PROTOZOA, NEMATODA AND LUMBRICIDAE IN THE RHIZOSPHERE OF HORDELYMUS-EUROPAEUS (POACEAE) - FAUNAL INTERACTIONS, RESPONSE OF MICROORGANISMS AND EFFECTS ON PLANT-GROWTH

Interactions among protozoa (mixed cultures of ciliates, flagellates a nd naked amoebae), bacteria-feeding nematodes (Pellioditis pellio Schn eider) and the endogeic earthworm species Aporrectodea caliginosa (Sav igny) were investigated in experimental chambers with soil from a beec hwood (Fagus sylvatica L.) on limestone. Experimental chambers were pl anted with the grass Hordelymus europaeus L. (Poaceae) and three compa rtments separated by 45-mu m mesh were established: rhizosphere, inter -mediate and non-rhizosphere. The experiment lasted for 16 weeks and t he following parameters were measured at the end of the experiment: sh oot and root mass of H. europaeus, carbon and nitrogen content in shoo ts and roots, density of ciliates, amoebae, flagellates and nematodes, microbial biomass (SIR), basal respiration, streptomycin sensitive re spiration, ammonium and nitrate contents, phosphate content of soil co mpartments. In addition, leaching of nutrients (nitrogen and phosphoru s) and leachate pH were measured at regular intervals in leachate obta ined from suction cups in the experimental chambers. Protozoa stimulat ed the recovery of nitrifying bacteria following defaunation (by chlor oform fumigation) and increased nitrogen losses as nitrate in leachate . In contrast, protozoa and nematodes reduced leaching of phosphate, a n effect ascribed to stimulation of microbial growth early in the expe riment. Earthworms strongly increased the amount of extractable minera l nitrogen whereas it was strongly reduced by protozoa and nematodes. Both protozoa and nematodes reduced the stimulatory effect of earthwor ms on nitrogen mineralization. Microbial biomass, basal respiration, a nd numbers of protozoa and nematodes increased in the vicinity of the root. Protozoa generally caused a decrease in microbial biomass wherea s nematodes and earthworms reduced microbial biomass only in the absen ce of protozoa. None of the animals studied significantly affected bas al respiration, but specific respiration of microorganisms (O-2 consum ption per unit biomass) was generally higher in animal treatments. The stimulatory effect of nematodes and earthworms, however, occurred onl y in the absence of protozoa. The sensitivity of respiration to strept omycin suggested that protozoa selectively grazed on bacterial biomass but the bacterial/fungal ratio appeared to be unaffected by grazing o f P. pellio. Earthworms reduced root biomass of H. europaeus, although shoot biomass remained unaffected, and concentrations of nitrogen in shoots and particularly in roots were strongly increased by earthworms . Both nematodes and protozoa increased plant biomass, particularly th at of roots. This increase in plant biomass was accompanied by a marke d decrease in nitrogen concentrations in roots and to a lesser extent in shouts. Generally, the effects of protozoa on plant growth consider ably exceeded those of nematodes. It is concluded that nematodes and p rotozoa stimulated plant growth by non-nutritional effects, whereas th e effects of earthworms were caused by an increase in nutrient supply to H. europaeus.