Ecosystem Insights from Gross Soil N Transformations in Four Luxembourg Beech Forests Along a Ph Gradient

Abstract

The balance of gross nitrogen (N) transformation processes in forests plays a pivotal role in regulating ecosystem N fertility. In Northwestern (NW) Europe, acidic and calcareous soils may differ in net mineralization, but internal N dynamics are often unquantified. We investigated gross N transformations in both the organic layer and the mineral topsoil (0-5 cm) of four Luxembourg beech forests along a natural pH gradient using 15N pool dilution. Also, abundances of bacteria, fungi and ammonia-oxidizers were measured with quantitative polymerase chain reaction (qPCR). Gross N turnover accelerated from acidic to calcareous soils. Gross mineralization rates increased 6-fold in the organic layer and 10-fold in the mineral topsoil. However, net N release did not increase correspondingly because gross immobilization rates also increased. High mineralization and immobilization at high pH were linked to higher microbial N demand and bacterial dominance, indicated by negative correlations with microbial C:N ratios and fungi-to-bacteria (F:B) abundance ratios. Autotrophic nitrification also increased with pH, associated with higher abundances of ammonia-oxidizers. Heterotrophic nitrification was much lower than autotrophic nitrification in calcareous soils but equally important in acidic soils. The mineral topsoil showed net N release, with low release of ammonium (NH4+) and nitrate (NO3-) in acidic soils, and high NO3- release in calcareous soils, contributing to high forest floor species richness at high pH. Our study underscores different mechanisms of microbial N transformations in fungi- and bacteria-dominated soils, and highlights the importance of immobilization and nitrification in regulating ecosystem N fertility.

Keywords: Gross N transformations, pH, microbial N demand, fungi-to-bacteria ratios, N fertility