In the previous section, the influence of soil disturbance on soil organisms was discussed and illustrated. Here, these ideas are explored again in terms of the effects of disturbance on five key processes that involve soil organisms: soil aggregation, degradation of organic matter, nitrogen cycling, nodulation of legumes and control of plant disease.
Some soil biological processes are carried out by groups of organisms (e.g. soil aggregation) and others by individual organisms (e.g. nodulation of a legume). The five following examples demonstrate how an understanding of the influence of disturbance on individual organisms can be used to gain insight into biological processes. The soil conditions at a point can be altered either by each organism or by the combined activities of several organisms. These small-scale changes in soil conditions create micro-sites in which very different microbial processes can occur near one another even when the environmental conditions required for each process are not the same.
The addition of organic matter to a soil can be considered a form of disturbance and it can change soil aggregation. This is because during the breakdown of organic matter, microbial organisms produce polysaccharides that can increase soil particle aggregation.
The effects of adding soil organic matter on soil aggregation were discussed previously (Tisdall and Oades 1980). The addition of glucose, ryegrass and purified cellulose each had different effects on soil aggregate stability. Glucose, which is readily available to soil organisms, rapidly increased aggregation, however this was not sustained. Addition of ryegrass increased soil aggregation more slowly, reflecting the longer time that the organic molecules in this form of organic matter take to be degraded. Pure cellulose, molecules that are more resistant to degradation than many molecules in ryegrass, had the slowest effect on soil aggregation but it had the longest-lasting effect.
Addition of organic matter also affects earthworm activity that may further contribute to soil aggregation. Earthworms ingest organic matter and soil and the result is an increase in soil aggregate stability. In the following example, the more organic matter that was added in the presence and absence of earthworms, the greater was the increase in soil aggregation (Degens 1997).
These two examples illustrate how disturbance of the soil environment by addition of different types and quantities of organic matter sets up a chain of events that enhances soil aggregation.
How can changes in soil moisture affect the rates of ammonification, nitrification and denitrification?
Soil organic matter is sometimes exposed to extreme climatic disturbances such as repeated cycles of wetting and drying or freezing and thawing. These cycles, which primarily affect soil moisture, can influence the rate of decomposition, and therefore have flow on effects for other biological processes and soil communities.
Changes in the quantity of water in soil pores dictate the types of organisms that are likely to be active. When soil pores are filled with water, conditions are anaerobic and denitrification is the dominant biological process. In contrast, when there is less soil pore water, but the soil is still moist, conditions are aerobic and ammonification and nitrification are the dominant processes (Linn and Doran 1984).
How does repeated freezing and thawing accelerate decay of forest leaf litter? Taylor and Parkinson (1988) posed the following hypotheses that were then tested in experiments: (i) decomposition of leaves repeatedly frozen and thawed should be faster than for leaves only frozen once, (ii) leaves subjected to the freezing and thawing cycles should suffer structural damage, allowing more rapid uptake of water and leaching, and (iii) leaves of different tree species respond the same way to freezing and thawing.
These two examples illustrate how water availability changes conditions in soil and alters rates of microbial processes involved in nutrient cycling.
The addition of some pesticides to soil can influence the rates of ammonification and nitrification. However, the magnitude of the impact of a pesticide on these processes depends on the extent to which organisms are directly or indirectly affected. For example, the application of a pesticide to an orchard inhibited nitrification without affecting ammonification (Helweg 1988). This initially resulted in the accumulation of ammonium that would otherwise have been converted to nitrate if the activity of the nitrifying bacteria had not been reduced. This difference in effects of pesticides may occur because the processes of ammonification and nitrification that usually occur together involve different groups of soil organisms. In this case only the nitrifying bacteria were affected. Over time however, the activity of the nitrifying bacteria increases, resulting in near normal levels of nitrate in the soil and a corresponding decline in ammonium.
In another study, the long-term application of herbicides to agricultural soils had little effect on either ammonification or nitrification (Hart and Brookes 1996). In this study, five pesticides (including fungicides and herbicides) were applied to separate plots of spring barley over a 19 year period. Ammonification and nitrification were largely unaffected by the application of any of the pesticides. It is possible that organisms adapted to the presence of the pesticides over this period and that the microbial community is different to what it was at the start of the experiment 19 years previously. Such changes are difficult to measure.
