Effect of bio-tillage on the least limiting water range of clayey red soil

Abstract

Poor soil physical properties related to the least limiting water range (LLWR) limit the productivity of clayey red soil (Ultisol) under a subtropical monsoon climate in southern China. This study evaluated the effects of bio-tillage on LLWR and identified the key factors influencing LLWR through a field experiment. The treatments included no plant, two cultivars of oilseed rape (Brassica napus L. cv. Huashuang 4 and Brassica napus L. cv. Xinan 28), one-year-old and perennial lucerne (Medicago sativa L. cv. Ladino), and one-year-old and perennial vetiver (Vetiveria zizanioides L. cv. Wild), used as cover crops prior to summer maize. Key parameters measured included plant root morphological traits, and soil bulk density, field capacity (FC), wilting point (PWP), available water content (AWC), penetration resistance (PR) and air-filled porosity (AFP) were determined. The two rape cultivars exhibited the shallowest root distribution (limited to 20 cm depth) and the lowest root surface density (RSD, ∼16.61 cm²·cm⁻³) and root volume density (RVD, ∼0.58 cm³·cm⁻³). In contrast, lucerne and vetiver demonstrated greater root development, with deeper root penetration (>60 cm), and higher RSD and RVD, with vetiver showing the highest values (RSD ∼24.01 cm²·cm⁻³, RVD ∼0.96 cm³·cm⁻³). Lucerne and vetiver treatments increased AWC and AFP but reduced PR. Soil planted with vetiver had lower FC (0.35–0.48 cm³·cm⁻³) and PR (1362–3297 kPa) than soil planted with lucerne, while soil planted with lucerne had a lower PWP (0.25–0.35 cm³·cm⁻³) than soil planted with vetiver. All crops improved LLWR at 0–20 cm depth, but vetiver increased LLWR below the depth of 20 cm due to its higher root length density (RLD) and RSD. Path analysis revealed that PR had the strongest direct negative effect on LLWR (coefficients from −1.0528 to −1.7642), while redundancy analysis showed a strong correlation between LLWR and the RSD (12.00 %) and RLD (11.33 %) of perennial vetiver, with weaker correlation to root diameter (7.00 %). Bio-tillage reduced PR through root growth, enhancing LLWR particularly at depth of 20–40 cm, with perennial vetiver showing the most significant improvement due to its deeper rooting depth and denser root distribution.