N₂-fixation is linked to the ability to encroach in African savanna trees

This dataset includes nitrogen (N) dynamics for 12 southern African savanna tree species (six encroachers, six non-encroachers) under manipulated atmospheric CO2 (~398 ppm ambient, ~545 ppm elevated) and soil moisture (well-watered vs water-limited) in open-topped chambers. Measured traits include nodule mass fraction, leaf and stem δ15N, and percentage of N derived from N2-fixation (% Ndfa). The data capture both short- and long-term N allocation strategies and enable exploration of how CO2 and water treatments affect N dynamics across encroaching and non-encroaching species.

Methods
The dataset comprises 12 southern African savanna tree species (nine Vachellia, three Senegalia), with six encroachers and six non-encroachers. All species are facultative N2-fixers. Seeds were obtained from SilverHill Seeds (Cape Town, South Africa), heat-treated at 80 °C for 10 minutes to break dormancy, germinated in November 2021, and transplanted into individual cylindrical pots (20 cm diameter × 75 cm height) containing sieved savanna soil from Fort Beaufort, Eastern Cape, South Africa.

Plants were grown in 16 open-top chambers at Rhodes University Elevated CO2 Facility (Makhanda, South Africa) under ambient (398 ppm) or elevated (545 ppm) CO2~. Soil moisture treatments included well-watered (daily irrigation to ~0.30 volumetric soil moisture) and water-limited (ambient rainfall, ~0.24 SMC). Treatments were fully crossed with approximately five replicates per species per treatment.

Plants were destructively harvested at 11 months. Above- and belowground biomass were separated, oven-dried, and weighed. Nodules were collected, dried, and weighed. Leaf and stem tissues were milled and analyzed for δ15N using isotope ratio mass spectrometry. Percent nitrogen derived from fixation (% Ndfa) was calculated using shoot δ15N and reference non-nodulating plants. Soil δ15N was measured from 2 g topsoil samples collected from 30 pots.

N-dynamic traits in the dataset include nodule mass fraction (NMF), leaf δ15N, stem δ15N, and % Ndfa. Phylogenetic relationships among species were incorporated using a time-calibrated tree based on plastid markers (trnL-F, matK, trnH-psbA). Phylogenetic signal (Pagel’s λ) and bivariate trait relationships are provided to account for shared evolutionary history (file = dated.tree)