50 years of continuous no-tillage, stubble retention and nitrogen fertilization enhanced macro-aggregate formation and stabilisation in a Vertisol

Abstract

Farm management practices such as no-tillage, stubble retention, and nitrogen (N) application have a substantial potential to enhance soil aggregation in agricultural soils. This is especially important for Vertisols, which may experience extended waterlogging due to low infiltration rates and low hydraulic conductivity. We evaluated the long-term impacts of the tillage method (no tillage, NT, and conventional tillage, CT), stubble management practices (stubble retention, SR, or stubble burning, SB) and N-fertilization on aggregate size distribution, and distribution of total C and N within different aggregate-size fractions in a Vertisol. Soil samples were collected from 0–0.1 m depth at the Hermitage Research Station in Queensland, Australia, after 50 years of management. We found that the soil under NT with SR and 90 kg N fertilizer application (NTSRN90) contained the highest proportion of large macro-aggregates (31.0 %), the greatest mean weight diameter (MWD) (1.79 mm), and the highest percentage of water stable aggregates (WSA, 90.2 %), whilst the lowest values (3.15 %, 0.71 mm and 72.7 %, respectively) were found in the CTSBN0 treatment. Therefore, reducing soil disturbance by primary tillage operations (NT) and the retention of crop residues (SR) and fertilizer application stimulated the formation of soil macro-aggregates from micro-aggregates. Retention of higher amounts of residues to the system in the SR and N fertilized treatments increased the macro-aggregate occluded particulate organic C (POC) and N (PON), which likely provided loci for aggregate formation and binding agents for stabilisation. Larger aggregates stored more organic C and N, and had lower water retention at saturation (−10 kPa), than smaller aggregates or the silt + clay fraction, thus having the potential to reduce the frequency of waterlogging in the soil under NT, SR, and N fertilization. Therefore, long-term practice of no-tillage along with stubble retention and N fertilisation could significantly enhance the macro-aggregation of fine-textured Vertisols, with this then ensuring the sustainability of the agricultural production system by creating favourable water retention and movement, and porosity for better management for crop production.