Aggregate‐occluded black carbon in soil

Summary The great stability of black carbon (BC) in soils may not be solely attributable to its refractory structure but also to poor accessibility when physically enveloped by soil particles. Our aim was to elucidate the intensity of physical entrapment of BC within soil aggregates. For this purpose, the A horizon of a forest, and of a grassland soil, and of three soils under tillage, were sampled at the experimental station Rotthalmünster, Germany. Black carbon was assessed in water‐stable aggregates and aggregate‐density fractions using benzene polycarboxylic acids as specific markers. The greatest BC concentrations made up 7.2% of organic carbon and were found in the < 53 μm fraction. The smallest BC concentrations occurred in the large macroaggregate fractions (> 2 mm). This pattern has been sustained even after tillage. The C‐normalized BC concentrations were significantly greater ( P < 0.05) in the occluded particulate organic matter (OPOM) fractions than in the free particulate organic matter (FPOM) and the mineral fractions. This enrichment of BC compared with organic carbon in the OPOM fractions amounted to factors of 1.5–2.7. Hence, BC was embedded within microaggregates in preference to other organic carbon compounds. Only 2.5–3.5% of BC was located in the OPOM fraction < 1.6 g cm −3 , but 22–24% in the OPOM fraction with a density of 1.6–2.0 g cm −3 . This suggests that BC possibly acted as a binding agent or was selectively enriched during decomposition of protected SOM, or both. Physical inclusion, particularly within microaggregates, could therefore contribute to the long mean‐residence times of soil‐inherent BC.