Simultaneous effects of plants and earthworms on mineralisation of N-15-labelled organic compounds adsorbed onto soil size fractions

The simultaneous impact of three successive crops of wheat (Triticum aestiv um L.) and of the earthworm (Lumbricus terrestris L.) on the mineralisation of N-15-labelled organic compounds adsorbed to different soil size fractio ns (sand and organic residues >50 mum; silt 50-2 mum; coarse clay 2-0.2 mum and fine clay <0.2 <mu>m) was studied under controlled conditions in the g reenhouse. Unplanted soils (UPS) were used as controls. In planted soils wi thout earthworm (PS) total plant biomass decreased with each cropping by up to 50%. However, in planted soils with earthworms (PES) the total plant bi omass loss was only 17%. This pattern was explained by the earthworm effect . Compared to the unplanted soils, the planted soils had an increased (mean +37%) mineralisation of N-15 adsorbed onto fine clays and a partial transf er of N-15 to Silt and coarse clay. The quantities of 15N mineralised and t ransferred were higher in the planted soils with earthworms, indicating an amplification of the phenomenon in the presence of earthworms. The simultan eous effect of the rhizosphere and the drilosphere did not lead to increase d mineralisation of N-15 adsorbed onto coarse clays and silts but instead a greater transfer of N associated with the fine fractions towards the coars er fractions.