DIFFERENTIAL FERTILIZER N IMMOBILIZATION IN TWO TILLAGE SYSTEMS INFLUENCES GRAIN N CONCENTRATION

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Abstract
Differential N immobilization between conventional tillage (CT) and no-till (NT) systems has been suggested as a possible mechanism for differences in crop response to N. To examine this effect, immobilization of N fertilizer was compared in NT and CT cereal production systems at two sites in southern Alberta (Lethbridge and Vauxhall) from 1983 to 1985. Fertilizer N, labelled with 15N, was applied at rates ranging from 25 to 50 kg N ha−1 to winter wheat (Triticum aestivum L. 'Norstar'), barley (Hordeum vulgare L. 'Galt'), or spring wheat (Triticum aestivum L. 'Chester') in various rotations. Nitrogen immobilization was subsequently quantified by 15N analysis of crop tissues and soil N fractions. The amount of N immobilized was strongly influenced by climatic variables. In 1984, characterized by drought in the spring but significant precipitation during June, mean N immobilization losses for winter wheat and barley in the CT treatments averaged much less than those in NT treatments (20 vs. 34% of N applied). Conversely, in 1985, which was characterized by a wet spring and a very dry summer, N immobilization losses were comparable in the two barley tillage treatments (mean = 29% of N applied). Lack of precipitation early in the season appears to restrict immobilization less in CT than in NT treatments because of greater fertilizer-residue contact in the latter. Despite the trends in immobilized N, grain yields were often higher in the NT than in the CT treatments, indicating that yield differences were likely related to the availability of moisture rather than to N fertility. Restricted N availability resulting from immobilization losses, however, reduced grain N concentration in NT relative to CT treatments. Key words: No-till, conventional till, N immobilization, N response, residual N