Evolution of root biomass doubly labelled (¹⁴C, ¹⁵N) in a plant-soil system: study on an annual cycle of maize cultivation

the aspects of the evolution of ¹⁴C and ¹⁵N labelled root residues of maize, in the soil-plant system, are studied by means of field and laboratory experiments and isotopic techniques. These methods permit the quantification of the nitrogen fluxes between the different compartments of the plant-soil system. During the cultivation period of maize, or a brown calcareous alluvial soil, 55 % of the nitrogen of the fertilizer is exported to the above ground part of the plant. The proportion of the nitrogen of added present in the roots is quite similar to the part immobilized by microbial activity (15 %). Only a part of the remaining nitrogen in eliminated out of the profile with draining water, therefore the fertilizer is lost neither by this way nor by volatilization or denitrification. Any loss of nitrogen after cropping comes from the mineralization - organization of added nitrogen: 64 % of the nitrogen immobilized in the roots is drained after mineralization. The root biomass induces an increase of the microorganism populations. These experiments showed a very interesting result: in a soil under a culture, the nitrogen from residual roots is preferentially used by the young roots and its translocation to the green parts is negligible. Two hypotheses are pointed out in order to explain this result: 1) the presence in the roots of the new maize of microorganisms (endomycorrhizas) which first consume the root debris; 2) the absorption by the roots of small organics compounds, which are stored in their structures. (author)