MANAGEMENT-PRACTICES AND SOIL BIOTA

The soil biota consist of a large number and range of micro- and macro -organisms and are the living part of soils. They interact with each o ther and with plants, directly providing nutrition and other benefits. They regulate their own populations as well as those of incoming micr oorganisms by biological control mechanisms. Microorganisms are respon sible for organic matter decomposition and for the transformations of organically bound nitrogen and minerals to forms that are available to plants. Their physical structure and products contribute significantl y to soil structure. Management practices have a significant impact on micro- and macro-organism populations and activities. Stubble retenti on, an increasing trend in Australia, provides an energy source for gr owth and activity. Significant increases in the sizes and activities o f microbial biomass, including heterotrophic microorganisms, celluloly tic microorganisms, nitrogen-fixing bacteria and nitrifying and denitr ifying bacteria have been observed. In addition, increases in protozoa and meso- and macro-fauna have been seen. Stubble retention provides a means of maintaining or increasing organic matter levels in soils. T he way in which stubbles are managed may impact further on the activit ies of the soil biota and may alter the population balance, e.g. bacte rial:fungal ratios. In general, no-tillage results in a concentration of microorganisms closer to the soil surface and causes least disrupti on of soil structure compared with conventionally tilled soils. Some p lant diseases increase with stubble retention and with no-tillage, par ticularly where the next crop is susceptible to the same disease as th e previous crop. However, the general increase in microbial population s resulting from stubble retention can exclude pathogens through compe titive inhibition and predatory and parasitic activity. Cropping seque nces may be used to break disease cycles. Crop rotations that include legumes may provide additional nitrogen and stimulate mineralization p rocesses. Coupled with no-tillage in stubble retention systems is an i ncreased usage of herbicides to control weeds. Continued herbicide use has been shown to significantly depress some groups of microorganisms and some of their activities but, in Australia, little information is available about the effects of herbicides on microbial populations. A lthough we know that micro- and macro-organisms are vital in maintaini ng ecosystem function, our knowledge about them is still very limited. New techniques in molecular microbial ecology promise further advance s. Much more detailed information about the effects of specific manage ments on the size and activities of populations is needed. Soils and t heir processes are extremely complex and, in order to develop appropri ate management practices, integration of new and existing information is necessary. This is now being made possible through computer simulat ion modelling.