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Fire and nutrients interact to influence the global distribution and dynamics of the savanna biome, but the results of these interactions are both complex and poorly known. A critical but unresolved question is whether short-term losses of carbon and nutrients caused by fire can trigger long-term and potentially compensatory responses in the nutrient stoichiometry of plants, or in the abundance of dinitrogen-fixing trees. There is disagreement in the literature about the potential role of fire on savanna nutrients, and, in turn, on plant stoichiometry and composition. A major limitation has been the lack of fire manipulations over time scales sufficiently long for these interactions to emerge. We use a 58-year, replicated, large-scale, fire manipulation experiment in Kruger National Park (South Africa) in savanna to quantify the effect of fire on (1) distributions of carbon, nitrogen, and phosphorus at the ecosystem scale; (2) carbon: nitrogen: phosphorus stoichiometry of above- and belowground tissues of plant species; and (3) abundance of plant functional groups including nitrogen fixers. Our results show dramatic effects of fire on the relative distribution of nutrients in soils, but that individual plant stoichiometry and plant community composition remained unexpectedly resilient. Moreover, measures of nutrients and carbon stable isotopes allowed us to discount the role of tree cover change in favor of the turnover of herbaceous biomass as the primary mechanism that mediates a transition from low to high 'soil carbon and nutrients in the absence of fire. We conclude that, in contrast to extra-tropical grasslands or closed-canopy forests, vegetation in the savanna biome may be uniquely adapted to nutrient losses caused by recurring fire.

The expansion of tropical forest into savanna may potentially be a large carbon sink, but little is known about the patterns of carbon sequestration during transitional forest formation. Moreover, it is unclear how nutrient limitation, due to extended exposure to fire-driven nutrient losses, may constrain carbon accumulation. Here, we sampled plots that spanned a woody biomass gradient from savanna to transitional forest in response to differential fire protection in central Brazil. These plots were used to investigate how the process of transitional forest formation affects the size and distribution of carbon (C) and nitrogen (N) pools. This was paired with a detailed analysis of the nitrogen cycle to explore possible connections between carbon accumulation and nitrogen limitation. An analysis of carbon pools in the vegetation, upper soil, and litter shows that the transition from savanna to transitional forest can result in a fourfold increase in total carbon (from 43 to 179 Mg C/ha) with a doubling of carbon stocks in the litter and soil layers. Total nitrogen in the litter and soil layers increased with forest development in both the bulk (+68%) and plant-available (+150%) pools, with the most pronounced changes occurring in the upper layers. However, the analyses of nitrate concentrations, nitrate:ammonium ratios, plant stoichiometry of carbon and nitrogen, and soil and foliar nitrogen isotope ratios suggest that a conservative nitrogen cycle persists throughout forest development, indicating that nitrogen remains in low supply relative to demand. Furthermore, the lack of variation in underlying soil type (>20 cm depth) suggests that the biogeochemical trends across the gradient are driven by vegetation. Our results provide evidence for high carbon sequestration potential with forest encroachment on savanna, but nitrogen limitation may play a large and persistent role in governing carbon sequestration in savannas or other equally fire-disturbed tropical landscapes. In turn, the link between forest development and nitrogen pool recovery creates a framework for evaluating potential positive feedbacks on savanna-forest boundaries.