Abstract
Nitrogen availability and uptake capacity are key factors influencing forest growth and development in N-limited terrestrial ecosystems. With the aim to determine how species and forest management affect tree N nutrition, we conducted root N uptake experiments as well as soil N analyses at three forest stands with different native and introduced tree species (i.e. Larix principis-rupprechtii Mayr., Quercus aliena var. acutiserrata Maxim. ex Wenz. and Picea wilsonii Mast.) and two management approaches (i.e. monoculture versus mixed stand) in the Qinling Mountains of China. Across the native and introduced species studied, in general, investigated trees take up both, organic and inorganic N compounds, but prefer organic N (Gln- and Arg-) over inorganic NH4 (+)-N. The introduced conifer species (L. principis-ruprechtii) showed higher root N acquisition capacities compared to a native conifer species (P. wilsonii) under N-limited conditions. Moreover, the mixed forest stand with L. principis-ruprechtii and Q. alinea var. acutesserata accumulated more nitrogen in soil pools and showed improved C and N retention capability through the whole soil profile as compared to the monocultures of P. wilsonii or L. principis-ruprechtii. Similar acquisition strategies were observed for specific N sources (i.e. organic versus inorganic) across all investigated tree species. Still the introduced species Larix exhibited a superior root N acquisition capacity and, therefore, may be a good candidate for afforestation programs in the studied region. The present results underpin the significance of forest management practices that achieve a mixed species structure with broadleaved tree species such as Quercus for restoration of soil C and N pools in order to stabilize forest ecosystems and to achieve sustainable forest development.