Temperature Regime of a Charcoal-Enriched Land Use Legacy Soil

The addition of charcoal to soils can distinctly alter the soil's physical and chemical properties. Such effects occur in soils deliberately amended with charcoal (biochar) and as a land use legacy in historic charcoal hearth sites. One aspect of soil property modification induced by charcoal addition that has rarely been studied is the effect on soil temperature and thermal properties. The objective of this study was to characterize the soil temperature regime of a relict charcoal hearth (RCH) site compared with a reference forest soil. Soil temperatures were monitored at three depths in two profiles over a period of 1 yr. Samples from both profiles were analyzed to characterize the bulk density, soil organic matter (SOM) content, charcoal content and thermal conductivity of the soils. The monitoring results reveal distinct differences between the temperature regimes of the profiles, with the RCH soil exhibiting higher daily and seasonal temperature variations at a depth of 15 cm but lower variations at a depth of 80 cm than the reference soil. The laboratory results show that these differences are related to a clearly lower thermal conductivity in the RCH soil, which is associated with the low bulk density and high contents of SOM. The results confirm that charcoal addition can clearly affect soil temperature, and they show that the effects can propagate beyond the charcoal-amended soil horizons. The observed modifications in the soil temperature regime might contribute to several poorly understood effects of charcoal addition on biogeochemical processes in soils.