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Résumé :
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To date, most of the soil compaction research conducted in Morocco has focused on cash crops (sugarbeets and sugar cane) which are grown in rotation with wheat, the staple crop and the second most-cultivated cereal in the country. Information on the effect of soil tillage, and the existing subsurface compaction, on wheat production in irrigated areas is inadequate. The objective of this study was to investigate the effect of subsurface compaction on the nitrogen (N) uptake, growth and yield of wheat under field conditions. A four-year study was conducted in two locations : 1980 and 1981 in Minnesota, and 1982 and 1983 in Morocco. Soil compaction was artificially created at different depths : 5, 10, 15, and 25 cm. Two wheat genotypes were used : Olaf in Minnesota, and Nesma 149 in Morocco. The effect of compaction on wheat was measured via several intermediary factors. (1) soil physical properties, (2) root growth characteristics, (3) nitrogen uptake, (4) shoot growth, and (5) yield and its components. The results indicated that artificial compaction created a subsurface compacted horizon with higher soil strength and higher mechanical impedance and reduced pore space. The increase in mechanical impedance restricted root growth in terms of where and how far they grew and expanded in the soil profile. However, wherever roots were, their ability to take up N was not affected by compaction. The effect of compacting was a decrease of 7 to 23% in grain yield and 10 to 27% in nitrogen accumulated in the total biomass at maturity. The decrease was attributed to a reduction in the number of shoots per unit area, and not to a decrease in individual shoots. The reduction in the number of shoots might be attributed to a reduction in the amount of available soil N to the roots. Such a reduction could be due to limitations in the ability of the roots to explore larger soil volumes to intercept soil N resources, and/or a restriction of NO(,3-) movement to the roots.
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