الفهرس 
ACKNOWLEDGEMENTOnly 14 pages are availabe for public view
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Abstract
Despite the importance of assessing soil spatial variability, it was rarely the focus of research. Moreover, the impact of agricultural soil management on spatial variability at the farm scale has received little research attention. A better understanding of the spatial distribution patterns of available soil phosphorus is required to achieve the best phosphorus management in crop systems. Furthermore, it is very important to choose the correct sampling method to determine soil fertility with high accuracy because soil tests are useless unless soil samples are taken correctly and accurately to represent the studied field. The lack of understanding of the spatial distribution of phosphorus at the farm scale, as well as the failure to accurately map its spatial distribution as a result of not taking soil samples correctly, all of which represent gaps in soil management under Egyptian conditions, therefore, the current study was conducted in an attempt to address these gaps.
Five fields were chosen for the current study, each measuring 100 x 50 m and covering an area of 0.5 ha. Of these five chosen fields, two fields represented the cultivated clayey soils (old cultivated lands), two fields represented the cultivated sandy soils (newly reclaimed lands), and one field represented the virgin sandy soil (never cultivated land). from May to June 2019, soil samples were collected from all fields examined. A systematic sampling grid (10×10 m) was employed to identify soil sampling sites. All fields studied (cultivated and virgin) were sampled at four depths, which are 0–10, 10–20, 20–30, and 30–40 cm. For each tested soil depth, 50 simple soil samples (one sample from each sampling point) and 50 composite soil samples (five simple samples from each sampling point) were collected. Each composite soil sample was formed by mixing five simple soil samples from the vicinity of each sampling site. Consequently, for each investigated field, a sum of 200 simple and 200 composite soil samples was collected from all depths studied. Thus, a total of 2000 soil samples were collected from the five investigated fields. These samples were analyzed to determine, assess, characterize, and map the spatial variability of available soil phosphorus (Olsen-P) in the soil fields investigated.
In the current study, another soil sampling design was performed to determine the impact of cultivation on some selected physicochemical properties of sandy soil. To achieve this aim, a comparative study was carried out on the soil properties of virgin sandy soil (never cultivated) and cultivated sandy soil (cultivated for 5 years). Both fields (cultivated and virgin) were sampled at four depths: 0–10, 10–20, 20–30, and 30–40 cm. For each tested soil depth, 20 composite soil samples were formed from 50 simple soil samples, for a total of 80 composite samples from the four studied depths for each field. These composite samples were analyzed to determine the cultivation impacts on some selected soil physicochemical properties such as available soil phosphorus (Olsen-P), soil pH, electrical conductivity (EC), calcium carbonate, organic matter, and cation exchange capacity (CEC).
Several sampling designs were applied to determine which are best for collecting representative soil samples to assess and map the spatial variability of available phosphorus. Data on soil properties were subjected to classical statistics using SPSS v.26. Furthermore, for mapping soil attributes, a geo-referenced database of soil properties was created using the sampling point coordinates and soil samples values. The geostatistics analyses were carried out by ArcGIS using the geostatistical analyst extension.
The important results obtained from this study and some conclusions can be summarized in the following sections:
1. Available soil phosphorus levels in the studied fields
The results revealed that available soil phosphorus levels varied considerably among cultivated clay soil fields, cultivated sandy soil fields, and virgin sandy soil, as well as within the same soil type across the different depths studied.
According to the results, the highest phosphorus levels were observed in cultivated sandy soils, followed by clayey soils, while the lowest levels were found in virgin sandy soil.
Also, the results revealed that high phosphorus levels were found in the surface soil layers of cultivated soils (0-10 and 10-20 cm), which may be due to the low mobility of phosphorus downward and the higher organic matter content of these layers than in subsoil layers.
Furthermore, the results showed that the available phosphorus levels decreased with soil depth increased through all studied soil types.
Overall, the cultivated clayey soils and virgin sandy soil had a low variation in phosphorus content compared to the cultivated sandy soils. This could be because phosphorus distribution in cultivated clayey soils and virgin sandy soil is more homogeneous than in cultivated sandy soils.
Moreover, the findings demonstrated that cultivated soils (clayey and sandy) had greater Olsen-P levels than virgin sandy soil. This could be due to the application of manures and P-fertilizers in cultivated soils, as well as crop residue decomposition in these soils
2. Soil Olsen-P availability
The higher class (˃ 10 mg kg-1) of Olsen-P availability dominated at the surface soil depths (0–10 cm) in cultivated clay soils, as well as at depths of 0–10 and 10–20 cm in cultivated sandy soils, while the lower class (˂ 5 mg kg-1) dominated at the depth of 30–40 cm in cultivated Sand-2 site (S2) and at all depths in virgin sandy soil.
Overall, soil phosphorus availability classes decreased as soil depth increased in all cultivated soil types.
Furthermore, cultivated sandy soils had higher percentages of the high class (˃10 mg kg-1) of Olsen-P than clayey soils.
In comparison to cultivated clay and sandy soils, the virgin sandy soil had the lowest class (˂5 mg kg-1) of Olsen-P as the sole class of soil phosphorus availability at all depths tested.
Also, the differences in the percentage values of phosphorus availability classes between simple and composite samples were slightly higher in clay soils than in the cultivated sandy soils.
3. Geostatistical analysis and mapping of soil phosphorus spatial distribution
The results showed that some soil phosphorus datasets did not show normal distributions, while the majority of the datasets showed normal distributions. Therefor a log transformation was applied to bring the distributions of the datasets that did not show a normal distribution closer to the normal distribution.
Based on the performance cross-validation of the semivariogram models used, of the eleven models tested, eight were chosen as the best-fitted models for mapping the spatial variability of the available soil phosphorus across the four depths investigated for all fields.
