Microbial Biomass
Key points
- Microbial biomass (bacteria and fungi) is a measure of the mass of the living component of soil organic matter.
- The microbial biomass decompose plant and animal residues and soil organic matter to release carbon dioxide and plant available nutrients.
- Farming systems that return plant residues (e.g. no-tillage) tend to increase the microbial biomass.
- Soil properties such as pH, clay, and the availability of organic carbon all influence the size of the microbial biomass.
Background
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 a soil profile and most of the nutrient release also occurs here (figure 1). Generally, up to 5 % of the total organic carbon and N in soil is in the microbial biomass. 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, and in Western Australia can hold 20 – 60 kg N/ha.
Figure 1: Microbial biomass nitrogen and release of nitrogen decreasing with depth (Murphy et al., 1998).
Microbial biomass is also an early indicator of changes in total soil organic carbon (
C). Unlike total organic C, microbial biomass C responds quickly to management changes. In a long term trial at Merredin, no significant change in 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., 2006a).
In soil the microbial biomass is usually ‘starved’ because soil is too dry or doesn’t have enough organic C. The amount of labile carbon is of particular importance as this provides a readily available carbon energy source for microbial decomposition. Soils with more labile C tend to have a higher microbial biomass.
Important sources of organic carbon as food for the microbial biomass are crop residues and soluble compounds released into the soil by roots (root exudates).
Figure 2: Microbial biomass carbon over a year from a soil near Meckering, WA.
Factors affecting microbial biomass
The microbial biomass is affected by factors that change the water or carbon content of soil, and include soil type, climate and management practices. Rainfall is usually the limiting factor for microbial biomass in southern Australia (figure 2). Soil properties that affect microbial biomass are clay, soil pH, and organic C (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.
Management of crop residues influences microbial biomass as they are one of the primary forms of organic carbon 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 carbon available to them (table 1).
Table 1: The effect of 17 years of retaining or burning stubble on microbial biomass carbon at different soil depths at Merredin, WA (Hoyle et al., 2006b).
Soil Depth (cm) Microbial biomass carbon (kg/ha)
Stubble retained Stubble burnt
0 – 10 229 165
10 – 20 112 93
20 – 30 69 58
Tillage practices that are less disruptive to soil can increase the microbial biomass. Less disruptive tillage increases the microbial biomass by increasing labile carbon in soil (figure 5). These management practices also protect soil aggregates and do not break fungal networks, which are an important habitat for the microbial biomass in soil.
Figure 4: Microbial biomass in soils with different clay contents and under different management. Soils with more clay generally have a higher microbial biomass because they retain more water and often contain more organic carbon.
The type of crops in a rotation can affect the microbial biomass. The residues of legume crops can increase microbial biomass due to their greater N contents. Rotations that have longer pasture phases increase microbial biomass because soil disturbance is reduced (figure 4). This may not be the case in sandy soils, where the lack of clay means organic matter is broken down rapidly. This leaves the microbial biomass ‘starved’.
Figure 5: Increased nitrogen content in labile organic matter and the microbial biomass with no-till compared to rotary till in a 9-year field trial at Wongan Hills, WA (Cookson et al., 2008).