These two sets of experiments investigated the impact of pesticides on nitrogen cycling and showed how a pesticide alters one process in the nitrogen cycle but not another depends on the effect on the individual organisms. It is possible that some bacteria involved in ammonification in the orchard soil were affected, but this may not have been detected because other organisms would have been present to carry out the same function. It was also shown that long-term application of several pesticides did not affect mineralisation and nitrification in a field experiment. In this case, the nitrifying bacteria were apparently not inhibited whereas some heterotrophic organisms were inhibited without preventing restricting ammonification.
Under some field circumstances delayed or limited nodulation of lupin has been observed. Two investigations of the process of nodulation of lupin indicated that poor nodulation of lupins could result from (i) low temperature (Peltzer et al. 2002) and (ii) low iron (Fe) (Tang et al. 1992) levels in the soil at the time when nodulation normally takes place.
For low temperature (Peltzer et al. 2002):
• Nodulation occurred in lupin at 25 and 12 oC, but not at 7 oC
• Transfer of root exudates from plants grown at 25 oC induced nodulation in the plants grown at 7 oC.
• The roots of plants grown at 7 oC were unable to produce enough molecular signals to stimulate the bacterial infection process on the root.
• The root exudates from plants grown at 25 oC contained enough molecular signal to induce nodulation at 7 oC.
• High resolution microscopic examination of the roots indicated that the infection process was not initiated at low temperature and that the stage in the nodulation process that was inhibited by low temperature was the infection stage.
For low iron (Tang et al. 1992):
• Nodulation occurred in lupin at levels of iron adequate for plant growth, but not those with severe iron deficiency.
• A split root system was set up with half the roots in adequate iron and half the roots in severe iron deficient conditions. Both root halves were inoculated with the appropriate bradyrhizobia.
• Nnodulation did not occur on roots in the iron-deficient side, indicating that iron was not transported throughout all the roots.
• High resolution microscopic examination of the roots indicated that the infection process was initiated, but that the nodules did not develop in the absence of iron.
• The stage in the nodulation process that was inhibited by low iron was the nodule development stage.
Both low temperature and low levels of iron inhibited nodulation of lupin, but the cause was different in each case. At low temperature, the nodulation process was blocked before the bacteria entered the root. This was due to the low temperatures inhibiting the production of molecules by the plant that activates the nodulation genes in the bacteria. This could occur soon after sowing if frosts were prevalent.
In contrast, the low iron levels caused problems at a later stage of nodule development. The bacteria infected the root as normal and a few root cells started to multiply. But, because of the lack of iron this process was interrupted and the nodule did not form.
Soil disturbance from processes such as cultivation can have a major impact on plant disease. This is because cultivation changes the position in the soil profile of spores or other forms of pathogenic organisms. This apparently simple process can either increase or reduce the likelihood of disease, depending on whether the pathogens are brought up near the soil surface and roots, or are buried more deeply.
Reduced cultivation by direct drilling has been used in combination with deeper seed sowing to control Pleiochaeta root rot of lupins in south-western Australia (Sweetingham 1996). Direct drilling ensures that the spores of Pleiochaeta remained in the surface layers of the soil. Deeper sowing of the seed ensures that the roots grow below the most active zone of the pathogenic fungi. Combining the two practices is an effective way to minimise root rot.
In other areas, tillage has been used to reduce root disease. For example, Rhizoctonia can be controlled in agricultural soils by modifying tillage practices to disrupt the network of hyphae in the soil (Rovira 1986). This reduces the inoculum potential of the fungus.
These two examples demonstrate that the growth of some plant pathogens can be minimised by changing cultivation practices and therefore, the amount of soil disturbance. In each case, the appropriate form of cultivation was based on an understanding of the life cycles of the pathogens involved.
• Changes in the soil environment, either resulting from disturbance or to environmental factors, influence the activity of individual organisms.
• Some soil biological processes are effected by groups of organisms (e.g. soil aggregation) and others by individual organisms (e.g. nodulation of a legume).
• Individual organisms within groups that have similar functions may be affected in different ways by soil disturbance.
• The effects of soil disturbance on biological processes that are performed by groups of organisms are the cumulative results of complex interactions between soil organisms in a dynamic environment.
• It is useful to understanding the effects of soil disturbance on processes involving individual organisms as well as groups of organisms. This can help identify where land management practices might be used either to enhance beneficial soil biological processes or to minimise detrimental processes.