Laboratory assistant working with plants, different kinds of soil and sand, testing and analyzing results

Microbial Biomass vs. Microbial Respiration

What is the difference between microbial biomass (MB) and microbial respiration rate (RR) ?

Both parameters are used to assess soil microbial health. The respiration assay measures the amount of carbon dioxide produced by the microbes in a given weight of soil. The soil is dried and then rewetted and put in an airtight jar that allows measurement of the amount of CO2 produced over 24 hours. The CO2 is produced by the activity of the microbes in the rewetted soil. Between 20% and 70% of the microbes die during drying but their dead bodies often provide nutrition for the survivors to use and regrow the population to its original level.

Respiration reflects the regrowing work that is being done. The respiration level is often mistakenly believed to predict microbial biomass (MB), but it does not. Microbes in a low pH or toxic soil have to work harder, therefore, their respiration rate is higher, just as your respiration rate in the gym is higher than when you are watching TV. Outside of the U.S. the respiration rate (RR) is only considered in relation to MB and this q-value RR/MB is used to determine the level of stress in a soil. If RR is high for the MB, the soil is in trouble.

MB, as measured by microBIOMETER®, correlates with chloroform fumigation—always and microscopic evaluation of soil. It is an excellent predictor of soil health because the size of the microbial population correlates with the nutrition available in the soil. If the soil is deficient in carbon, nitrogen, phosphorus or any other mineral, or contains toxins, MB will be low. In fact, MB is low in any soil that is compacted, has a low pH or is overly dry, because microbes need oxygen and moisture and the correct pH for enzymatic activity.

In nature, the plant uses 30% of its food production to feed a microbial population that will mine the soil for the N, P, K, S etc. that it needs. Interestingly MB is low in soil treated with high levels of mineral fertilizers; researchers have shown that the stimulus for the plant to grow a microbial population is its need for nitrogen and phosphorus. If these are artificially supplied the plant is not stimulated to feed the microbes that usually provide these nutrients to the plant. And since the microbes are at least half of the immune system of the plant, you now need lots of pesticides to protect the plant.