The best semivariogram models chosen were J-Bessel, Rational Quadratic, K-Bessel, Tetraspherical, Gaussian, Stable, Hole Effect, and Exponential. This finding indicated that, among the eleven Ordinary Kriging (OK) models tested, there was no single fit model for mapping all phosphorus datasets at all tested depths in the same field or across fields investigated.
Thus, the chosen model as the best-fit model differed according to the soil depth, sample type, and soil type. Furthermore, these results indicated that the chosen models are the best-fitting models for mapping the spatial variability of soil phosphorus under the current study.
The current study found that the spatial variability of Olsen-P in the cultivated study sites (C1, C2, S1, and S2) varied greatly across the soil depths investigated and that simple and composite soil samples had very similar spatial distribution patterns at the same soil depth.
Furthermore, the soil surface layers (0–10 and 10–20 cm) had the greatest variation in the phosphorus spatial distributions, while the subsurface layers (20–30 and 30–40 cm) had the lowest variability in all studied fields.
Moreover, cultivated sandy soils sites had a higher spatial variation in phosphorus distribution than cultivated clay soils sites, which may be attributed to the management practices applied at these sites.
Also, results showed that the spatial distribution of soil phosphorus could vary even within a small scale of soil (0.5 ha).
In addition, the current study suggests that either simple or composite soil samples can be used for mapping the spatial distribution of soil Olsen-P at the investigated depths (0–10, 10–20, 20–30, and 30–40 cm) using a 10 x10 m sampling grid, as the differences between them are low.
The results also showed that soil phosphorus datasets in all studied sites and at all depths investigated had a spatial dependence ranging from strong to moderate. Depending on the semivariogram parameters the majority of the soil phosphorus spatial distributions had strong spatial dependence.
Strong spatial dependency on soil phosphorus may be attributed to intrinsic (inherent) soil factors such as parent material, texture, pH, lime content, and mineralogy, while both intrinsic and extrinsic (farming management practices) factors influence moderate spatial dependency.
4. Soil Olsen-P levels in the study sites according to soil sample type
Overall, although there were no significant differences according to the T-test between values of soil phosphorus based on soil sample types, the findings of the current study indicated that the type of soil samples should be considered, particularly for cultivated soils.
Moreover, the reason for the lack of statistically significant differences between the two types of soil samples (simple and composite) used in mapping soil phosphorus may be due to sampling at small distances using a systematic grid of 10×10 m.
Therefore, the simple soil samples may be suitable for mapping phosphorus spatial variability at surface layers (0–20 cm) of cultivated clayey soils but not for the same depths of cultivated sandy soils. Furthermore, both sample types can be used for mapping spatial variability of soil phosphorous in virgin sandy soils.
5. Soil Olsen-P levels in the study sites across tested soil depths
Based on the statistical parameter data, there are variations in soil Olsen-P levels across the different soil depths tested in both clay and sandy soil fields.
The findings indicated that there are statistically significant differences in soil Olsen-P levels in cultivated soils (sandy and clayey) between non-consecutive soil depths and successive depths of 0-10 and 10-20 cm, but not between successive depths in cultivated soils and all studied depths in virgin sandy soil except for between the surface (0-10 cm) and subsurface depths (30-40 cm).
According to the statistical parameter data, it can be said that there are differences between soil Olsen-P levels across different soil depths under the two types of soil samples. These differences are big among the non-successive soil depths, but they are small among the successive depths.
6- The performance assessment of different soil sampling designs in mapping soil Olsen-P spatial variability.
The performance of the applied sampling designs varied according to soil types (sandy or clay), soil sample types (simple or composite), and soil depth (surface layer or subsurface layer). In clay soils, the performance of applied sampling designs was better than in sandy soils, as well as at soil subsurface layers than at soil surface layers in both soil types.
The findings of the current study showed that the choice of a soil sampling design that can be used to create accurate spatial variability maps for soil Olsen-P depends mainly on the degree of variability present within the study area. In this context, whenever the soil spatial variability is high, a sampling design with a large number of samples is the best, and vice versa.
The outcomes of this study suggest that the number of soil samples should not be fewer than 12 per 0.5 ha to produce accurate maps of the soil phosphorus variability across soil depths of 0–40 cm in clay soils (old cultivated soils on the Nile banks), as well as not be fewer than 20 samples per 0.5 ha in sandy soils (newly reclaimed soils outside the Nile banks) at the same depth (0–40 cm).
Furthermore, this study represents a good attempt at providing information on comparing different sampling designs to facilitate choosing the optimal one for mapping the spatial variation patterns of phosphorus in soils of different textures across different depths under two soil sample types.
7- Impact of short-term cultivation on some selected properties of sandy soils
The results showed that the available phosphorus levels in the cultivated soil were higher than in the virgin soil by 16, 9, 8.5, and 6 times, with an increase in the organic matter content of 16.8, 12.4, 11.9, and 7.9 times at depths of 0-10. , 10-20, 20-30, and 30-40 cm, respectively.
Moreover, compared to virgin soil, cultivated soils showed decreases in salinity of -8.9%, -56.4%, -66.3%, and -71.8%, at depths of 0-10, 10-20, 20-30, 30 -40 cm, respectively.
Furthermore, some properties of the cultivated soil improved, especially in the topsoil layers, such as pH reduction, calcium carbonate decrease, and CEC increase, while the degree of soil texture remained unchanged.
Understanding the ongoing changes in soil physical and chemical properties within the cultivation process is required to maintain satisfactory soil quality and sustainable soil productivity. Therefore, continuous monitoring of the effects of different soil management strategies in the short term is recommended.