MICROBIAL NUMBERS AND ACTIVITY IN DRIED AND REWETTED ARABLE SOIL UNDER INTEGRATED AND CONVENTIONAL MANAGEMENT

During an 8-week microplot experiment, effects of moisture regime and farm management on microbial numbers and activity were studied. Under integrated management (reduced input farming), bacterial numbers, O2 consumption and N mineralization, respectively, were 1.6, 2.1 and 1.8 times higher than under conventional management (high input farming). These differences may be attributed to 1.3 and 1.4 times higher contents of organic matter and total N in the integrated microplots. One month of drying from a water potential of -0.03 to -0.12 MPa, and subsequent rewetting to - 0.01 MPa, did not affect bacterial numbers significantly. However, the relatively small decrease in water potential caused a significant decrease in O2 consumption and N mineralization. After rewetting, respiration increased from 1.3 to 1.5 fold, and N mineralization from 3 to 5 fold. Concurrently, the frequency of dividing-divided cells (FDDC) increased from 10 to 16% in the conventional and to 23% in the integrated microplots. This suggests that the FDDC, which is determined by direct microscopy and requires no incubation, can be used as an index of in situ bacterial growth rate in soil. For marine bacteria, mathematical relationships have been established between specific growth rate (mu) and FDDC. If it is assumed that these relationships are also valid for soil bacteria, FDDCs of 16 and 23%, respectively, may indicate specific growth rates of about 1 and 2 day-1. Bacterial production rates based on FDDC (8.5-45 mu g C g-1 day-1) were 3-8 times higher than those based on O2 consumption rates determined by 2-week incubations. Uncertainties of the methods are discussed.