Cover crop effects on soil physical quality under initial harvester traffic in a cotton cropping system

Machinery traffic–induced compaction has detrimental effects on soil structure and physical properties, leading to the degradation of soil physical functionality. However, the potential of cover crops to mitigate the deleterious effects of cotton harvester traffic in sandy soils remains poorly investigated. This study aimed to evaluate how different cover crops grown either as sole crops or in mixtures can mitigate the adverse effects of cotton harvester traffic on the physical quality of a tropical sandy loam Oxisol in southeastern Brazil. The experiment was established in 2015 using a randomised complete block design with five cover crop treatments: Ruzigrass ( Urochloa ruziziensis ); Millet ( Pennisetum glaucum ) + Ruzigrass; Millet + Sunn hemp ( Crotalaria juncea ); Mixture (Ruzigrass + millet + radish ( Raphanus sativus ) + sunn hemp); and a control treatment (Fallow - weed). The harvester passed only once over each experimental plot, which were subdivided into trafficked and non-trafficked zones. Undisturbed soil samples were collected from the 0.00–0.10 and 0.10–0.20 m soil layers. Soil bulk density and degree of compaction, total porosity, mesoporosity and macroporosity, plant-available water, saturated and unsaturated soil hydraulic conductivity, matrix and relative soil aeration capacity, soil water storage capacity, and an index of soil resistance to compaction were determined for both soil layers. The results showed that the first harvester traffic substantially degraded soil physical quality, particularly in the 0.00–0.10 m layer, with increases in soil bulk density of up to 49% and reductions of 35% in saturated soil hydraulic conductivity, 28% in mesoporosity, 44% in macroporosity and 55% in relative soil aeration capacity, compared with non-trafficked soil, irrespective of the cover crop. The soil resistance to compaction index indicated that 96% of the increase in the degree of compaction occurred after the first machine pass, with the treatment Fallow exhibiting a 97% increase. The Millet + Ruzigrass treatment exhibited the lowest soil bulk density (1.57 Mg m⁻³) and degree of compaction (84%), as well as the highest values of total porosity (0.41 m³ m⁻³), saturated soil hydraulic conductivity (8.58 cm h⁻¹) and plant-available water (0.20 m³ m⁻³) following harvester traffic. The Mixture treatment showed the highest unsaturated soil hydraulic conductivity at a matric potential of −100 hPa (0.149 cm day⁻¹). Overall, the results suggest that the adoption of cover crops, particularly the Millet + Ruzigrass and Mixture, represents an agronomically efficient strategy to mitigate the negative impacts of harvester traffic on the soil physical quality. Improvements in soil physical attributes promoted by cover crops are essential for maintaining soil physical functionality, ensuring greater soil water and air availability. • Cover crops reduced soil compaction caused by harvester traffic. • Increased soil compaction enhanced water storage capacity but impaired soil aeration. • Increased soil bulk density markedly decreased macroporosity and saturated hydraulic conductivity. • Soil bulk density increase due to harvester traffic in the 0.10–0.20 m corresponded to 50% of that observed in the 0-0.10 m.