FIELD DECOMPOSITION OF LEAF LITTERS - EARTHWORM-MICROORGANISM INTERACTIONS - THE PLOWING-IN EFFECT

We describe a new decomposition model including the consequences of ea rthworm mechanical activity, especially the ploughing-in effect. In an experiment we tested this ploughing-in effect on the disappearance of leaf litter from four tree species (sessile oak, Quercus petraea L., holm oak, Quercus ilex L., sweet chestnut, Castanea sativa Mill and be ech, Fagus sylvatica L.) during two 2y exposure at the site of Anduze which contained the highest earthworm biomass. Our experiment, carried out with leaf litter placed in litterbags allowing (0.5 cm mesh size) or preventing (0.1 cm mesh size) the access of earthworms, confirms ( 1) the rates of litter decomposition observed in the first experiment and (2) the important effect of earthworms on litter decomposition;(fr om 20.5% to 30.2% depending on the litter). The breakdown of the diffe rent litters by soil fauna varied from 24.2% to 40.6% depending on the litters after 2y exposure in the field at Anduze and the litter decom position rate was increased by a mean factor of about 5.2. We describe a technique for detecting litter contamination attributed to earthwor m casts. This technique is based on the change of the litter total wei ght loss (TWL)-to-litter total C loss (TCL) ratios which varied from 0 .652 to 0.915 in the presence of earthworms and were close to 1 (from 0.952 to 0.995) in the absence of earthworms. We assumed that this lar ge change in these ratios indicated a mineral contamination of litters due to earthworm casting. This hypothesis was confirmed by the litter ash contents varying from 2.6% to 9.6% with earthworms and from 1.3% to 2.1% without earthworms only. These results point out the ecologica l importance of contamination of the litter by casts during decomposit ion. (C) 1998 Elsevier Science Ltd. All rights reserved.