Termites constitute 10% of all animal biomass in the tropics, and up to 95% of soil insect biomass. They are the major determinants of soil structure and the biological processes that underpin soil health. Termites have been shown to have spectacular effects on soil hydraulic function, leading to increased rainfall use efficiency and pasture production.

Right stocking rates equal pasture health Results from the research so far are showing that conservative stocking maintains the ability of soils to capture rainfall. Under conservative stocking, soil macro-invertebrates such as termites, earthworms and ants create large holes in the soil that facilitate the infiltration of rainwater (macro-pores). However, under high rates of pasture use, macro-invertebrate activity declines, reducing soil porosity and rainfall capture. This in turn reduces the length of the growing season for pastures. We have found that simple assessments of macro-invertebrate activity are good on-ground indicators of the state of soil health.

Many indicators of how grazing affects land condition are based on assessments of the effects of chang­ing soil health rather than the causes.

But a team from the Tropical Savannas CRC and CSIRO has gone to the source, by researching processes that maintain the soil’s ability to capture rainfall.

They’ve found that providing the right conditions for earthworms, termites and ants will all improve tropical pastures.

Further, these macro-invertebrates can also provide an early-warning system for soils on a downward spiral. The research team, Tracy Dawes-Gromadzki, Garry Cook, Adam Liedloff, Austin Brandis, explains.

Australia’s tropical savannas are characterised by low fertility soils, highly variable rainfall and long dry seasons.

The amount of rainfall captured and stored by the soil is the key driver of plant growth, but this can vary considerably across the landscape. If the soils capture the first rains of the wet season effectively, the longest possible growing season for pastures is ensured.

However, landscapes that don’t capture this rain effectively will have a longer dry season, which in turn will cause further declines in soil health and productivity. In contrast to southern Australia, high intensity storms account for most of the rainfall in tropical savannas. Therefore we need to maintain high soil infiltration rates to ensure that as much of this water as possible is retained in the landscape.

How is soil health important in savannas?

Soil health, like human health, comprises two important aspects. Firstly, there is the absence of disease and secondly fitness, or general well being. In north Australian savannas, the fitness of soils, and in particular their ability to capture and retain rainfall, is the most important aspect of soil health and the major factor that can limit plant production.

In contrast, where the physical properties of soils are either satisfactory or can be cost-effectively manipulated (e.g. by tillage) such in the cropping lands and sown pastures of southern Australia, the disease aspect of soil health can become a more important limitation.”

Question: How can we tell if a paddock has high macropore densities and therefore good water infiltration and soil health? Search for signs of the presence of macro-invertebrate activity. Results from the current research show that termite sheeting (shown above), earthworm casts and ant nests on the soil surface provide an indication of the levels of macro-invertebrate activity and the presence of macropores. Assessment is easy and provides a simple direct measure of soil health.

Role of macro-invertebrates

The critical role of termites and earthworms in maintaining hydrological function of tropical savanna soils is widely recognised. Soil macro-invertebrates such as termites, earthworms and ants create large holes in the soil (macro-pores) that facilitate the infiltration of rainwater.

These macro-invertebrates increase water infiltration, facilitate gaseous exchange and provide a favourable environment for root growth. Increased infiltration rates can reduce surface run-off and soil erosion.

Macro-invertebrate activity also increases litter decomposition, carbon and nutrient turnover, nutrient storage and soil turnover. These animals are most active in patches of perennial vegetation, and help the patches capture and store run-off water. Grazing animal management can greatly influence the sustainability of these perennial vegetation patches.

Under conservative stocking, the activity of soil macro-invertebrates is high, but under high rates of pasture use, activity declines, leading to reduced soil porosity and reduced rainfall capture. This in turn reduces the length of the growing season for pastures. We have found that simple assessments of macro-invertebrate activity are good on-ground indicators of the state of soil health.

