Soil bacteria: a valuable but fragile resource

Photo of Gupta Vadakattu

[Photo by Brad Collis: Exploring biomass: Gupta Vadakattu with research plantings.]

Dr Gupta Vadakattu, Senior Research Scientist with CSIRO Land and Water, explains the role of non-symbiotic nitrogen-fixing bacteria and the conditions that affect their performance.

Contributions from biological nitrogen fixation processes, both symbiotic (for example Rhizobium-legume symbiosis) and non-symbiotic nitrogen fixation (soil bacteria) are desirable for both the economic and environmental sustainability of crop production - especially in Australia"s low-input dryland farming systems. However, the potential for non-symbiotic nitrogen fixation by free-living bacteria can vary according to the availability of energy sources (decomposing stubble) and environmental conditions.

In research by CSIRO"s divisions of Plant Industry and Land and Water, nitrogen-fixing bacteria (such as Azospirillum brasilense, Azotobactor vinelandii) occurred in all soils sampled through southern Australia, regardless of soil type or land use.

Populations were generally higher in clay soils compared to sandy soils, but land use appeared to have the greatest impact on the size of these bacteria populations. For example, populations were lower in paddocks under legume (sown lucerne pasture) compared to paddocks sown to crops with high carbon:nitrogen (C:N) ratios (wheat, oat, rice, sorghum, millet). The significant factor for increasing populations of nitrogen-fixing bacteria is the availability of crop residues with wide C:N ratios.

Nitrogenase (nitrogen-fixing) activity consumes large amounts of microbial energy and therefore nitrogen- fixing bacteria will most likely use available nitrogen sources before resorting to fixation. Also, due to the low levels of biologically-available organic carbon in Australian soils, significant amounts of non-symbiotic nitrogen fixation can only occur near decomposing crop residues and in the rhizosphere (root zone).

Therefore stubble retention is essential to gain maximum benefits from non-symbiotic nitrogen fixation. Cellulose and hemicellulose (major components of cereal stubble) and their decomposition products can serve as the energy source for all nitrogen-fixing bacteria.

The level of carbon decomposition and therefore rates of non-symbiotic nitrogen fixation can also be determined by the degree of tillage. Other significant factors that can affect this are soil temperature, availability of soil water, soil texture and pH.

Due to the sensitivity of the nitrogenase enzyme to oxygen and the dependence on moisture to reduce oxygen levels in soil, highest rates of non-symbiotic nitrogen fixation in dryland agriculture soils mainly occur at moisture contents close to or higher than field capacity.

In clay soils, micro-sites of low oxygen availability are present at moisture levels less than field capacity and still support non-symbiotic nitrogen fixation.

Data collected showed that summer-dominant rainfall zones (northern NSW) potentially supported high levels of nitrogen-fixing bacteria (25 to 35 kilograms of nitrogen per hectare from January to June). Soil moisture was not a limiting factor in this region at this time.

In contrast, moisture availability seemed to be the key regulating factor for non-symbiotic nitrogen fixation during this period in southern and western agricultural zones (less than 10 kg/ha from January to June).

These areas have a Mediterranean-type climate with hot dry summers and cool wet winters. Hence in summer after harvest, soil moisture not only limited general microbial activity but also the use of residual carbon in crop residues for nitrogen-fixing activity. With the onset of winter rains, potential nitrogen-fixing bacteria activity increased in the Mediterranean-type regions but temperatures were not ideal and so activity was still limited.

Cereal crop yields in the low-rainfall Mallee region of SA and the WA wheat-belt, range from one to twotonnes a hectare only, so the levels of available carbon from crop residues could also limit non-symbiotic nitrogen fixation in this zone.

Nitrogen budget calculations based on 17 years of data for the long-term farming system trial at Avon, SA, indicated that up to 20kg of nitrogen/ha/year was attributed to nitrogen-fixing bacteria.

Integration of data using spatial analysis tools can help to estimate the potential for non-symbiotic nitrogen fixation, and key regulating factors in cropping regions. Such information is useful for agronomists and extension officers to help explain changes in nitrogen status within paddocks or within specific farming systems and help in providing more accurate advice on nitrogen fertiliser requirements, particularly in low-input farming systems.

It should also be useful for researchers to select the most potentially responsive areas for non-symbiotic nitrogen fixation studies.

For more information: Dr Gupta Vadakattu,, Dr Margaret Roper, CSIRO Plant Industry,