No detectable upper limit for carbon in agricultural soils

SOIL offers enormous carbon storage potential – and provides a win-win for farmers because improved sequestration capacity builds alongside soil health and improved nutrient and water cycling.

Soil also offers immediate carbon storage benefit, where, for example, a Sitka spruce plantation often degrades soil health and releases carbon emissions in the shorter term. This means that intensive forestry can take years or even several decades to reach carbon neutrality before the positive carbon storage from tree growth can be realised.

The scientific study of soil carbon sequestration is less advanced than woodland sequestration, and in academic and policy circles there’s been debate over whether a short term boost in soil carbon sequestration can continue in the longer term.

However, a new study by German researchers has found no detectable upper limit of mineral-associated organic carbon in temperate agricultural soils.

They say that “Soil organic carbon (SOC) sequestration is a promising climate change mitigation option. In this context, the formation of the relatively long-lived mineral-associated organic carbon (MAOC) is key.

“To date, soils are considered to be limited in their ability to accumulate mineral-associated organic carbon, mainly by the amount of clay and silt particles present.”

Using the comprehensive German Agricultural Soil Inventory, the researchers selected 189 samples with a wide range of SOC (5–118 g kg−1) and clay contents (30–770 g kg−1) to test whether there is a detectable upper limit of mineral-associated organic carbon content.

They found that the proportion of mineral-associated organic carbon was surprisingly stable for soils under cropland and grassland use across the whole range of bulk SOC contents.

They say that “soil texture influenced the slope of the relationship between bulk soil organic carbon and mineral-associated organic carbon, but no upper limit was observed in any texture class.”

Carbon content in the fine fraction (g C kg−1 fraction) was also negatively correlated to fine fraction content (g kg−1 bulk soil).

They say that “both findings challenge the notion that mineral-associated organic carbon accumulation is limited by soil fine fraction content.”

Read the full paper

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