Field-scale soil quality indicators and the determination and evaluation of the spatial variability of soil quality

In this study, the physical, chemical, and biochemical properties of soils in a small field-scale (12 ha) area in the Harran Plain were determined, their spatial distributions were modeled, predictions were made for unsampled points, a minimum data set was created for soil quality assessments, and soil quality was calculated and spatial variations were mapped. For this purpose, disturbed and undisturbed soil samples were collected from 54 different points at surface (0-10 cm) and subsurface (10-20 cm) depths in the field. The results showed that the aggregate stability of the soils was low and the bulk density was 1.28 g cm-3 in the surface and 1.34 g cm-3 in the subsoil. Total porosity ranged from 48.4% to 68.7%, water-filled pore volume from 52.5% to 83%, and macro/micro pore volume from 0.17 to 0.37 cm3 cm¯3. Soil pH ranged from 7.44 to 7.99 in the surface and 7.44 to 7.91 in the subsoil, and electrical conductivity ranged from 1.63 dS m-1 to 1.54 dS m-1 in the surface and 1.54 dS m-1 in the subsoil. The average organic matter content was 1.35% and the lime content was 31%. Available phosphorus ranged from 4.44 to 13.90 mg kg-1 in the surface and 3.04 to 11.42 mg kg-1 in the subsoil, exchangeable potassium ranged from 351.2 to 631.9 mg kg-1 in the surface and 83.1 to 605.6 mg kg-1 in the subsoil. The low organic matter content negatively affected aggregate stability and water holding capacity. Great variations were detected in soil enzyme activities of beta-glucosidase, alkaline phosphatase, and urease activities within the study area. Microbial biomass carbon showed a homogeneous distribution at 0-10 cm and 10-20 cm depths. Mineral nitrogen content ranged from 105.9 to 937.0 mg kg-1 and 100.5 to 778.8 mg kg-1 in 0-10 cm and 10-20 cm soil depths, respectively. Potentially mineralizable nitrogen (PMN) ranged from 6.36 g kg-1 to 56.22 g kg-1 in 0-10 cm soil depth and 6.03 g kg-1 to 46.73 g kg-1 in 10-20 cm soil depth. Soil quality indices revealed that the soils were functioning only half of their potential. Spatial modeling data showed that PMN had strong spatial dependence at both depths, but was influenced by other soil properties. It was determined that PMN values increased with increasing soil organic matter content, soils with high microbial activity had higher PMN values, and sandy soils generally had lower PMN values. Semivariogram models of PMN at 0-10 cm and 10-20 cm depths were well-described by spherical or exponential models, and nugget/sill ratios were determined as 15.9% and 2.7%, respectively. Areas with high and low PMN were observed at both depths, and PMN showed consistent variability over short distances at both depths. These findings highlight that PMN is an important indicator for evaluating nitrogen dynamics and plant nutrient availability in soil. The findings generally revealed that the variability of soil properties, even at a small field scale, leads to significant variability in soil quality. The findings obtained within the scope of this study are extremely useful in terms of evaluating the effect of agricultural practices to be applied in the field in the following years on soil quality.