Fire is an important component of many ecosystems as it impacts biodiversity, biogeochemical cycles and primary production. In wetlands, however, fire interacts with hydrological regimes and other ecosystem characteristics to determine soil C gains or losses and rates of nutrient cycling. But, the long-term effects of fire legacies on wetland soil chemistry have not been studied yet in the context of hydroperiod. We used the Florida Everglades as a model landscape to study how fire regimes, hydroperiod, and soil types collectively contribute to long-term carbon (C), nitrogen (N), and phosphorus (P) stocks in both short- and long-hydroperiod 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 surveys (n = 539) across different ecosystem and soil types within Everglades National Park. Macrophyte and periphyton P concentrations (n = 150) collected from 2003 to 2016 in fire-affected sites were analyzed as well.
Results/Conclusions
We found that hydroperiod is the main driver of soil C accumulation in both marl and peat soils, but fire played a substantial role in nutrient cycling. Particularly in marl soils, soil P concentrations were significantly 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%, thereafter, the longer the time since last fire, the more P-depleted the soil (41% lower). We believe that in short-hydroperiod wetlands the effect of fire remains more localized, whereas in long-hydroperiod wetlands post-fire nutrients mineralization becomes locally not detectable, primarily due to higher water flows and consequent lateral transport. Macrophyte P tissue concentration was 50% higher only in the first year post-fire. Thereafter, P accumulated in soils for about a decade. Macrophyte live biomass decreased with the long-term absence of fire (23% lower), suggesting that fire might help plant growth by recycling nutrients, and by removing litter accumulation which forms a physical obstacle to photosynthesis. Given its substantial effect on nutrient cycling, we advocate for fire management that uses fire return intervals to minimize depletion of soil nutrients and promote positive feedbacks to the wetland ecosystem. The outcomes of shifting fire regimes, thus disturbance regimes, in a time where changing climate may affect wetlands hydrological regimes, should, however, be carefully considered in order to preserve ecosystem resilience.