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

Abstract

Pyrogenic organic matter (PyOM), formed in wildfires and prescribed burnings or produced intentionally in the form of biochar for soil amendment purposes, is attracting a growing scientific and practical interest due to its important role in the global carbon cycle and agronomic applications as 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 environments. In this paper, we report results of a study utilizing X-ray microtomography and elemental analysis for chemical and structural changes of PyOM formed in prescribed burning events and aged thereafter from 1 to 71 years in boreal forest soil. Our results indicate that changes in elemental composition occurred at decadal timescale and an apparent steady state was reached after ca. 30 years after PyOM formation and exposure to the environment. PyOM was able to retain its porous structure originating from the cellular structure of the initial wood tissue at this timescale. However, structural analysis revealed several influences of aging on the pore structure such as formation of surface coating layers, pore fillings and fractures. These changes may alter the pore size distribution and accessibility of the pores and further influences of PyOM on soil functions such as transfer and retention of water and nutrients in PyOM pores.