From Leaf to Topsoil, a Semi-quantitative Assessment of the Organic Matter Variations in Riparian Forests on a Lake Shore

The water level of Lake Neuchâtel (Switzerland) was lowered 150 years ago, initiating soil formation and plant colonization on the previously submerged lands. Since then, four riparian forest types have been differentiated, with contrasted humidity and soil conditions. This study aimed to describe the processes underlying variations in organic matter (OM) composition and content, and to evaluate the importance of water level on soil development. This investigation was conducted in a semi-quantitative way, using a holistic approach. We measured OM production (wood, dead wood, and annual litter) and OM decomposition rate (tea bag index, soil respiration), and soil macrofauna was inventoried in parallel with descriptions of humus forms. Measured values varied with the plant composition of these forests and with water fluctuations. Two different organic matter pathways characterized the four forest types. Forests with waterlogged soils displayed fast decomposition, low soil respiration, low macrofauna biomass, and water-bound humus forms. Conversely, forests with a low water table showed slower decomposition rates, higher summer respiration rates, higher macrofauna biomass, and Mull-type terrestrial humus forms. Hence, there was a paradox between decomposition rate and respiration, where decomposition was more efficient in the most humid forests, although soil CO2 emissions were lower than in drier forests.