The mystery of SOC, POC & MAOC!

The importance of Soil Organic Matter (SOM) and Soil Organic Carbon (SOC) is well recognised. But the understanding of processes by which it is formed and maintained is less so. A recent paper by Tang et al. (2026) helps to answer some important questions for croppers and graziers in southern Queensland and northern NSW.

The study examined Particulate Organic Carbon (POC – carbon that breaks down over the short term) and Mineral Associated Organic Carbon (MAOC – carbon that breaks down over the long term) along a national rainfall gradient of 500 – 600 mm. This includes Australia’s major cropping regions as well as grazing lands in both temperate and sub-tropical climates.

Figure 1 – Sampling locations along the 500 mm – 600 mm rainfall gradient. Coarse refers to soils higher in sand whilst fine refers to soils higher in clay.

Their analysis untangled some of the major drivers of SOC, POC and MAOC. Climate (Mean Annual Temperature) and soil texture (Percentage clay) explained much of the variability. Natural followed by grazing lands stored the most SOC, MAOC and POC. Not surprisingly cropping soils had the lowest SOC.

Carbon stabilises itself by different pathways in temperate versus subtropical soils. In the SQNNSW region MAOC dominates (approximately 80% of SOC) with overall lower levels of SOC than in temperate regions, primarily due to higher temperatures and variable rainfall. This compares with soils in temperate regions where the amount of POC tends to be much higher and forms a greater proportion of SOC. Sandier soils, with less capacity to form MAOC, also have a higher proportion of POC compared to clay soils. 

For subtropical cropping soils in particular, fertiliser use can assist in increasing MAOC, however this is balanced by overall declines in both POC and overall SOC.

Increasing mineral-organic interactions through increasing root exudates (through vigorous crops/pastures and/or longer growing seasons) combined with amendments (e.g. organic and/or fertiliser applications) can help build and maintain the more stable MAOC.

One of the most interesting findings of the study was that there does not appear to be a fixed upper limit to MAOC, but rather different efficiencies by which carbon inputs are transformed and retained.

For land managers interested in improving or maintaining SOC this study highlights the importance of understanding your context. The major drivers of SOC levels are climate and soil texture – factors over which we have no control. Subsequent factors are how much carbon is being released into the soil by plants – which managers have some control over, and land use.

Now that we know that MAOC is dominant in subtropical soils, both in cropping and grazing, the challenge before us is to refine the practices that can help maximise MAOC formation.

Link to the paper

• SOC stabilisation shifts from carbon accumulation in temperate soils to mineral association in subtropical soils - ScienceDirect

Reference

TANG, Y., TRAN, T. K. A., MINASNY, B., BAKHSHANDEH, S., DU, M., FRANCOS, N., HUANG, Y.-C., JANG, H. J., NG, W. & XUE, P. 2026. SOC stabilisation shifts from carbon accumulation in temperate soils to mineral association in subtropical soils. Soil Biology and Biochemistry, 110101.