MICROBIAL BIOMASS DYNAMICS DURING THE DECOMPOSITION OF LEAF-LITTER OFPOPLAR AND EUCALYPTUS IN A SANDY LOAM

Soil microbiological properties during decomposition of leaf litter of poplar (Populus deltoides) and eucalyptus (Eucalyptus tereticornis) w ere studied under laboratory conditions. Microbial biomass C and ninhy drin-N were measured at different intervals up to 90 days following in corporation of poplar and eucalyptus leaves separately @ 20 and 100 t ha-1. In general, the net increase in total biomass C or ninhydrin N f ollowing amendment was larger in the soils which received poplar leave s than in the soils that received eucalyptus leaves. The amounts of bi omass C, at day 90, in the soils which received eucalyptus leaves @ 20 and 100 t ha-1 was about half and one-third, respectively, that of th e soils that received poplar leaves at the same rates. Similarly, the field soils naturally receiving eucalyptus leaf litter contained about half the amounts of biomass C or ninhydrin N of the soils that receiv ed poplar leaf litter. In contrast, the amounts of organic C and total N were more in soils which received eucalptus leaves both in the labo ratory experiment and under field conditions than in the soils that re ceived poplar leaves, indicating that the decomposition of eucalyptus leaves in soils was slower than that of poplar leaves. The ratio of bi omass C/soil organic C in soils receiving eucalptus leaves was about 2 -4 times lower than those in soils with no admendment or soils receivi ng poplar leaves. These results, therefore, suggest that the alleloche micals released into soil during decomposition of eucalyptus leaves ha d a toxic effect on soil microorganisms and may thus affect the nutrie nt cycling and hence soil fertility.