Effect of environmental exposure on charcoal density and porosity in a boreal forest

Although charcoal's density and porosity shape its environmental roles (e.g. controlling its landscape movement and determining the internal pore space available as microbial habitat), the rate of change of these properties in the environment remains largely unknown. With time, charcoal pores may fill or charcoal particles may shatter, altering the ecosystem services delivered. In this study we examined the effects of environmental exposure on the density and porosity of charcoals pyrolyzed at two different temperatures (350 and 520°C). Fresh charcoal made from the dominant local tree (Pinus sylvestris) was returned in litter incubations to Norwegian boreal forest soils for 20months in three different placements: (i) aboveground, (ii) in the humus layer, and (iii) under the humus layer in contact with the mineral subsoil. By varying soil horizon placement we were able to separate the effects of infill from the effects of environmental disturbance on charcoal density and porosity. Environmental exposure changed charcoal density and porosity, and the response varied with environmental placement. Charcoal placed in soil layers increased in porosity by ~1-2% after 20months' incubation. This is likely because open indentations on the charcoal surface became partially occluded, creating more detectable pore space. In contrast, the porosity of charcoal incubated aboveground decreased slightly (~1-2% over 20months). Because there were no minerals or humic substances to infill the aboveground charcoal samples, this porosity reduction was likely caused by breakage of particles induced by weathering. When charcoal particles cleave through pores, internal pore space is destroyed. The small changes observed here indicate that environmental exposure did not trigger rapid shifts in charcoal density and porosity. In addition, these physical properties appear not to have reached equilibrium after 20months incubation, suggesting that the effect of environmental exposure on charcoal's physical properties occurs on the timescale of years to decades.