The soil moisture regime of charcoal-enriched land use legacy sites

Abstract

The stratigraphy and properties of soils can be significantly altered by past land use, even in areas that have been continuously used for forestry. An example of such a pedological legacy of past forest use is the soils associated with relict charcoal hearths (RCHs). The physical properties and moisture regime of RCH soils and their ecological implications have hardly been studied, although RCHs are widespread in many forest areas and might be used as model sites to study long-term effects of soil amendment with biochar. The objective of our study was to characterize the soil moisture regime and hydraulic properties of RCH soils through comparison to reference forest soils on sandy substrates in the northeastern German lowlands. RCH and reference soils were studied in woodlands around the historic iron works in Peitz (Brandenburg, Germany), combining laboratory analyses of bulk density, pore size distribution, and saturated hydraulic conductivity with sensor-based monitoring of soil moisture contents and matric potentials. The laboratory analyses reveal a low bulk density and high porosity of the RCH substrates, which is mainly related to larger volumes of coarse and fine pores. Soil moisture monitoring shows higher water contents in RCH soils under relatively wet conditions and lower water contents under dry conditions, as well as strong hysteresis effects, especially after dry periods. The results therefore affirm that the legacies of charcoal production increase spatial and temporal variations in soil moisture, which in turn can cause increased variability in ecological site conditions in charcoal production areas. They furthermore reveal that the high porosity of charcoal-enriched substrates is not necessarily associated with higher water retention and plant-available water contents, and that the pore size distribution and water retention in RCHs differ from those characteristically found for biochar-amended soils.