Why can Palhinhaea cernua (lycophyte) grow closer to fumaroles in highly acidic solfatara fields?

Palhinhaea cernua, a lycophyte, and Dicranopteris linearis, a fern, are commonly observed in solfatara fields in Kyushu, Japan, but their distribution trends are different. The aim of this study was to determine why P. cernua is more abundant in areas closer to fumaroles from both a soil and plant perspective. Samples of P. cernua and D. linearis, as well as their respective growing soils, were collected, and the mineral properties, including the concentration of various mineral elements and inorganic anions and δ¹⁵N, were determined. P. cernua was better adapted to soil with lower pH, higher soluble aluminum concentrations, and poorer calcium and phosphorus concentrations than D. linearis. A positive correlation was observed between shoot nitrogen concentration and both shoot sulfur concentration and soil water-soluble sulfur concentration in P. cernua, implying the involvement of sulfur in nitrogen acquisition in P. cernua. The results also suggested that D. linearis mainly uses soil NO₃-N, while P. cernua uses NH₄-N, which is predominant and excessive in the solfatara fields, particularly near the fumaroles. This high preference for NH₄-N in P. cernua was confirmed through a cultivation experiment. While D. linearis prefers NO₃-N and distributes further from fumaroles, P. cernua may have survived in the solfatara fields by utilizing NH₄-N and sulfur, which are abundant near fumaroles where competition from other plant species is minimal.