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Crop and Soil Sciences Department, Plant and Soil Sciences Building, Michigan State University, East Lansing, MI 48824-1325 USA
rasse{at}astro.ulg.ac.be
Although alfalfa (Medicago sativa L.) stands have been reported to improve soil physical properties, little is known about the specific influences of above- and belowground alfalfa components on soil physical properties. A 2-yr study was conducted to investigate alfalfa root and shoot mulch modifications of soil physical properties and water movement in the root zone of a Kalamazoo loam soil (fine-loamy, mixed, mesic Typic Hapludalf) in southwest Michigan. Four treatments were considered: bare fallow (BF), bare fallow with alfalfa shoot mulch (BFSM), alfalfa with shoots removed and roots remaining (AR), and alfalfa with alfalfa shoot mulch (ASM). Volumetric soil water contents were measured by time domain reflectometry (TDR). Development of fine roots was monitored by minirhizotron technology. Alfalfa root systems increased saturated hydraulic conductivity (Ksat) by 57%, total and macroporosities by 1.7 and 1.8%, respectively, and water recharge rate of the soil profile by as much as 5.4% per day. These effects of alfalfa roots on soil porosity were mainly attributed to increased amplitudes of wetting and drying cycles and high rates of root turnover in the Ap horizon. Ksat was significantly correlated with macroporosity
. Mean weight diameter (MWD) of aggregates from bare fallow soils was 20% higher when alfalfa shoot mulch was applied. Our results suggest that aggregate stability was more affected by C sources from shoot mulch and root turnover than by factors specific to root activities such as physical enmeshment of aggregates and increased soil wetting and drying cycles.
Abbreviations: AR, alfalfa with shoots removed and roots remaining ASM, alfalfa with alfalfa shoot mulch BF, bare fallow BFSM, bare fallow with alfalfa shoot mulch Ksat, saturated hydraulic conductivity MWD, mean weight diameter MR, minirhizotron RIM, root-induced macropore TDR, time domain reflectometry
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