Plant nitrogen-use strategies and their responses to the urban elevation of atmospheric nitrogen deposition in southwestern China.

The elevation of nitrogen (N) deposition by urbanization profoundly impacts the structure and function of surrounding forest ecosystems. Plants are major biomass sinks of external N inputs into forests. Yet, the N-use strategies of forest plants in many areas remain unconstrained in city areas, so their responses and adapting mechanisms to the elevated N deposition are open questions. Here we investigated concentrations and N isotope (δN) of total N (TN) and nitrate (NO) in leaves and roots of four plant species in subtropical shrubberies and pine forests under N deposition levels of 13 kg-N ha yr and 29 kg-N ha yr at the Guiyang area of southwestern China, respectively. The δN differences between plant NO and soil NO revealed a meager NO reduction in leaves but a preferentially high NO reduction in roots. δN mass-balance analyses between plant TN and soil dissolved N suggested that soil NO contributed more than reduced N, and dissolved organic N contributed comparably with ammonium to plant TN, and the study plants preferred NO over reduced N. The elevation of N deposition induced root but not leaf NO reduction and enhanced the contribution of soil NO to plant TN, but plant NO preference decreased due to much higher magnitudes of soil NO enrichment than plant NO utilization. We conclude that plants in subtropical forests of southwestern China preferred NO over reduced N, and NO was reduced more in roots than in leaves, anthropogenic N pollution enhanced soil NO enrichment and plant NO utilization but reduced plant NO preference.