Soybean root elongation in relation to water and mechanical stresses in intact and disturbed structures of an Oxisol

Process-based root growth simulation models detail soil-root interactions, but do not incorporate the role of soil structure dynamics. Our objective was to parameterize the soybean root elongation under mechanical and hydric stress levels for soybean roots grown in soil with preserved (Field) and packed structure (Packed).

In a long-term field experiment under the no-tillage system, soil samples from an Oxisol with preserved and non-preserved structures were used to evaluate soybean root growth responses. Soil samples with non-preserved structure were reconstructed with under six bulk densities (1.02; 1.10; 1.18; 1.26; 1.34; 1.42 Mg m^-3) considering the field range of degrees of compactness and used for soybean cultivation. Soil penetration resistance (mechanical stress), soil water content, soil porosity and degree of soil water saturation (hydric stress) were evaluated. Root elongation rates of soybean seedlings were measured. The soybean relative root elongation was the response variable used in the model, assessed by combining the effects of water and mechanical stress on root elongation, allowing for a qualitative comparison of soil structures (Field and Packed).

Field structure exhibited superior performance under conditions of high physical constraints, particularly in scenarios of elevated soil penetration resistance. An increase in soil penetration resistance from 1 MPa to 3.5 MPa resulted in a 52% reduction in relative root elongation under Field structure conditions, whereas in packed samples this reduction was more pronounced, reaching 74%. Under conditions of low soil penetration resistance (0.5 MPa) and a water saturation degree of 60%, relative soybean root elongation in packed samples was 29% higher than in field structure. However, the relative elongation between both structural conditions became similar as soil penetration resistance increased to 1.3 MPa. Furthermore, under near-saturation conditions (i.e., 90%), soybean root elongation in field structure was nearly three times higher than that estimated in packed samples.

Soil structure drives biophysical soil-root interactions, mitigating mechanical (soil penetration resistance) and hydric (degree of saturation) stresses on soybean root elongation under no-tillage, confirming our hypothesis via a semi-empirical model. Field structure under no-tillage facilitates root elongation through pore network, enhancing tolerance to water excess and deficit versus packed conditions. Degree of saturation provides practical water stress assessment, linked to soil penetration resistance via Busscher equation, capturing non-linear interactions for soybean root elongation responses to soil physical stresses. Incorporating soil structure effects into simulation models enhances root growth prediction, supporting effective agricultural practices under soil conservation management systems.