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SOIL MANAGEMENT

Extraction of Subsoil Nitrogen by Alfalfa, Alfalfa–Wheat, and Perennial Grass Systems

Dep. of Plant Sci., Univ. of Manitoba, Winnipeg, MB, Canada R3T 2N2

Corresponding author (m_entz{at}umanitoba.ca)

Received for publication July 14, 2000. The role of alfalfa (Medicago sativa L.) in extracting NO₃–N from deep soils of areas with cold, short growing seasons, such as western Canada, is not well understood. A study was established in 1990 to determine NO₃–N extraction ability to 300 cm; initial soil NO₃–N concentrations were high (>8 mg kg^-1). Systems included continuous alfalfa; annual rotations of spring wheat (Triticum aestivum L.), field pea (Pisum sativum L.), and barley (Hordeum vulgare L.); a native-grass system [big bluestem (Andropogon gerardi Vitman) and western wheatgrass (Agropyron smithii Rydb.)]; and continuous fallow. The annual rotation effectively lowered NO₃–N to <2.3 mg kg^-1 in the 30- to 90-cm depth. By the 4th yr, alfalfa had reduced NO₃–N concentrations to <3.8 mg kg^-1 for the 30- to 240-cm increment. The greatest NO₃–N extraction benefits of alfalfa were realized in the 4th yr at a maximum soil depth of 270 cm. Subsoil NO₃–N concentration increased in the continuous alfalfa between the 4th and 6th yr. Greater NO₃–N extraction occurred with the native-grass treatment compared with continuous alfalfa in the 0- to 120-cm soil depth; however, similar extraction patterns existed below 120 cm. A system involving 4 yr of alfalfa followed by two wheat crops resulted in the lowest subsoil NO₃–N concentration, even lower than the continuous alfalfa and native-grass systems. It was concluded that subsoil NO₃–N extraction with alfalfa was maximized when alfalfa was rotated with annual crops.

Abbreviations: AA-WWWW, wheat following 2 yr of alfalfa • AAAA-WW, wheat following 4 yr of alfalfa

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