Using a novel electrical measurement approach to measure the effects of liming on rooting parameters in German beech forests

Hundreds of thousands of hectares have been limed in German forests in the last three decades to mitigate the effects of soil acidification. To understand the long-term impacts of liming on tree rooting behaviour and the implications for soil organic matter, we used a novel electrical approach to quantify rooting parameters of mature beech forests and compared it to the standard monolith excavation approach. At each of the three experiment sites located in northern Germany, we looked at rooting behaviour in limed plots (overall eight tons of lime per hectare applied in the 1980s and 1990s) in comparison to adjacent control plots. First, we used the standard monolith excavation approach to determine fine root biomass at the stand level. With an electrical approach called the “earth impedance method” (EIM) we subsequently estimated tree absorbing root surface area (ARSA; this is the contact area where roots take up nutrients and water). This experimental, non-destructive approach uses a low frequency alternating electric current that flows from the roots to the soil and vice versa, and the electrical impedance (resistivity) is recalculated to estimate ARSA for the sample tree. We measured the ARSA of six mature trees per plot (12 trees per site).

To summarize the results of the sampling approaches, (1) both root estimation approach measurements were positively correlated, validating the EIM; (2) the ARSA was positively correlated with tree size at each site, further substantiating the rapid and cost effective EIM; (3) however this method is vulnerable to variables that effect electrical conductivity, such as soil moisture and the thickness and makeup of the organic horizons.

Overall, no significant differences between limed and control plots were detected with either measurement approach, suggesting that despite improved soil pH conditions the tree root systems in limed plots remain relatively constant in size and capacity to take up nutrients and water.