Nutrient dynamics and the role of legume trees in the coastal Atlantic forest of southeast Brazil

Background/Question/Methods

Among several botanical families present in the coastal Atlantic Forest, the Fabaceae family has a significant ecological role not only due its abundance and wide distribution, but as well as for having an important role in the terrestrial nitrogen (N) cycle. Some species of this family live in symbiosis with rhizobia bacteria that are able to fix N from the air and provide it to the host plant in exchange for carbohydrates. Additionaly, independently of the ability to fix N, Fabaceae species have a higher foliar N content than other families. Studies conducted by our group have shown that Atlantic tropical forest located in lower altitudes (100 m a.s.l., Lowland Forest) has a higher N availability, while, at higher altitudes (1,000 m a.s.l., Montana Forest) the N availability is lower. Under this scenario the main objective of this study was to investigate the role of Fabaceae on the N cycle in the Lowland and Montana coastal Atlantic Forests of Southeast Brazil. To achieve this goal, the foliar N and C contents and the foliar stable isotopic composition of nodulating Fabaceae (F+), non-nodulating Fabaceae (F-), and of non-Fabaceae (NF) species were determined in 207 individuals sampled at both altitude.

Results/Conclusions

The overall average δ¹⁵N value was lower in F+ leaves (0.4 ± 1.2 ‰) than in the NF (1.6 ± 1.8‰) but not different when compared to F- (1.4 ± 1.3 ‰). The overall average foliar N content in F+ was higher than observed in F- and NF. On the other hand, phosphorus (P) concentrations did not differ between F+, F- and NF. The Ca content was higher in NF than in F+, with no significant difference in relation to F-. The C:N ratio was higher in NF when compared to F+ and F-. Average value of N:P ratio was higher in F+ and F- in comparison to NF. Comparing the two forests, Lowland forest individuals had higher δ¹⁵N values, P and Ca contents, and lower C:N and N:P ratios, but no significant difference in the N content was found in relation to Montana Forest individuals. The average foliar δ¹⁵N of F+ was approximately 1‰ lower than NF and  F-. Therefore, there is no clear signal that F+ are actively fixing N. However, it was confirmed that Fabaceae leaves have different concentrations of nutrients over other species which probably interfere in biogeochemical processes that regulates the N dynamics, as decomposition.