MINERALIZATION OF N-15-LABELED ORGANIC-COMPOUNDS ADSORBED ON SOIL SIZE FRACTIONS - EFFECT OF SUCCESSIVE WHEAT CROPPING

The effect of growing three successive wheat (Triticum aestivum L.) crops on the mineralization of N-15-labelled organic compounds adsorbed to different soil size fractions (sand and organic residues > 50-mu-m; silt 50-2-mu-m; coarse clay 2-0.2-mu-m and fine clay < 0.2-mu-m) was studied. Unplanted soils were used as controls. From the first to the third crop, plant biomass decreased; root-, stem- and leaves-plus 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 N-15 adsorbed to fine clay and (2) a transfer of N-15 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.