Soil Physical Resilience to Compaction as Influenced by a Legacy of Different Organic Amendments in the Field

Soil organic carbon affects the response of agricultural soils to compaction, yet few studies assess the impact of contrasting organic amendments applied at different rates. A long-term field experiment with different rates of slurry (0, 10, 20 and 40 Mg ha−1) and compost (0, 35, 100 and 200 Mg ha−1) was used to create a soil organic carbon gradient. Loose soils were sieved, equilibrated to −5 kPa and subjected to rapid uniaxial compression followed by wetting/drying cycles to assess soil resistance and resilience. Soil organic carbon increased with the amendment rate across slurry and compost-amended soils. At equivalent organic carbon levels, slurry and compost-amended soils showed similar responses, showing the benefits of carbon amendments to decreasing compaction risks in the field. All compost treatments and the 40 Mg ha−1 slurry were more compressible than the control and lower slurry rates, with compression indices > 0.38 for soils of 100–200 Mg ha−1 compost (carbon content 55–69 g kg−1) and 0.35 for those receiving 40 Mg ha−1 slurry, compared with 0.30 in the control, 10 and 20 Mg ha−1 slurry (organic carbon content 35–45 g kg−1). Structural recovery, measured as recompression index, was greater for 100–200 Mg ha−1 compost (0.15) and 40 Mg ha−1 slurry (0.12) treatments than those with lower soil organic carbon contents. In these soils, the recovery of porosity after the second 200 kPa stress was only 3%–6% lower than after the first, indicating sustained resilience aided by wetting–drying cycles. Microporosity and saturated hydraulic conductivity increased with soil organic carbon. Soils with higher organic carbon content were more resistant to the initial 50 kPa stress, less resistant to a higher stress of 200 kPa, showed greater rebound and exhibited greater resistance to aggregate collapse than those with lower organic carbon contents.