Soybean root traits after 24 years of different soil tillage and mineral phosphorus fertilization management

International audienceLegume crops are widely used in conservation agricultural systems, which are associated with minimum soil tillage, due to their nitrogen-fixing capabilities. However, tillage and fertilization regimes may affect the vertical distribution of legume roots and root traits, hence nutrient and water uptake by altering soil properties in the long term. This study aimed to investigate how tillage and P fertilization affect soybean (Glycine max, L.) root distribution and morphology in a long-term experiment. A 24-year rain-fed corn-soybean rotation was established in 1992 on a clay loam soil in L’Acadie, Quebec, Canada. The split-plot design (four replicates) comprised tillage systems [moldboard plough (MP) and no-till (NT)] as main plot factors and P fertilization [0 (0 P), 17.5 (0.5 P) and 35 (1 P) kg P ha−1 every two years during the corn (Zea mays L.) phase] as sub-plot factors. Soybean roots and shoots were sampled in 2015, after 24 years, at flowering stage. Root samples were taken by collecting 5.25-cm diameter cores to a depth of 60 cm at 5 cm, 15 cm and 25 cm perpendicular to crop row. Soil cores were cut into 0–5, 5–10, 10–20, 20–30, 30–40 and 40–60 cm layers. After washing and separating the soil and roots, root traits (biomass, length, surface and diameter, and the proportions of primary, secondary and tertiary roots) were quantified using the WinRHIZO software. Tillage and P fertilization regimes showed no significant effect on soybean root traits. Roots under NT had a relatively higher root length density (RLD) of 1.95 cm cm−3 for a 60-cm soil profile compared to roots under MP (1.55 cm cm−3); RLD was relatively smaller at the highest P rate (1.57 cm cm−3) compared to the control and half rate treatment (1.82 and 1.86 cm cm−3, respectively). However, the interaction between tillage and P fertilization significantly influenced the vertical distribution of soybean roots. Roots under NT primarily accumulated at 0–10 cm, containing 44% of the total root length (24% under MP); by contrast, 36% of root length under MP and 21% under NT accumulated at 10–20 cm. However, the difference in vertical root distribution between NT and MP was mitigated as P fertilization increased. Soybean roots under NT showed higher RLD and greater root accumulation in the upper layers than MP possibly in response to nutrient availability and stratifications with higher nutrient contents in the top layers (0–10 cm) after 24 years of continuous NT practice