Functional differences between woodland savannas and seasonally dry forests from south-eastern Brazil: Evidence from 15N natural abundance studies

Nitrogen availability and N-cycling dynamics across ecosystems play a critical role in plant functioning and species distribution. Measurements of N-15 natural abundance provides a way to assess ecosystem N dynamics, and the range of nitrogen stable isotope values (delta N-15) for plants in an ecosystem can indicate divergent strategies for N uptake. We tested the hypotheses that the N-rich seasonally dry forest would have higher soil and leaf delta N-15 and a smaller range of leaf delta N-15 values compared to the N-poor cerradao (savanna woodland). We measured N concentration and delta N-15 in two soil depths and leaves of 27 woody species in cerradao and 26 in seasonally dry forest. As expected, total soil N concentration decreased while soil delta N-15 value increased with soil depth. Regardless of soil depth, seasonally dry forest soils had higher delta N-15 and total N concentration compared to cerradao soils. Foliar delta N-15 values varied from -6.4 parts per thousand to 5.9 parts per thousand in cerradao and from -2.3 parts per thousand to 8.4 parts per thousand in seasonally dry forest plants. Phylogenetically independent contrasts analysis and comparisons of delta N-15 mean values of the most abundant species and species co-occurring in both sites confirmed the hypothesis of higher delta N-15 for seasonally dry forest in comparison to cerradao. These results corroborate the expectation of higher soil and leaf delta N-15 values in sites with higher soil N availability. However, except for the most abundant species, no across-site leafsoil (delta N-15 leaf delta N-15 soil) differences (delta N-15) were found suggesting that differences in leaf delta N-15 between cerradao and seasonally dry forest are driven by differences in soil delta N-15. Variation of leaf delta N-15 was large in both sites and only slightly higher in cerradao, suggesting high diversity of N use strategies for both cerradao and seasonally dry forest communities.