Structural and chemical changes in pyrogenic organic matter aged in a boreal forest soil

Pyrogenic organic matter (PyOM) is formed during wildfires and prescribed burnings or produced intentionally in the form of biochar for soil amendment purposes. It is attracting a growing scientific and practical interest due to its important role in the global carbon cycle and agronomic applications as a soil enhancer. Most of the studies on the physicochemical properties of PyOM have been conducted using fresh biochars even though the characteristics of PyOM are expected to alter due to aging processes in soil environment. In this paper, we report the results of a study that utilized X-ray microtomography and elemental analysis to investigate the chemical and structural changes in the PyOM formed during prescribed burning events and aged thereafter for 1-71 years in a boreal forest soil. Our results indicate that changes in elemental composition occurred at decadal timescales, and an apparent steady state was reached ca. 30 years after PyOM formation and exposure to the environment. At such timescales, PyOM was able to retain its porous structure originating from the cellular structure of the initial wood tissues. However, structural analysis revealed several effects of aging on the pore structure, such as the formation of surface coating layers, pore fillings, and fractures. These changes may alter pore size distribution and accessibility of the pores and further alter the influence of PyOM on soil functions, such as the transfer and retention of water and nutrients in PyOM pores.