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.