Spatial heterogeneity and vertical stratification of soil nutrients in northwestern Indian forests

Soil nutrient dynamics in forest ecosystems are governed by complex interactions among vegetation cover, pedogenic gradients, and landscape-scale heterogeneity, yet depth-explicit assessments across contrasting land covers remain scarce in semi-arid alluvial systems. This study presents the first regional-scale, depth-stratified (0-30, 30-60, and 60-90 cm) assessment of twelve soil physicochemical parameters across Non-Degraded Forest and Degraded Forest land covers in the forests of Punjab and Chandigarh, northwestern India. A total of 954 soil samples from 318 locations were analysed, and linear mixed-effects modelling (LMEM) was employed to partition variance among spatial hierarchy, soil depth, and land-cover type. Soil pH ranged from 6.82 (Dasuya) to 8.76 (Ferozepur), and organic carbon from 0.27% to 1.11%, with the highest values in sub-montane northern divisions. Variance partitioning revealed that spatial hierarchy (Division/Grid random effects) accounted for the largest share of total variance (29–75%), followed by soil depth, which significantly structured organic carbon (15.23%), nitrogen (7.54%), and several micronutrients; land-cover effects were comparatively minor (0.13–2.70%). Soil depth significantly influenced pH, organic carbon (OC), nitrogen (N), potassium (K), zinc (Zn), manganese (Mn), iron (Fe), copper (Cu), and boron (B) ( p < 0.001), while land cover significantly affected only pH and copper. Depth-stratified Pearson correlation analysis revealed strong pH-Fe ( r = -0.42 to -0.63) and pH-Mn ( r = -0.43 to -0.60) antagonisms across all profiles, confirming alkalinity-driven micronutrient immobilisation characteristic of calcareous soils. Micronutrient inter-linkages (Mn-Fe, Fe-Cu) were strongest in Non-Degraded Forest surface horizons and progressively weakened with depth, with Degraded Forest subsoils showing minimal nutrient coupling. These findings demonstrate that landscape-scale pedogenic heterogeneity and vertical biogeochemical gradients exert stronger control over nutrient distribution than categorical vegetation status alone, and underscore the need for depth-explicit soil monitoring frameworks and organic matter conservation strategies in degraded alluvial forest soils of northwestern India.