Nutrient-Specific Spatial Dependence and Heterogeneity of Cultivated Soils in an Arid Irrigated Agroecosystem

Spatially explicit knowledge of soil nutrient heterogeneity in arid irrigated agroecosystems remains limited, constraining precision fertilization. In Wuwei City (Hexi Corridor, northwestern China), nutrient management has largely relied on coarse regional averages, while validated geostatistical characterization of cultivated soils is lacking. This study aimed to quantify the variability, interrelationships, and spatial dependence of four key plough-layer nutrients, soil organic matter (SOM), total nitrogen (TN), available phosphorus (AP), and available potassium (AK), across 638 cultivated-land sites sampled in 2022. Descriptive statistics, Pearson correlation, semivariogram modelling, and ordinary kriging with independent validation were conducted to characterize nutrient structure and predictive performance. All nutrients exhibited substantial variability (CV 44.8-97.1%), with AK showing the highest dispersion and weakest spatial continuity. SOM and TN were strongly correlated (r = 0.954), indicating near-collinearity and shared regulation of organic matter. Nugget-to-sill ratios (0.559-0.734) indicated predominantly moderate spatial dependence, while AP exhibited a correlation range of approximately 90 km, reflecting regional-scale gradients superimposed on local management effects. These results demonstrate nutrient-specific spatial structures within the same agroecosystem and underscore the limitations of uniform fertilization practices. Spatially differentiated nutrient management, particularly for K, is recommended, and integration of environmental covariates is needed to enhance predictive precision.