Long-term crop residue management drives soil aggregation and carbon–nitrogen retention in a calcareous rice–wheat system of the eastern Indo-Gangetic Plains

Abstract

Rising food demand and intensive agricultural practices that reduce soil organic matter and soil structural stability threaten sustainability. Long-term crop residue management is considered a key strategy for restoring soil health and enhancing carbon sequestration in the calcareous soils of Indo-Gangetic Plains; however, its effects on aggregate-mediated carbon (C) and nitrogen (N) retention across soil depths remain poorly understood. This study evaluated the impact of 30 years of residue management on soil aggregation, aggregate-associated organic carbon and nitrogen, bulk soil properties, and soil organic carbon (SOC) and total nitrogen (TN) stocks in a rice–wheat cropping system (RWCS). Five treatments comprising uncultivated soil (UN), no residue addition (R 0 ), and 25% (R 1 ), 50% (R 2 ), and 100% (R 3 ) residue addition, respectively, were assessed across four soil depths (0–5, 5–15, 15–30, and 30–60 cm). Residue addition reduced bulk density and increased mean weight diameter, geometric mean diameter, macroaggregate proportion (Ma), and aggregate ratio, with the greatest improvements observed under R 3 . Aggregate-associated SOC and TN increased consistently with residue addition across all depths and aggregate size fractions. Higher Ma formation promoted the accumulation and physical protection of C and N, resulting in significant improvement in SOC and TN in bulk soil, stratification ratios, and enzyme activities. The total SOC stock increased from 34.25 Mg ha⁻¹ under R0 to 62.15 Mg ha⁻¹ under R 3 , while SOC sequestration rate increased from 0.11 to 1.04 Mg ha⁻¹ year⁻¹. Redundancy analysis revealed strong associations between crop residue input, aggregate stability, aggregate-associated C and N, and SOC and TN stocks. Partial least squares structural equation modeling demonstrated that residue-derived C inputs enhanced SOC and TN stocks primarily through better soil aggregation. These findings underscore long-term residue addition as a viable strategy for improving soil structure, C and N retention, and SOC sequestration in calcareous RWCS under comparable agro-climatic conditions.