Changes in humus forms and soil N pathways along a 130-year-old pure beech forest chronosequence

Abstract

We investigated changes in humus morphology and soil potential net N mineralization pathways along a pure beech even-aged forest chronosequence, composed of four stages (15, 65, 95, and 130 years old) on Luvisol. To quantify the respective contributions of bacteria and fungi to soil N processes, we used acetylene and a fungicide (captan). We analyzed the data in order to highlight correlation patterns between soil N cycling and humus morphology. We sampled the OL layer (unmodified leaf fragments), the FH layer (mixture of coarse plant fragments with fine organic matter) and the A layer. Within the organic layers, N mineralization was two times higher in mature stands than in younger ones. Nitrification decreased sharply within the OL and the A layers along the chronosequence but was stable in the FH layer. The relative contributions to ammonification by bacteria and fungi remained constant along the chronosequence in both the OL and the A layers. In the FH layer, fungal contribution to ammonium production increased along the chronosequence. Nitrification was mainly autotrophic. We observed strong correlations between morphological and soil N variables. Our results suggest that pure beech forest maturation promotes soil ammonium production; its efficiency is probably controlled by fungal activity. They also suggest the potential use of selected macro-morphological variables as indicators of both soil N mineralization rates and the nature of N pathways.