DYNAMICS OF MICROORGANISMS, MICROBIVORES AND NITROGEN MINERALIZATION IN WINTER-WHEAT FIELDS UNDER CONVENTIONAL AND INTEGRATED MANAGEMENT

To reduce environmental problems, integrated farming has been proposed , which may involve a considerable reduction of fertiliser-N input. A reduced fertiliser-N input must be compensated for by a higher N miner alisation from organic matter. To reduce losses and to facilitate opti mal use of the N mineralised for crop growth, knowledge of the effects of management on soil organisms and on their role in N cycling is nee ded. Therefore, biomass and activity of bacteria, biomasses of fungi, bacterivorous amoebae, flagellates and nematodes, and in situ N minera lisation were monitored during a full year in a winter wheat field und er conventional management (CONV) and integrated management (INT). Fun gal biomass was about 100-fold lower than bacterial biomass. The avera ge bacterial biomass was not significantly higher in INT than in CONV, whereas amoebae and nematodes were 64% and 22% higher, respectively. Average N mineralisation was 30% higher in INT. The differences are at tributed to the approximately 30% higher organic matter content of INT . Bacterial biomass and frequency of dividing-divided cells (FDDC) wer e relatively low in December and January, probably owing to temperatur es just above 0 degrees C. At about 5 degrees C in February and March, relatively high FDDC values and a doubling of bacteria occurred. Duri ng summer, FDDC values were relatively low and bacterial numbers were stable, probably owing to nutrient limitation. After harvest and skim ploughing, rapid increases in FDDC and bacteria were found. In the non -fumigated INT field, protozoan peaks coincided with the bacterial pea k, whereas in CONV bacterivorous fauna were drastically reduced by soi l fumigation. Nevertheless, the bacterial peaks were similar in CONV a nd INT, indicating that bacteria were not controlled by bacterivores. Nitrogen mineralisation was relatively low in winter. The increased ba cterial growth in February and March, and in September appeared to enh ance immobilisation rather than mineralisation of N. During the growin g season from April to the end of August, bacterial growth was relativ ely low and N mineralisation was relatively high. This probably result ed from bacterivore feeding and from substrate- or nutrient-limited ba cteria with a low growth efficiency. Considerable mineralisation rates after harvest confirmed the need for measures to stimulate immobilisa tion during periods without crop uptake.