Rasse, D. P. 1997. Alfalfa and corn root modifications of soil nitrogen flux and retention. Dissertation, Michigan State University, East Lansing, Michigan, USA.
Sustainable nitrogen-conservative agroecosystems are increasingly sought by agronomists and environmentalists. This study was designed to evaluate the global hypothesis that temporal continuity of root systems within the soil profile diminishes nitrate leaching and builds easily mineralizable nitrogen pools within root-zone soils. Effects of alfalfa root systems on soil physical properties, soil nitrogen leaching and corn nutrition were investigated in two separate experiments conducted on Kalamazoo loam soils (fine-loamy, mixed, mesic Typic Hapludalf) in southwestern Michigan from 1994 to 1996. In the first field experiment, soil nitrogen dynamics were compared under alfalfa and bare fallow. The second field experiment was conducted in conventional and no tillage plots under corn – alfalfa rotation equipped with undisturbed, in situ, large monolith lysimeters. Measurements in this study included: 1) soluble soil mineral nitrogen from suction lysimeters, and fixed soil mineral nitrogen from soil core extractions, 2) volumetric soil water contents by time domain reflectometry, 3) root biomass extracted from deep probe samples, 4) root demographics by minirhizotron technologies, 5) soil physical measurements, and 6) plant biomass and yields. The presence of alfalfa root systems significantly increased saturated hydraulic conductivity, total, and macro-porosities of soils compared to bare fallow. Living alfalfa root systems efficiently prevented nitrate leaching throughout the year, by generally keeping soil solutions below 1 mg L-1 of NO3-N. Second year alfalfa stands accumulated 26 and 76 kg N ha-1 in the crowns and roots respectively. Decomposing alfalfa shoots had little impact on soil nitrate contents compared to root systems, but promoted greater nitrate leaching and denitrification rates. Mineral nitrogen released from spring spray-killed alfalfa and associated soil mineralization was estimated at 115 kg N ha-1 in late November 1996. All nitrogen fertilizer applied to corn, planted directly after spray-killed alfalfa, remained in the soil as mineral nitrogen which was vulnerable to spring leaching. Corn root densities per soil horizon, planted after alfalfa, mimicked alfalfa root demographics. An excess of 40% of the corn roots recolonized alfalfa root induced macropores.
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