Long-term nutrient inputs and soil quality dynamics in vertisol-dominated rice–wheat systems

This study evaluates the long-term effects of fertilizer management combined with organic matter amendments on soil physical, chemical, and nutrient attributes in a rice–wheat system cultivated on Vertisols. Across treatments, bulk density ranged between 1.32 and 1.45 Mg m−3 and particle density between 2.35 and 2.39 Mg m−3, with no consistent shifts due to prolonged management. The FYM-inclusive regimen (YT 6/2.5 t ha−1 + FYM; designated as T5) consistently produced the lowest bulk density, followed by treatments T3 (YT 5/2 t ha−1), T2 (GRD), and T4 (YT 6/2.5 t ha−1), whereas the control (T1) exhibited the highest bulk density. Incorporating FYM with inorganic fertilizers reduced bulk density, aligning with known improvements in soil structure and porosity. Particle density remained largely unchanged across organic and inorganic inputs, indicating that basic soil mineralogy was not markedly altered by the treatments. Soil porosity followed a parallel trend, with organic-amended plots especially those including FYM exhibiting the highest porosity and the control the lowest. Overall porosity increased with organic matter additions and varied fertilizer regimes, reflecting enhanced SOM, aggregation, and shifts in pore-size distribution. Infiltration rates ranged from 0.38 to 0.67 mm h−1, with the FYM-inclusive treatment (T5) achieving the highest rate significantly above others; the control recorded the lowest. The rise in infiltration under integrated organic–inorganic inputs is consistent with prior evidence. Water holding capacity (WHC) improved with elevated fertilizer rates, with WHC in kharif from 34.98% to 41.92% and in rabi from 34.88% to 41.57%, and the FYM-inclusive regime (T5) yielding the greatest WHC. Soil pH (7.73–8.03) and electrical conductivity (0.13–0.18 dS m−1) showed no consistent treatment effects, while cation exchange capacity (38.88–41.95 cmol(+) kg−1) was highest under FYM-inclusive management, reflecting increased exchange sites from higher organic matter. Soil organic carbon increased significantly with both inorganic fertilizer alone and in combination with FYM, with the highest SOC pre-rice 0.60% and post-wheat 0.62% under T5, and the lowest under control. Available macronutrients (N, P, K) were greatest under FYM-inclusive regimes and remained superior to the control, indicating sustained nutrient supply; micronutrients followed a similar pattern, with the strongest pre-crop levels under FYM-inclusive management, though treatment differences were moderate overall. Collectively, integration of FYM with inorganic inputs enhances soil physical structure, WHC, CEC, SOC, and nutrient pools without altering intrinsic particle density or baseline pH and EC, underscoring the value of integrated nutrient management for sustained soil fertility and potential yield gains in rice–wheat rotations on Vertisols.