INPUT OF FERTILIZER-DERIVED LABELED-N TO SOIL ORGANIC-MATTER DURING A GROWING-SEASON OF MAIZE IN THE FIELD

Fertilizer N was applied as N-15-labelled ammonium nitrate to a maize crop grown under field conditions in north-western France. After labelled-N was supplied (in May), plant and soil samples (to 80 cm depth) were collected at the 10-leaf stage (in June), flowering (in August) and harvest (in October). At harvest, applied N was recovered quantitatively in the plant and soil system (100 +/- 6%): 71 +/- 4% in above-ground plant parts, 26 +/- 3% in the soil organic phase and 3 +/- 3% as residual fertilizer in the soil. From mid-May to late-June, microbial immobilization accounted to a large extent for fertilizer N input to soil organic matter (20 kgN ha-1). Recently-immobilized N in the topsoil (0-35 cm) was associated mainly with the clay particle-size fraction: 47, 32, 17 and 4% with fine clay, coarse clay, silt and fine sand fractions, respectively. From late-June to mid-August (when maize displays its maximum root growth rate) another 20 kg ha-1 of fertilizer N were incorporated as soil organic N. From mid-August to the end of the growing season in October, no significant variation in the amount of fertilizer-derived organic N in the soil was recorded. Pathways of in situ input of fertilizer N to soil organic matter were approached by comparing soil organic N labelling with C-13 natural labelling of the same soil samples by maize C. Particle-size fractions > 200-mu-m incorporated labelled-N mainly through maize underground biomass production. At harvest, 20% of the fertilizer-derived organic N present in the soil was located in these fractions as root material. Microbial immobilization of fertilizer N, associated with native C, contributed largely to the N-labelling of the < 50-mu-m fraction. Fertilizer-derived organic N incorporated into this fine fraction displayed a similar distribution, amongst clay and silt subfractions, within each of the three growth periods. Expressed on a whole-soil basis, fine clay < 0.2-mu-m and fractions > 200-mu-m were more enriched in recently-immobilized N than in total N, reflecting active associated soil organic matter.