Tunsisa Hurisso and Steve Culman, School of Environment and Natural Resources, Ohio State University
In short, the answer is usually both.
Soil organic matter is a mixture consisting of various compounds (e.g. simple sugars, cellulose, proteins, etc.) derived primarily from plants and microbes. It represents 1-6% by mass of agricultural soils, but plays a disproportionate role in soil function. Organic matter in soils acts like a sponge, holding nutrients (sequestration or stabilization) that become available to plants when organic matter is broken down (mineralization) by the collective action of the soil food web (mainly by bacteria and fungi). In addition to nutrients, organic matter also enhances the soil’s water holding capacity, making farmlands more resilient to periods of drought. In contrast, when organic matter is depleted due to repeated plowing and/or removal of crop residues from the field, the ability of a soil to hold water and nutrients will be greatly diminished. Therefore, growers should strive for both organic matter stabilization and mineralization processes to ensure short-term crop productivity and to build long-term soil resilience.
Total organic matter is commonly-measured in standard soil tests. But the majority of soil organic matter is not biologically available, as it is physically protected inside soil aggregates or chemically associated with clay minerals. Consequently, total organic matter cycles slowly over time and can be insensitive to changes in management practices. The rapidly cycled, active organic matter fraction has a turnover time of weeks to a few years and is very sensitive to management practices. Therefore, measurements of the soil active organic matter can help shed insight into organic matter dynamics of soil, reflecting both stabilization and mineralization processes.
In a recently published article, a team of researchers lead by Ohio State University examined the relationship between two rapid and inexpensive tests of soil active organic matter across a wide range of soil types. They ran permanganate oxidizable-carbon (POXC; aka ‘active carbon’) and respiration (aka ‘mineralizable carbon’) on more than 1000 soil samples collected from farmer fields and long-term cropping system trials across the United States. Their work found that both POXC and soil respiration tests were correlated, but the relationship was differentially affected by management practices. Measurements of POXC were more related to practices that promote organic matter stabilization, such as reduced tillage, compost and biochar applications. Whereas soil respiration measurements were more related to practices that promote organic matter mineralization, such as tillage, leguminous cover crops and manure applications. The researchers suggested that the integrated use of POXC and respiration tests can serve as a very useful framework to assess soil organic matter dynamics (stabilization vs. mineralization) in farmers’ fields.