Differing nitrogen use strategies of two tropical rainforest late successional tree species in French Guiana: Evidence from 15N natural abundance and microbial activities

Previous studies in lowland tropical rainforests of French Guiana showed that, among non-N2-fixing trees, two groups of late successional species contrasting in their leaf 15N natural abundance coexist, suggesting two different main ways of nitrogen acquisition. Two abundant late-successional species typically co-occurring in rainforests in French Guiana, namely Eperua falcata and Dicorynia guianensis, were chosen as representative of each group. Stable isotope techniques and measurements of potentials of microbial N transformation were performed to assess to what extent leaf 15N natural abundance of these species could be related to (i) δ15N signatures of soil mineral N sources and (ii) the capacity of soil to express nitrification and denitrification (both processes being directly involved in the balance between NH4+ and NO3–). Soil δ15N-NH4+ was roughly similar to leaf δ15N of D. guianensis (around 3.5‰), suggesting a preferential use of NH4+, whereas in E. falcata, leaf δ15N values were closer to root δ15N-NO3− values (0.2 and −2.0‰, respectively), suggesting a preferential use of NO3−. These differences in N source utilization were not accompanied by differences in availability in soil NO3− or in intensity of microbial functions responsible for soil N mineral evolution. However, (i) under both tree species, these functions showed clear spatial partitioning, with denitrification occurring potentially in soil and nitrification in the litter layer, and (ii) E. falcata fine roots colonized the litter layer much more strongly than D. guianensis fine roots. This strongly suggests that (i) the contrasted leaf δ15N values found in the two late-successional species reveal distinct N acquisition strategies and (ii) the ability of roots to predominantly exploit the litter layer (E. falcata) or the soil (D. guianensis) may constitute an important explanation of the observed differences. A complementarity between tree species, based on mineral N resource partitioning (itself resulting from a spatially structured location of the microbial functions responsible for the balance between NH4+ and NO3–), can thus be supposed.