Wetland plant and microbial communities are sensitive to changes in flooding and salinity, giving rise to distinct zonation patterns along elevation gradients in many coastal ecosystems. Such changes in community composition can alter plant-soil feedbacks regulating important ecosystem functions like decomposition. Local management practices, such as prescribed fire, may further alter the ecosystem consequences of these shifting plant-soil feedbacks along coastal elevation gradients. To examine the effects of fire on decomposition among different zones, we characterized plant and soil microbial community composition and quantified above- and below-ground decomposition of a standard cellulose substrate in plots spanning a coastal elevation gradient (0.11 – 0.98 m NAVD 88) at Grand Bay National Estuarine Research Reserve, Mississippi, USA. This gradient encompassed four distinct plant zones: salt marsh, brackish marsh, freshwater marsh, and pine woodlands. Half of these plots were subjected to a spring burn (fire plots), while the other half were left unburned (control plots). In each zone for both fire treatments, we examined relationships among plant and soil microbial communities, hydro-edaphic conditions, and decomposition using non-metric multidimensional scaling and Procrustes analyses. We also tested the effects of fire and plant zone on decomposition using analysis of covariance, with elevation as the covariate.
Results/Conclusions:
Plant and microbial communities differed among zones along the coastal elevation gradient, and ordination alignment between the two was relatively similar across zones, particularly for brackish and salt marsh. Fire effects on community composition were minimal and largely restricted to plant cover at upper elevations. Differences in plant and microbial communities were driven by changes in salinity and flooding, and were strongly related to patterns of soil organic matter and plant biomass. These community shifts influenced ecosystem function, as aboveground decomposition decreased, and belowground decomposition increased, with increasing elevation. Decomposition was affected by prescribed fire, with more mass remaining aboveground, but less remaining belowground, in burned plots than in unburned plots, likely because fire increased soil temperature and decreased soil moisture by reducing plant cover. Thus, the distinct plant and microbial communities along this coastal elevation gradient were strongly related to decomposition, and prescribed fire modified this function through its effects on plant cover or hydro-edaphic conditions. These results suggest that land management practices or climate change drivers that affect hydro-edaphic conditions can contribute to community shifts, and consequently, the associated plant-soil feedbacks governing organic matter accumulation and nutrient cycling in coastal ecosystems.