Effect of integrated nutrient management practice on soil aggregate properties, its stability and aggregate-associated carbon content in an intensive rice–wheat system

• Organic substitution of N improved aggregate properties and size distribution.
• Kind and source of organic inputs strongly influence aggregation and C accumulation.
• Higher C content in microaggregates-in-macroaggregates implies potential of C stabilization in rice–wheat system through organics.
• Crop residues incorporation emerges as viable option for improving soil structural stability and soil C.

We studied the impact of integrated nutrient management practices on the physical properties and structural stability of soil aggregates, and the associated C contents after 18 years of rice–wheat rotation on a sandy loam soil at Project Directorate for Farming Systems Research, Modipuram. Treatments included fertilizer nutrients (NPK), NPK with Zn and/or S; and partial (25%) substitution of N with farmyard manure (FYM), sulphitation press mud (SPM), green gram residue (GR) or rice/wheat residue (CR) in various combinations. Soil aggregate properties and its stability, aggregate associated and particulate fractions of C at 0–7.5, 7.5–15 and 15–30 cm depths were studied to document C sequestration potential of different nutrient management options. The aggregate strength and density were lower with organic substitution (p < 0.05) while water retention by aggregates at field capacity was 2–4% higher with organic inputs. Macroaggregates (>0.25 mm) constituted 58–92% of water stable aggregates and varied significantly among treatments and soil depths. Organic material incorporation improved soil aggregation and structural stability and resulted in higher C content in macroaggregates. The strong linear positive response to C additions indicated C sequestration potential in soils, with preferential location in macroaggregates. However, the kind and source of organic inputs strongly influenced both the soil aggregation and C accumulation in aggregates. A combination of GR in rice and FYM in wheat significantly improved C content in macroaggregates, and residue incorporation was beneficial compared to 100% N application through inorganic fertilizer or GR to rice. Coarse particulate organic matter (cPOM, >0.25 mm) accounted most of the increase in C content within macroaggregates and was substantially higher with CR incorporation. A relatively higher C content in microaggregates-within-macroaggregates (isolated following Six et al., 2002a) in organic-amended soil implies potential in bringing higher C stabilization in intensive rice–wheat system through combination of inorganic and organic fertilizers and crop residues.