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

ABSTRACT

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 (δ15N) of total N (TN) and nitrate (NO3-) in leaves and roots of four plant species in subtropical shrubberies and pine forests under N deposition levels of 13 kg-N ha-1 yr-1 and 29 kg-N ha-1 yr-1 at the Guiyang area of southwestern China, respectively. The δ15N differences between plant NO3- and soil NO3- revealed a meager NO3- reduction in leaves but a preferentially high NO3- reduction in roots. δ15N mass-balance analyses between plant TN and soil dissolved N suggested that soil NO3- contributed more than reduced N, and dissolved organic N contributed comparably with ammonium to plant TN, and the study plants preferred NO3- over reduced N. The elevation of N deposition induced root but not leaf NO3- reduction and enhanced the contribution of soil NO3- to plant TN, but plant NO3- preference decreased due to much higher magnitudes of soil NO3- enrichment than plant NO3- utilization. We conclude that plants in subtropical forests of southwestern China preferred NO3- over reduced N, and NO3- was reduced more in roots than in leaves, anthropogenic N pollution enhanced soil NO3- enrichment and plant NO3- utilization but reduced plant NO3- preference.