PLANT-GROWTH AND THE MINERALIZATION OF ADSORBED C-14-LABELED AND N-15-LABELED ORGANIC-COMPOUNDS

The effect of wheat growth (Triticum aestivum L.) on the mineralization of C-14- and N-15-labelled organic compounds adsorbed on different soil size fractions (silt, 50-2-mu-m; coarse clay, 2-0.2-mu-m; fine clay, < 0.2-mu-m) was studied during 74 days of plant growth. Unplanted soils amended with the same soil size fractions were used as controls. The distribution of C-14 and N-15 in microbial biomass, plant and soil was determined in planted and unplanted soils. The mineralization of C-14 and N-15 from the soil size fractions and the decomposition rates constant (k) were always higher in planted soils and occurred in the following order: fine clay > silt > coarse clay. In planted soils, the microbial biomass-total C, -total N, -C-14 and -N-15 were related (1) to the wheat phenological stage (maximum at earing time) and (2) to the nature of the adsorbed organic matter. The planted soils amended with fine clay were always the most active. Compared to the unplanted fine clay-soil, the increase of mineralization of adsorbed C-14 and N-15 were respectively 5.5 and 6.8% more. 68% of this C-14 excess were released as (CO2)-C-14 and 32% were incorporated in microbial biomass. In the same way, 66% of this N-15 excess were assimilated by plants and 34% incorporated in microbial biomass.