Investigation of short-term effects of forage radish on water and solute transport through a tracer test and inverse modeling

The benefits of using cover crops for improving soil and water quality are well known. Less clear is whether cover crops, especially those forming a taproot system, can favor solute transport down to the groundwater by modifying soil hydraulic properties and solute dynamics. In this study, we employed 12 lysimeters to conduct a comparative analysis between a taproot cover crop, specifically forage radish (FR), and bare soil (BS), under three water table management conditions. Our objective was to evaluate whether the enhancement of root-derived macroporosity could have modified water and solute dynamics, and offset the benefits provided by FR that is commonly used to mitigate solute leaching. A tracer solution of bromide (Br-) was added to lysimeters, and solute flux concentrations were determined at different depths during a 25-day test. Soil moisture and pressure heads were monitored. Water and solute transport parameters were estimated by inverse modeling using HYDRUS-1D. A complementary laboratory experiment was performed to quantify the effect of FR root apparatus on the macropore structure by using noninvasive X-ray microtomography (mu CT). Results showed that the growth of FR within the lysimeters induced alterations in water and solute dynamics compared with BS. This is primarily attributed to its proficiency as solute scavenger, with an uptake capacity of up to 47% of the total injected tracer. Our comparative analysis instead revealed subtle differences in soil structure and hydraulic properties brought about by the presence of FR. Major changes were observed for the saturated hydraulic conductivity (Ks), which increased from an average of 8.4-49.8 cm day-1 within the 20-45 cm layer in BS and FR, respectively. Additionally, there was a difference in immobile water content (theta im), with the values in FR averaging 21% lower than those in BS. These modifications can be attributed to the formation of fissures and channels, primarily concentrated in the proximity of taproot development, without extending into deep preferential flow pathways. These structural changes were supported by the nondestructive mu CT analyses. Upon aggregating the effects observed, solute movement to groundwater was not affected by FR compared to BS conditions. Taproot served as solute scavenger, offsetting the risk of leaching at -60 cm, below its fleshy portion. Taproot-induced soil hydraulic changes were mainly observed in the 0-45 cm layer. Forage radish caused an increase in the macroporosity where the taproot mainly developed.