No detectable upper limit of mineral‐associated organic carbon in temperate agricultural soils

• Published in: Global change biology 2023-08, Vol.29 (16), p.4662-4669
• Main Authors: Begill, Neha, Don, Axel, Poeplau, Christopher
• Format: Article
• Language: English
• Subjects: Agricultural land; Agriculture; Carbon; carbon saturation; Carbon Sequestration; Clay; Clay minerals; Climate change; Climate change mitigation; Grasslands; Minerals; Mitigation; national soil monitoring; Organic carbon; organic matter fractionation; Organic soils; Soil; Soil properties; Soil texture; Soils; Texture
• ISSN: 1354-1013; 1365-2486
• DOI: 10.1111/gcb.16804

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 MAOC, mainly by the amount of clay and silt particles present. Using the comprehensive German Agricultural Soil Inventory, we 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 MAOC content. We found that the proportion of MAOC was surprisingly stable for soils under cropland and grassland use across the whole range of bulk SOC contents. Soil texture influenced the slope of the relationship between bulk SOC and MAOC, but no upper limit was observed in any texture class. Also, C content in the fine fraction (g C kg−1 fraction) was negatively correlated to fine fraction content (g kg−1 bulk soil). Both findings challenge the notion that MAOC accumulation is limited by soil fine fraction content per se. Mineral‐associated organic carbon (MAOC) is the stabilised fraction of soil organic carbon (SOC). Its accrual in the soil can help mitigating climate change. So far, the capacity of soils to store MAOC was believed to be limited by the amount of mineral surfaces available. Here, we provide evidence that up to a total SOC content of about 12%, MAOC is a surprisingly constant fraction of total SOC. This questions the notion of a maximum capacity of soils to store MAOC, at least for temperate soil within the tested range of SOC contents.