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 mineralisation from organic matter. To reduce losses and to facilitate optimal 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 needed. Therefore, biomass and activity of bacteria, biomasses of fungi, bacterivorous amoebae, flagellates and nematodes, and in situ N mineralisation were monitored during a full year in a winter wheat field under conventional management (CONV) and integrated management (INT). Fungal biomass was about 100-fold lower than bacterial biomass. The average 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 attributed to the approximately 30% higher organic matter content of INT. Bacterial biomass and frequency of dividing-divided cells (FDDC) were relatively low in December and January, probably owing to temperatures just above 0 degrees C. At about 5 degrees C in February and March, relatively high FDDC values and a doubling of bacteria occurred. During 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 peak, whereas in CONV bacterivorous fauna were drastically reduced by soil fumigation. Nevertheless, the bacterial peaks were similar in CONV and INT, indicating that bacteria were not controlled by bacterivores. Nitrogen mineralisation was relatively low in winter. The increased bacterial growth in February and March, and in September appeared to enhance immobilisation rather than mineralisation of N. During the growing season from April to the end of August, bacterial growth was relatively low and N mineralisation was relatively high. This probably resulted from bacterivore feeding and from substrate- or nutrient-limited bacteria with a low growth efficiency. Considerable mineralisation rates after harvest confirmed the need for measures to stimulate immobilisation during periods without crop uptake.