Fire causes a stronger release of nutrient limitation in higher than lower phosphorus-limited subtropical wetlands

Andrea Nocentini and John S. Kominoski, Department of Biological Sciences, Institute of Environment, Florida International University, Miami, FL, Joe O'Brien, Southern Research Station, USDA Forest Service, Jed Redwine, National Park Service, SE Region of the National Park Service, Miami, FL

Background/Question/Methods
Fire is an integral component of many ecosystems worldwide, as it drives unique ecological processes. By mineralizing, entirely or in part, the fuel provided by live and dead organic matter from plants, fire affects the concentrations of available nutrients and stoichiometric ratios in different above- and below-ground components of ecosystems. In oligotrophic, phosphorus (P)-limited wetlands, fire has the potential to ease nutrient limitation by disproportionately releasing P into the soils relative to N. At the eastern boundary of Everglades National Park (Florida, US), characterized by short-hydroperiod marshes, we set up n = 9 plots at an extremely P-limited site (LP), and n = 9 plots at a moderately P-enriched site (HP). All plots were burned in February 2020. Pre- and post-burn carbon (C), nitrogen (N), and P concentrations were measured in surface water, periphyton mats, detrital flocculent material, soil, and plant roots and leaves. Fire intensity was also measured at n = 11 plots by deploying radiometers during the burn. We tested the following questions: i) How does variation in fire intensity affect nutrient cycling in wetlands characterized by distinct levels of P limitation? ii) Does fire alter nutrient limitation by changing P concentrations in the soil and other compartments?
Results/Conclusions
Maximum fire intensity was highly variable among plots, ranging from 1714 to 43513 W. Two days post-burn, soluble reactive P in surface waters was 0.25 ± 0.04 at LP and 0.33 ± 0.06 µg L-1 at HP plots, whereas it was not detected pre-burn at any of the plots. Post-burn, soil total N increased by 144% at LP and by 55% at HP plots (P<0.05). Soil total P increased by 198% at LP and by 71% at HP plots post-burn (P<0.01). Soil organic matter, determined as ash-free dry mass, increased by 130% only at LP plots (P<0.01). Interestingly, post-burn soil organic matter content and P concentrations were not different between LP and HP sites, whereas pre-burn they were both lower at LP plots (P<0.05). Changes in soil total N and total P between pre- and post-burn correlated more strongly to fire intensity at the LP site (r= 0.86 for soil N; r=0.85 for soil P) than at the HP site (r= 0.14 for soil N; r=0.71 for soil P). Fire has the capacity to ease P-limitation in more strongly P-limited wetlands. Thus, prescribed fire can be differentially employed to manage biogeochemical cycling of wetlands with distinct nutrient levels.