Inorganic and Organic Nitrogen Acquisition by a Fern Dicranopteris dichotoma in a Subtropical Forest in South China

The fern Dicranopteris dichotoma is an important pioneer species of the understory in Masson pine (Pinus massoniana) forests growing on acidic soils in the subtropical and tropical China. To improve our understanding of the role of D. dichotoma in nitrogen (N) uptake of these forests, a short-term N-15 experiment was conducted at mountain ridge (MR, with low N level) and mountain foot (MF, with high N level). We injected N-15 tracers as 15 NH4, (NO3)-N-15 or N-15-glycine into the soil surrounding each plant at both MR and MF sites. Three hours after tracer injection, the fern D. dichotoma took up (NH4+)-N-15 significantly faster at MF than at MR, but it showed significantly slower uptake of (NO3-)-N-15 at MF than at MR. Consequently, (NO3-)-N-15 made greater contribution to the total N uptake (50% to the total N uptake) at MR than at MF, but N-15-glycine only contributed around 11% at both sites. Twenty-four hours after tracer injection, D. dichotoma preferred (NH4+)-N-15 (63%) at MR, whereas it preferred (NO3-)-N-15 (47%) at MF. We concluded that the D. dichotoma responds distinctly in its uptake pattern for three available N species over temporal and spatial scales, but mainly relies on inorganic N species in the subtropical forest. This suggests that the fern employs different strategies to acquire available N which depends on N levels and time.