Extraction of subsoil nitrogen by alfalfa, alfalfa-wheat, and perennial grass systems
2001
Entz, M.H. | Bullied, W.J. | Forster, D.A. | Gulden, R. | Vessey, J.K.
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.
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