Comparing indicators of N status of 50 beech stands (Fagus sylvatica L.) in northeastern France

We compared different potential indicators of nitrogen (N) availability across 50 beech forests growing on a wide range of soils in northeastern France. Among the 50 sites measured, high elevation acidic soils had the highest potential net N mineralization in the A horizon (PNM0–5 cm), while low elevation neutral and calcareous soils had the lowest (PNM0–5 cm). We found that (PNM0–5 cm) was negatively correlated with soil pH (R2 = 0.47***) and positively correlated with microbial C/N (R2 = 0.34***). However, when high elevation sites were excluded from analyses, the relationship between PNM0–5 cm and soil pH as well as microbial C/N became weaker (R2 = 0.23*** for both variables). We found no relationship between PNM0–5 cm and organic N concentration, soil C/N, or vegetation-based indices for N availability (Ellenberg N and Ecoplant C/N). Bivariate linear regression analyses showed that 69% of the variability in percent nitrification (%Nitrif) was explained by both soil pH (0–5 cm) and soil C/N. Percent nitrification was strongly correlated with vegetation-based indices for N availability. The Ellenberg N and R (pH index) values together explained 74% of the variation in %Nitrif. No relationship was found between %Nitrif and soil δ15N (natural abundance in 15N). Of the 76 plant species evaluated, the probability of presence of 61 plant species was significantly correlated with %Nitrif while the probability of presence of 27 plant species only was correlated with PNM0–5 cm. From these results, we believe that the use of plant community composition or the combination of soil pH and C/N are robust indicators of N availability.