Properties of successive horizons in a thick forest floor (mor) reflect a sequence of soil acidification

Successive horizons of thick forest floors may be considered as a litterfall decomposition sequence, but the evolution of physico-chemical properties along such a sequence is poorly known. Here, we have characterized the physico-chemical properties within a thick forest floor and evaluated their potential impacts on the release of Dissolved Organic Carbon (DOC). Seven sub-horizons of a mor forest floor were sampled in a 200 year-old beech stand on Podzol. We performed total analysis, measured the concentrations of exchangeable cations and determined the concentration of ions and DOC in water extracts. From Ol to Oh, sharp gradients were observed: (1) total concentrations of Ca, Mg and Mn decreased, while total concentrations of Si, Al, K and Na increased, (2) exchangeable Ca, Mg, K and Na concentrations decreased while exchangeable H and Al concentrations increased and (3) leachate pH value dropped from 6.3 to 4.2. These gradients can be compared to the evolution of physico-chemical properties of a soil undergoing acidification. However, mineral incorporation into forest floor likely prevented a decline in total K and Na concentrations. Such a vertical heterogeneity of physico-chemical properties within the forest floor can affect DOC release: low pH value and high Mn concentrations in Of could promote DOC production by lignin degradation, while high Al concentrations in Oh could reduce DOC solubility.