Rhizosphere-Scale Quantification of Hydraulic and Mechanical Properties of Soil Impacted by Root and Seed Exudates

Abstract

Using rhizosphere-scale physical measurements, we tested the hypothesis that plant exudates gel together soil particles and, on drying, enhance soil water repellency. Barley ( L. cv. Optic) and maize ( L. cv. Freya) root exudates were compared with chia ( L.) seed exudate, a commonly used root exudate analog. Sandy loam and clay loam soils were treated with root exudates at 0.46 and 4.6 mg exudate g dry soil and chia seed exudate at 0.046, 0.46, 0.92, 2.3 and 4.6 mg exudate g dry soil. Soil hardness and modulus of elasticity were measured at −10 kPa matric potential using a 3-mm-diameter spherical indenter. The water sorptivity and repellency index of air-dry soil were measured using a miniaturized infiltrometer device with a 1-mm tip radius. Soil hardness increased by 28% for barley root exudate, 62% for maize root exudate, and 86% for chia seed exudate at 4.6 mg g concentration in the sandy loam soil. For the clay loam soil, root exudates did not affect soil hardness, whereas chia seed exudate increased soil hardness by 48% at 4.6 mg g concentration. Soil water repellency increased by 48% for chia seed exudate and 23% for maize root exudate but not for barley root exudate at 4.6 mg g concentration in the sandy loam soil. For the clay loam soil, chia seed exudate increased water repellency by 45%, whereas root exudates did not affect water repellency at 4.6 mg g concentration. Water sorptivity and repellency were both correlated with hardness, presumably due to the combined influence of exudates on the hydrological and mechanical properties of the soils.