The microbial biomass consists mostly of bacteria and fungi, which decompose crop residues and organic matter in soil. This process releases nutrients, such as nitrogen (N), into the soil that are available for plant uptake. About half the microbial biomass is located in the surface 10 cm of soil and most of the nutrient release also occurs here (figure 1). Generally, up to 5% of the total organic C and organic N in soil exists in the microbial biomass component of soil organic matter. When microorganisms die, these nutrients are released in forms that can be taken up by plants. The microbial biomass can be a significant source of N, in some cases holding more than 60 kg N/ha.
Figure 1: Both microbial biomass nitrogen and release of nitrogen decrease with depth (Murphy et al., 1998).
Microbial biomass is also an early indicator of changes in total organic C. Unlike total organic C, microbial biomass C responds quickly to management changes. In a long term trial at Merredin, Western Australia, no significant change in total organic C was detected between stubble burnt or retained plots after 17 years. Microbial biomass C in the same plots had increased from 100 to 150 kg C/ha (Hoyle et al., 2006).
In soil the microbial biomass is usually ‘starved’ because soil is too dry or doesn’t have enough organic C. The amount of labile organic C is of particular importance as this provides a readily available carbon energy source for microbial decomposition (see ‘Labile Organic Carbon’ fact sheet). Soils with more labile C tend to have a higher microbial biomass. Fresh plant residues and soluble compounds released into the soil by roots (root exudates) are important sources of energy © for the microbial biomass.
The microbial biomass is affected by factors that change soil water, temperature or carbon content, and include soil type, climate and management practices. Rainfall is usually the limiting factor for microbial biomass in southern Australia (figure 2).
Figure 2: Microbial biomass carbon over a year from a soil near Meckering, Western Australia.
Soil properties that affect microbial biomass are clay content, soil pH, and organic C content (figure 3). Soils with more clay generally have a higher microbial biomass as they retain more water and often contain more organic C (figure 4). A soil pH near 7.0 is most suitable for the microbial biomass.
Figure 3: The main soil properties affecting the microbial biomass and factors influenced by it.
Figure 4: Microbial biomass in topsoils (0–10 cm) with different clay contents and under different management.
Management of plant residues influences microbial biomass as residues are one of the primary forms of organic C and nutrients used by the microbial biomass. Retaining crop residues rather than burning them provides a practical means of increasing the microbial biomass in soil by increasing the amount of organic matter available to them.
Minimising tillage increases microbial biomass by protecting soil aggregates formed by fungal networks. The pore spaces in the aggregates are an important habitat for the microbial biomass in soil. Conversely a change to more disruptive practices can quickly deplete soil carbon in the topsoil, particularly microbial biomass carbon (figure 5).
Figure 5: Rapid changes occur to microbial biomass in topsoil (0–5 cm) after tillage practice is changed for 3 years, in a trial at Cowra, NSW. (Pankhurst et al., 2002).
Direct drilling can provide more efficient use of residues by microbial activity in 0–5cm layer compared with other tillage treatments as shown in figure 6. Stubble incorporation relocates residues deeper into soil, but the lack of aeration may limit decomposition and therefore microbial activity and microbial biomass carbon.
Figure 6: Microbial quotient is the ratio of microbial biomass to soil organic carbon and indicates how efficiently soil organic matter is being used by microorganisms (Pankhurst et al., 2002).
Type of crop can also affect the microbial biomass. The residues of legume crops can increase microbial biomass due to their greater nitrogen contents. Rotations that have longer pasture phases generally increase microbial biomass because soil disturbance is reduced and organic matter supply is increased (figure 4). However this may not be the case in very sandy soils, where the lack of clay means organic matter is broken down rapidly if there is sufficient moisture. This leaves the microbial biomass ‘starved’.
Hoyle FC, Murphy DV and Fillery IRP (2006) ‘Temperature and stubble management influence microbial CO2-C evolution and gross transformation rates’, Soil Biology and Biochemistry 38: 71-80.
Murphy DV, Sparling GP and Fillery IRP (1998) ‘Stratification of microbial biomass C and N and gross N mineralizsation with soil depth in two contrasting Western Australian Agricultural soils’, Australian Journal of Soil Research 36: 45-55.
Pankhurst CE, Kirkby CA, Hawke BG and Harch BD (2002) ‘Impact of change in tillage and crop residue management practice on soil chemical and microbiological properties in a cereal-producing red duplex soil in NSW, Australia’, Biological Fertility of Soils 35:189-196.
The New South Wales Department of Primary Industries has further soil information (online)
Author: Jennifer Carson (The University of Western Australia)
Revised for NSW: Sally Muir and Abigail Jenkins (New South Wales Department of Primary Industries), 2012
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.