The soil microbial biomass consists mostly of bacteria and fungi, which decompose crop residues and organic matter in soil. The microbial biomass typically makes up less than 5 % of total soil organic matter, but it plays a very large role in a number of key soil functions, including nutrient release, the maintenance of good soil structure and the suppression of plant pathogens. Changes to the microbial biomass can also be an early indicator of changes in total soil organic carbon. Unlike total organic carbon, microbial biomass carbon responds quickly to management changes, and can often be measured before changes in total organic carbon are detected.
One of the most important roles of the microbial biomass is the conversion of organic matter into mineral nutrients available for plant uptake. The microbial biomass is important for transforming nitrogen, phosphorus, sulphur, potassium, calcium, magnesium, manganese and zinc into forms that can be used by plants. If it weren’t for soil microbes, plant nutrients would remain ‘locked away’ in dead plant and animal tissue. About half the microbial biomass is located in the surface 10 cm of a soil profile and most of the nutrient release also occurs here (figure 1).
Figure 1: Microbial biomass nitrogen and release of nitrogen decrease with depth (Murphy et al., 1998).
The activity of the microbial biomass also has a direct impact on soil structure. As bacteria grow they produce polysaccharide gels, which help stick soil aggregates together. Fungi also helps stabilise soil aggregates as the long branching structures they produce, called filamentous hyphae, help entangle soil particles and bind them together.
A diverse and active soil microbial biomass can also be important for the suppression of plant pathogens. Soils that have been intensively cropped for decades have often lost much of their microbial diversity and become susceptible to disease. This is because many of the competitors of fungal pathogens and root-feeding nematodes have disappeared. A soil that has an active and diverse microbial biomass is more likely to have microbes present that can help soils resist disease. For example, some bacteria and fungi produce antibiotics that are detrimental to pathogens, or can inhibit the growth of pathogens by limiting their access to essential nutrients. Certain types of fungi are able to parasitise fungal pathogens, or can even produce structures that ensnare nematodes.
The microbial biomass is affected by factors that change the water or carbon content of soil, and include climate, soil type and management practices. The microbial biomass grow best in warm and moist conditions. Consequently areas with warm moist climates will have a greater microbial biomass than cold or dry areas.
Soil type can also influence the size of the microbial biomass. Soils with higher clay contents generally have a higher microbial biomass as they retain more water and often contain more organic C (figure 2). Soil pH is also important as microbial growth declines under conditions that are too acid or too alkaline. A soil pH near 7.0 is most suitable for microbial growth.
Figure 2: Microbial biomass in soils with different clay contents and under different management, corrected for years of cropping. Microbial biomass has declined in both soils after 30 years of cropping. However, the Waco soil has retained more of its microbial biomass carbon due to its higher clay content, which has helped preserve its stocks of organic carbon (Dalal et al. 1987).
Management has a major impact on microbial biomass in agricultural soils, due largely to its ability to impact the amount of organic carbon entering the soil. The amount of labile carbon present in soil is of particular importance for the microbial biomass. Labile carbon is carbon that is easily broken down by microorganisms, and is largely made up of crop residues and particulate organic matter (see Organic carbon pools factsheet). This carbon provides a readily available energy source for microbial decomposition, and soils with more labile C tend to have a higher microbial biomass. In many agricultural soils where concentrations of labile carbon are low, the microbial biomass is often ‘starved’ because it doesn’t have enough organic C.
Studies have shown that biological activity in Queensland cropping soils is generally low compared to other land uses, such as pasture or native vegetation, due to the low amount of labile carbon present. Management practices that can increase soil carbon and thus soil microbial biomass include:
Figure 3: Microbial biomass carbon decreases over the fallow period. Stubble retention is generally higher under stubble retained compared to stubble burnt systems (Wang, unpublished data).
Bell M, Seymour N, Stirling GR, Stirling AM, Van Zwieten L, Vancov T, Sutton G and Moody P (2006). Impacts of management on soil biota in Vertosols supporting the broadacre grains industry in northern Australia., Australian Journal of Soil Research 44(4): 433?451.
Dalal RC and Mayer RJ (1987) Long term trends in fertility of soils under continuous cultivation and cereal cropping in southern Queensland. VII Dynamics of nitrogen mineralisation potentials and microbial biomass., Australian Journal of Soil Research 25: 461-472.
Gonzalez-Quinones V, Stockdale EA, Banning NC, Hoyle FC, Sawada Y, Wherrett AD, Jones DL and Murphy DV (2011) Soil microbial biomass – Interpretation and consideration for soil monitoring., Soil Research 49: 287-304.
Murphy DV, Sparling GP and Fillery IRP (1998) Stratification of microbial biomass C and N and gross N mineralisation with soil depth in two contrasting Western Australian Agricultural soils., Australian Journal of Soil Research 36: 45-55.
The National Soil Quality Monitoring Program is being funded by the Grains Research and Development Corporation, as part of the second Soil Biology Initiative.
The participating organisations accept no liability whatsoever by reason of negligence or otherwise arising from the use or release of this information or any part of it.