Interactive effects of hydrology and fire drive differential biogeochemical legacies in subtropical wetlands

Fire is an important component of many ecosystems, as it impacts biodiversity, biogeochemical cycles, and primary production. In wetlands, fire interacts with hydrologic regimes and other ecosystem characteristics to determine soil carbon (C) gains or losses and rates of nutrient cycling. However, how legacies of fire interact with wetland hydroperiod to affect soil chemistry is uncertain. We used the Florida Everglades as a model landscape to study how fire regimes, hydroperiod, and soil types collectively contribute to long-term C, nitrogen (N), and phosphorus (P) concentrations and stoichiometric mass ratios (C:N, C:P, N:P) in both short- and long-hydroperiod subtropical wetlands that consist of marl and peat soils, respectively. We used fire records from 1948 to 2018 and hydroperiod from 1991 to 2003, and analyzed these data together with soil chemistry data collected during two extensive field surveys (n = 539) across different ecosystem and soil types throughout Everglades National Park. We also analyzed macrophyte and periphyton P concentrations (n = 150) collected from 2003 to 2016 in fire-impacted wetland sites. Hydroperiod was the main driver of soil C concentration in both marl and peat soils, but fire played a substantial role in nutrient cycling. Particularly in marl soils, soil P concentrations were affected by the absence of fire. In the first decade post-fire, we observed an amplification of P cycling with decreased soil C:P ratios by 95% and N:P ratios by 45%. After more than a decade post-fire, soil P became increasingly depleted (41% lower). Macrophyte P tissue concentration was 50% higher only in the first year post-fire, whereas periphyton P did not change. By recycling nutrients and through removal of litter accumulation, which forms a physical obstacle to photosynthesis, fire likely helps maintain high levels of macrophyte aboveground live biomass as well. Given its substantial effect on nutrient cycling, we advocate for fire management that uses fire return intervals that minimize depletion of soil nutrients and promote positive feedbacks to productivity in wetland ecosystems. In addition, coordinated management of fire return intervals and wetland hydroperiod can be used to set priorities for wetland soil nutrient concentrations and ratios.

Automated summary

Using the Florida Everglades as a model landscape, the study found that fire regimes, hydroperiod, and soil types collectively impact C, N, and P concentrations and stoichiometric ratios in wetlands. While hydroperiod primarily drives soil C concentration, fire plays a significant role in nutrient cycling, particularly affecting soil P concentrations in marl soils. Fire can help maintain high levels of macrophyte aboveground live biomass and nutrient cycling, emphasizing the importance of fire management for wetland ecosystems.