Plant Root Exudates Decrease Mobility of Smectite Colloids in Porous Media in Contrast to Humic Acid

Abstract

Plant root exudates are primarily composed of carbohydrates (CHs), amino acids (AAs), and organic acids (OAs). Little is known about how plant root exudates influence the stability and mobility of clay colloids in the soil profile. In this study, transport behaviors of K+–saturated smectite colloids dispersed in maize (Zea mays L.) artificial root exudate (ARE) solution, humic acid (HA) solution, and deionized water through water-saturated sand columns were investigated with solution ionic strengths of 0.1 and 10 mmol L−1 KCl and pH of 5, 7, and 9. Results showed that smectite colloids were more aggregated and transported less in ARE solution, followed by water and HA solution. This trend became less apparent with increasing pH but more pronounced with increasing ionic strength, suggesting that enhanced stability and mobility of HA-dispersed smectite colloids probably resulted from increased electrostatic and/or steric repulsions. The results for CH-, AA-, or OA-dispersed smectite colloids revealed that the AA fraction was primarily responsible for enhanced colloid retention because positively charged amine groups in AAs (especially lysine) might neutralize the negative surface charge of colloids and promote intersurface bridging. The residual colloids, after flushing with deionized water (thus eliminating secondary energy minima), decreased in the order of ARE-, water-, and HA-dispersed colloids, suggesting greater retention by primary energy minima and pore straining for the ARE-dispersed smectite colloids. Overall, in contrast to HA, plant root exudates decreased the stability and mobility of smectite colloids, thus facilitating the retention of clay colloids in root zones during water percolation events.