Soil Carbon Q & A with Dr. Judy

soil carbon

We recently received the following questions from one of our customers and below are the responses from Dr. Fitzpatrick.

Part of my research is surrounding the soil organic carbon results we attained from microBIOMETER®, and I am wondering if someone from your team could provide more information on what this means relative to total organic carbon (TOC) in a sample and if they are comparable?

The literature shows a strong correlation between available organic carbon and microbial biomass carbon (MBC). Since your compost is not soil, the available organic carbon in your sample would be TOC and would correlate. MBC by microBIOMETER® is even better than that: a big number tells you that you have carbon and all the nutrients needed by microbes and plants.

Since MBC has correlations to TOC is there a formula or percentage to convert MBC to TOC? Or approximately how much MBC makes up a TOC number?

There is no formula to correlate TOC with MBC. TOC includes carbon that we consider stored as well as carbon that is easily available to microbes. Increasing easily available carbon for example by applying compost will increase microbes and eventually increase TOC, but as microbes rarely exceed 1% of TOC, it would have little effect on TOC short term. In long term stable systems we see a correlation but the correlation is not the same for example in forest as in agriculture as the capacity to store TOC is different soils under different conditions. In studying the effect of long term (40 years) different management systems at U. of TN on MBC and TOC, MBC by microBIOMETER® correlated with the TOC demonstrating the effectiveness of sustainable practice on increasing TOC and the positive correlation with MBC levels.

Does a high MBC usually mean a higher F:B ratio? And if so, could we draw any conclusions about carbon sequestration capabilities from that?

Generally as the MBC increases there is an increase in fungi. The soil food web is a balanced community. Some communities are more fungal dominated some less, but similar communities tend to have the same F:B ratio. It is generally believed that fungi, especially mycorrhizal fungi, contribute more to carbon sequestration than bacteria. This may be because glomalin is carbon rich and tends to sequester.

To further my understanding of soil/compost mixtures. I performed two microBIOMETER® tests. One test was on “active compost” which is compost in a medium stage of decomposition, and generates some CO2 and another one “finished compost” which is cured, ready for usage, and low CO2 production. However, I found that they had similar amounts of MBC and F:B ratio. Is this normal?

A study with microBIOMETER® at University showed a higher F:B in finished compost. The higher respiration/MBC indicates that your unfinished compost is still being digested — working microbes make more CO2. Holding MBC stable in your finished product is good.