How much rain soaked in? High tech electronics and remote communications are being used to validate our indicators, by measuring the actual variation in rainfall capture across the landscape. Four stations are used for the automatic collection of soil water data. Each station is connected to sophisticated soil moisture measurement probes. Three stations communicate with a base station by radio signals, with all data collected monthly via the CDMA mobile network. Rainfall and flume data are automatically collected, with SMS messages sent to researchers when overland flow occurs to enable the quick collection of sediment samples.

Automated soil moisture data collection

The Tropical Savannas CRC project, Soil health in savannas , is examining the effects of stocking rates on soil health and landscape productivity within the Wambiana Grazing Trial near Charters Towers, Queensland, which is co-funded by the Queensland Department of Primary Industries & Fisheries and Meat and Livestock Australia. Previous research using soil infiltrometers contributed to understanding the impacts of stock management on soil–water.

However, the artificial and ‘one-off’ nature of this method restricts its applicability. This project developed an automated soil moisture data collection facility which gives a better method of assessing soil health, by directly measuring daily changes in soil water throughout the year.

In paddocks of different stocking rates, probes in different patch types (bare, grass and shrub patches) record the amount of soil water at four soil depths down to 0.5 metres. Integration with existing automated rain gauges and run-off flumes is allowing a full eco-hydrological understanding to be developed.

The project is integrating its understanding of the dynamics of soil health with measurements of pasture and livestock productivity and enterprise profitability. This information is also being incorporated into computer simulations such as the Tropical Savannas CRC Savanna.au model, to predict the outcomes of different management decisions on landscape ecohydrology, soil health and productivity.

Soil health indicators

Many indicators of how grazing affects land condition have been developed, but they are often based on assessments of the effects rather than the causes of changing soil health. The ability of indicators to provide early warning of soil health decline will be greater if they measure the cause rather than the effect.

For example, vegetative cover alone has been shown to be a poor indicator of soil-surface properties and water infiltration, because pasture quality and economic productivity can remain high over the short term while the ability of soils to capture and retain resources declines.

Heavy use of pastures can start a process of declining soil health that will reduce soil water availability and limit pasture regrowth and further reduce soil health. This negative feedback means that poor soil health is very difficult to reverse and such efforts are unlikely to be cost-effective.

By studying the biological processes that maintain the soil’s ability to capture rainfall, the project has shown that there is potential for early warning indicators of soil productivity decline which could be used to prevent degradation occurring. This project is developing and testing the validity of soil macro-invertebrates as indicators of soil health. As well as being the key drivers of nutrient cycling and hydrological processes in tropical soils, they also have many other attributes that make them ideal indicators of soil health. They are sensitive to a range of environmental stresses; easily measured and quantifiable; their functional role is easily understood; and they are simple to use, rapid and inexpensive. The assessment of broad groups of soil macrofauna such as termites, worms and ants can often permit the evaluation of the sustainability of a system without the need for taxonomic expertise.

Above: the percent of rainfall entering the soil within the different vegetation patch types considered in this study. As grazing intensity increases, the percentage of rainfall captured is reduced.

Results from our work are showing that simple measures of macro-invertebrate activity, such as the density of macropores in the soil surface and the proportion of the soil covered by worm casts and termite sheeting, correlate highly with the amount of rainfall captured during storms. Recently, in some of our restoration manipulation studies, the indirect introduction of soil macrofauna led to a 30% increase in soil–water infiltration and retention rates during the first rains of the wet season, and a 60% increase in litter decomposition rates.

Manipulation experiments are also under way to examine what effect the removal of specific soil macrofauna has on soil ecohydrology and nutrient dynamics.

Conclusion

Benefits from devising and implementing optimal grazing management that promote sustainable soil health through the management of vegetation and associated soil macrofauna, will include improved pasture productivity, drought survival, biodiversity, soil organic matter levels, improved soil structure and reductions in sediment, nutrient and water run-off. Most importantly, improvements in all these factors will strengthen rural enterprises through maintaining productive landscapes.