Charred plant fragment-aluminium complexes in buried humic horizons of a cumulative volcanic soil profile: Formation, accumulation and potential role

The objective of the present study was to investigate the formation, accumulation and potential role of charred plant fragment (CPF)-aluminium (Al) complexes in several buried A (Ab) horizons of a cumulative volcanic soil profile, focusing on CPFs, active Al components and soil age. Nineteen soil samples were collected from four buried humic horizons (2A-5A, from 7.3 cal ka BP to AD 1471), and their CPF-carbon (C), soil organic C (SOC), acid oxalate-extractable Al (Al-ox), pyrophosphate-extractable Al (Al-py, Al-humus complexes) and short-range ordered Al minerals (Al-ox - Al-py) were quantified. The proportion of CPF-C content (up to 15.8 g kg(-1)) to SOC content (up to 117 g kg(-1)) ranged from 2.3% to 15.1% (8.4% on average). The contents of active Al components (6.71-77.1 g kg(-1)) increased: Al-py < Al-ox - Al-py < Al-ox. No consistent trends with age were found for the accumulation levels of CPF-C, SOC and active Al components. However, significant positive correlations (p < 0.001) were found between the Al-py and CPF-C and SOC contents, suggesting that the accumulation and stabilisation of soil organic matter in these humic layers proceeded simultaneously with the formation of organo-Al complexes, including CPF-Al complexes. On the other hand, significant negative correlations (p < 0.01) were obtained between the Al-ox - Al-py and Al-py, CPF-C or SOC contents. From these correlations, it was assumed that the complexation of organic constituents, including CPFs, with Al competed with the synthesis of short-range ordered Al minerals (including allophane and imogolite), irrespective of soil age and climatic conditions with the range of more than 6000 years. This competitive relationship in the presence of CPFs was first described in the current study. Highlights Significance of charred plant fragment-aluminium complexes in humic horizons was discussed. There were no consistent trends with age for the accumulation levels of organic constituents. The accumulation of organic matter proceeded simultaneously with the formation of the complexes. The formation of the complexes competed (>6000 years) with the synthesis of aluminium minerals.