One teaspoon of soil can contain up to 4,000 different species of microbes and more individuals than there are people on Earth. The way they act together to promote soil health is crucial to maintaining the structure and fertility of soils.
Can soil microbes tell you how sustainable your land management is?
The answer may be 'yes,' according to new research from CSIRO Land and Water, Adelaide, which has found a positive link between average grain yield and the activity of soil microbes.
"This is the first time we have been able to sitively link grain yields with measures of soil biological condition, except for the obvious impact of root diseases on yield. This link is the first step towards developing practical, easy-to-use indicators of soil health," said project manager, Bernard Doube.
These results come from a three-year study of long-term agronomic trials in the Australian wheat belt (the Waite rotation trials, two in each of Queensland, NSW, Victoria, South Australia and Western Australia). The research was supported by growers through the GRDC.
The trials measure the effects on grain yields of soil management practices: tillage, stubble management, rotations and nitrogen fertiliser. Dr Doube used this window of opportunity to examine the corresponding changes in soil condition, especially the microbes in soil.
"If we can relate variations in soil condition to variations in average grain yield, we might have an indicator of soil health," Dr Doube said.
Dr Doube found positive links between average grain yields and the combinations of some biological factors in the soil. In particular, a measure of the rate of microbial activity called the qCO2 (the ratio of soil respiration to soil microbial biomass) and the proportion of biologically active organic matter in soil (the ratio of microbial biomass to % carbon, called the microbial quotient) were linked to grain yields.
"This means that soils with relatively high levels of microbial activity are likely to be the healthier soils," said Dr Doube.
"One teaspoon of soil can contain up to 4,000 different species of microbes and more individuals than there are people on Earth. The way they act together to promote soil health is crucial to maintaining the structure and fertility of soils."
A combination of factors appears to be the key
The data from the Waite rotation trial revealed no positive links between yield and any individual soil measures, including microbial biomass, organic matter levels and microbial activity.
To ascertain the effects of management practices on soil microbes, Dr Doube's group measured 16 different characteristics (and combinations) in soils taken from the long-term trials. These characteristics included:
- physical factors such as water-holding capacity
- chemical factors such as pH and electrical conductivity
- soil organic factors such as % carbon and particulate organic matter, and
- biological factors such as microbial biomass, soil respiration and nitrogen mineralisation.
The level of root disease in the soil was measured by a colleague, Herdina, of the CRC for Soil and Land Management, also in Adelaide, using a newly developed molecular probe for take-all. In one trial (Longernong, Victoria) soils from the rotation containing a pasture phase had a higher take-all reading than did soils from those rotations without a pasture phase.
Dr Doube found conservation management practices such as including a legume ley in the rotation, direct drilling and retaining stubble commonly caused improved soil structure and increases in organic matter levels and microbial biomass.
Crop rotation and the use of chemical fertiliser generally had a greater effect on organic matter levels and microbial biomass than did tillage or stubble management, Dr Doube said. However, no single soil factor consistently predicted the effects of conservation management on soil properties.
Complicating matters was the finding that changes in grain yield from year to year (due largely to weather) were far greater than changes due to soil management. But by using averaging procedures to allow for the year-to-year variation in yield, Dr Doube was able to detect important effects of soil management practices on grain yields in some trials.
The next step is to see if the same ratios of microbial biomass, soil respiration and % carbon are useful indicators of grain yield in other long-term trials, Dr Doube said. "If that is successful, we will need to develop other measures of soil microbial activity which are easy to use, cheap and accurate."
Contact: Dr Bernard Doube 08 8303 8400