0.05) with increasing reclamation years, with the soil salinity degree reduced from saline soil to slight salinity and then slightly increased to medium salinity.AN, AP, and SOC showed a significant increase of 24.87 mg/kg, 10.11 mg/kg, and 6.76 g/kg, respectively, with increasing reclamation years, while AK decreased gradually after reclamation and then increased at 20a.SQI increased from 0.307 to 0.327 with increasing reclamation years.In particular, the reclamation year 1a apparently enhanced the soil quality from 0.307 to 0.339.Overall, soil reclamation weakened the soil salinity, enhanced the soil nutrients, and improved the soil quality.This study provides technical support and guidelines for sustainable management and development for the Yellow River Delta.

Figure 1 .
Figure 1.Study area, which is located in the northeast of Shandong Province, China.

Figure 2 .
Figure 2. Piper diagram of composition of cations and anions in the soils for different reclamation years.

Figure 3 .
Figure 3. Vertical distribution of soil salt content (SSC) for different reclamation years (SSC is illustrated as a function of reclamation years and soil depth).

Figure 5 .
Figure 5. Principle component analysis (PCA) of soil properties for different reclamation years.

Table 1 .
Overall soil characteristics at depth of 0-100 cm for different reclamation years (average results and standard deviation).
Different letters within the same indicator show significance, and similar letters are not significant at p < 0.05 after one-way ANOVA.

Table 2 .
Eigenvalues and variance contribution of principal component analysis.