Plant nitrogen-use strategies and their responses to the urban elevation of atmospheric nitrogen deposition in southwestern China
2022
Hu, Chao-Chen | Liu, Xue-Yan
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.
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