Fire intensity and ecosystem oligotrophic status drive relative phosphorus release and retention in freshwater marshes

Abstract

AbstractEcosystems have been shaped by fire for millions of years. Many oligotrophic ecosystems rely on fire for biogeochemical cycling and maintenance of key processes. However, it is uncertain how fire intensity interacts with nutrient limitation to drive differential responses in postfire biogeochemical cycling and ecosystem recovery. In this study, we compare pre‐ and postfire carbon and nutrient pools in two adjacent wetlands characterized by either lower or higher phosphorus (P) inputs and thus different levels of P limitation. Carbon (C), nitrogen (N), and P pools and litter and root decomposition rates were measured starting 1 year prefire until 1 year postfire in lower‐P (LP) and higher‐P (HP) freshwater marshes of the Florida Everglades. Dominant vegetation biomass and vegetation composition were monitored as well. Fire energy release was measured to link the causal mechanism, heat transfer, to nutrient fluxes. We observed temporary increases in surface water, periphyton, and leaf tissue P concentrations in both wetland types following fire. Soil P increased and soil C:P and N:P ratios decreased postfire in the LP wetland but did not change in the HP wetland. Prefire soil P was greater in the HP than the LP wetland, but did not differ 1 month or 1 year postfire. Vegetation structure and composition were marginally affected by fire. Fire intensity was highly variable within both wetlands. Increasing fire intensity correlated with increasing accumulation of soil organic matter in the LP wetland, and with increases in decomposition rates in both wetland types, between pre‐ and postfire. Litter and root decomposition rapidly increased with increasing fire intensity, but seemed to stabilize after a certain fire intensity threshold was reached. Our results indicate that fire can temporarily ease P limitation in oligotrophic wetlands, by affecting soil nutrient stoichiometry and organic matter processing, while maintaining vegetation community composition. Further, we identified fire dosing, expressed as the fire radiative energy release, to be an effective control of postfire biogeochemical cycling. Hence, nutrient management should be incorporated into burn prioritization tools and, when applying prescribed fires, natural resource managers should also consider how fire attributes, such as fire intensity, impact nutrient cycling and other soil processes.