Long-Term Effects of Conservation Agriculture Practices on Soil Function: Evidence from a Nine-Year Field Experiment in the UK

The intensification of agricultural practices to maximise crop yields has often led to long-term soil degradation, characterised by reduced biological activity, disrupted soil structure, and nutrient depletion. Conservation agriculture (CA), comprising minimal soil disturbance, permanent soil cover, and crop rotation, offers a potential sustainable alternative for improving soil health. Although CA has shown promise in improving soil structure, organic matter content, and microbial activity in various regions, its long-term performance in temperate climates remains underexplored. Using a split-plot field trial established in 2014 on a sandy-loam soil at Sutton Bonington, UK, this study examines the effects of CA practices in years 7–9 after establishment (September 2020 to August 2023), specifically zero tillage and minimum tillage versus conventional ploughing, including with and without the effect of crop residue retention on the physical, chemical, and biological properties of soil. Soil assessments included conventional analyses and X-ray Computed Tomography (CT) to evaluate soil porous architecture. Averaged across the study period, surface (0–5 cm) bulk density was higher under zero tillage than under the tilled treatments. However, bulk density under zero tillage declined over time (significant tillage × sampling date interaction, p = 0.002), and by the 2022–2023 season, it was significantly lower than under tillage (1.02 vs. 1.03 and 1.08 g cm−3 under minimum tillage and ploughing, respectively). In residue-removed plots, X-ray CT showed greater pore connectivity under zero tillage, but no increase in CT-derived macroporosity. Zero tillage also increased earthworm populations (178 vs. 115 and 105 m−2), surface soil organic matter (5.8 vs. 5.6 and 4.9%), and, by 2022–23, soil organic carbon, while microbial biomass carbon was higher under both zero and minimum tillage than under ploughing (33.3 and 32.2 vs. 23.4 mg C kg−1). Residue retention had no significant effect on any measured property. These findings suggest that CA practices can enhance soil bio-physical properties in temperate environments, with the most notable benefits emerging after long-term adoption.