Compacted soil adaptability of Brassica napus driven by root mechanical traits

Soil compaction due to mechanized farming operations is a recurrent issue affecting crop growth and yield. Yet, how soil compaction affects plant functions and ecological strategies is poorly known. With Brassica napus, i.e. a widespread crop species as study object, we aim to understand (i) how soil compaction impacts root and shoot traits related to the plant's well-being, nutrient acquisition of Brassica napus with different mechanical robustness, as well as their trade-offs, and (ii) how such impacts vary among different cultivars. To do this, we cultivated six cultivars of Brassica napus in non-compacted (control) and compacted (treatment) soils, respectively, in a sand culture system. After harvesting, a series of mechanical, morphological and chemical traits of roots and/or shoots were measured. Results showed that soil compaction significantly limited root penetration depth and root system establishment in morphological traits, leading further to significant reduction in nutrients acquisition and plant biomass accumulation. However, soil compaction significantly increases the average root diameter and root/shoot ratio, and facilitate more root exudates secretion (e.g. organic acids and polysaccharides) of Brassica napus cultivars. The Brassica napus cultivars with large root mechanical traits (e.g. root tensile force, root tensile strength and modulus of elasticity) had higher root cellulose and lignin concentrations and showed a stronger response in maximum root depth and specific root length compared with Brassica napus cultivars with small root mechanical traits in compacted treatment, which resulted in the greater fine root length and more root exudates secretion at root-soil interface. Furthermore, deep rooting enhanced nutrients acquisition and further biomass accumulation in compacted soil. Totally, the Brassica napus cultivars with large root mechanical traits with more fine roots and root exudates were critical for Brassica napus root penetration into a deep soil layer in compacted soil.