Organic matter chemistry controls CO2 flush

For producers interested in monitoring biological soil health, a good indicator of general microbial activity is the CO₂ flush, which is the amount of CO₂ released after rewetting dried soils. While relatively inexpensive, rapid, and easy for laboratories to collect, CO₂ flush data is challenging to interpret because multiple factors may affect its magnitude. In particular, the amount and chemical composition of soluble organic matter may affect measurements.

In an article recently published in the Soil Science Society of America Journal, researchers report on using ultrahigh resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry to monitor chemical changes in soluble soil organic matter during the 48-hour period after rewetting of dried soil from a greenhouse experiment. Soluble soil organic matter, the most available pool to microbes, consists of thousands of different molecules, for which chemical formulas were assigned.

The largest fraction was chemically similar to lignin, suggesting that the soluble organic matter in rewetted soil was derived from soil organic matter rather than from dead microbial cells. Nitrogen-containing molecules were about 7% of the total, and this fraction was depleted to near zero by 48 hours.

The rate of CO₂ release was influenced more by the nitrogen-containing fraction than the total amount of soluble organic matter, supporting the use of the CO₂ flush as an indicator of soil nitrogen availability.