Vertical mobility of pyrogenic organic matter in soils: a column experiment

Abstract

Pyrogenic organic matter (PyOM) is a major and persistent component of soil organic matter, but its mobility and cycling in soils is largely unknown. We conducted a column experiment with a topsoil and subsoil of a sand and a sandy loam to study the mobility of highly 13C labeled ryegrass PyOM (>2.8 at. %), applied as a layer on a 7 cm long soil column, under saturated conditions. Further, we used fresh and oxidized PyOM (accelerated aging with H2O2) to identify changes in its migration through the soil with aging and associated surface oxidation. Due to the isotopic signature, we were able to trace the PyOM carbon (PyOM-C) in the soil columns, including density fractions, its effect on native soil organic carbon (nSOC) and its total export in percolates sequentially sampled after 1000–18 000 L m−2. In total, 4 %–11 % of the added PyOM-C was mobilized and <1 % leached from the columns. The majority of PyOM-C was mobilized with the first flush of 1000 L m−2 (51 %–84 % of exported PyOM-C), but its export was ongoing for the sandy soil and the loamy subsoil. Oxidized PyOM showed a 2–7 times higher mobility than fresh PyOM. In addition, 2-fold higher quantities of oxidized PyOM-C were leached from the sandy soil compared to the loamy soil. Besides the higher mobility of oxidized PyOM, its retention in both soils increased due to an increased reactivity of the oxidized PyOM surfaces and enhanced the interaction with the soil mineral phase. Density fractionation of the upper 0–2.3 cm, below the PyOM application layer, revealed that up to 40 % of the migrated PyOM was associated with the mineral phase in the loamy soil, highlighting the importance of mineral interaction for the long-term fate of PyOM in soils. The nSOC export from the sandy soil significantly increased by 48 %–270 % with addition of PyOM compared to the control, while no effect was found for the loamy soil after the whole percolation. Due to its high sorption affinity towards the soil mineral phase, PyOM can mobilize mineral-associated soil organic matter in coarse-textured soils, where organo-mineral interactions are limited, while finer-textured soils have the ability to re-adsorb the mobilized soil organic matter. Our results show that the vertical mobility of PyOM in soils is limited to a small fraction. Aging (oxidation) increases this fraction but also increases the PyOM surface reactivity and thus its long-term retention in soils. Moreover, the migration of PyOM affects the cycling of nSOC in coarse soils and thus influences the carbon cycle of fire-affected soils.