Nitrogen (N) and phosphorus (P) are essential plant nutrients that limit productivity in most, if not all, terrestrial ecosystems. Due to fundamental differences in the biogeochemistry of N and P, fire has the potential to alter the relative availability of N versus P both immediately following fire and over inter-fire cycles. We investigated the short- and long-term effects of fire on soil and plant nutrients in Florida scrub ecosystems. In addition, we conducted a nutrient addition experiment to test the hypotheses that: (1) plant productivity in recently burned scrubby flatwoods is N-limited; (2) plant productivity in intermediately burned scrubby flatwoods is P-limited; and (3) plant productivity in long unburned scrubby flatwoods is co-limited by N and P.

Results/Conclusions

In palmetto flatwoods, fire caused a greater increase in phosphate (PO₄^3-) than ammonium (NH₄⁺), resulting in a decrease in the soil available N:P ratio shortly after fire. Similarly, foliar %P of resprouting species increased more than foliar %N, resulting in a decrease in foliar N:P ratios shortly after fire. In scrubby flatwoods, PO₄^3-, but not total inorganic N, varied with time after fire, causing N:P ratios to be greatest at intermediate times after fire and lowest 13 years after fire. In surface soils, soil %C and %N, dissolved organic N, net N mineralization, and microbial N were all highest 13 years after fire. In recently burned scrubby flatwoods, shrubs appear to invest more in aboveground productivity, and Quercus inopina, the dominant oak, responded to P and N + P addition, but Serenoa repens, the dominant palmetto, responded to N addition. At intermediate times after fire, shrubs appear to invest more in belowground than aboveground productivity and show co-limitation by N and P with a stronger P-limitation, while in long unburned sites, scrubby flatwoods shrubs appear to invest in both aboveground and belowground productivity and show co-limitation by N and P. Overall, our research shows that fire affects soil and plant nutrients and limitation of primary productivity, but that reallocation of nutrients from below- to aboveground, fire frequency, species composition, and differences among species may mediate the direct effects of fire on nutrient availability and limitation in Florida scrub.