Abstract
The effect of growing three successive wheat ( Triticum aestivum L.) crops on the mineralization of 15N-labelled organic compounds adsorbed to different soil size fractions (sand and organic residues > 50 μm; silt 50-2 μm; coarse clay 2–0.2 μm and fine clay < 0.2 μm) was studied. Unplanted soils were used as controls. From the first to the third crop, plant biomass decreased; root-, stem- and leavesplus ears-biomass decreased respectively by 75.8, 74.0 and 39.0%. Compared to the unplanted soils, the cropped soils showed (1) an increase (37.0%) in the mineralization of 15N adsorbed to fine clay and (2) a transfer of 15N to silt and coarse clay (equivalent to gains of respectively 25.9 and 8.4%). The gains in these fractions are attributed to microbial transformation of easily-mineralizable molecules, linked to fine clay, to more humified compounds which became adsorbed to silt and coarse clay. Thus, the presence of a crop enhances the ability of the microflora to compete for nitrogen.
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