Temporal and spatial variations in nitrogen transformations in deciduous forest ecosystems along an urban–rural gradient

• Published in: Soil biology & biochemistry 2004-02, Vol.36 (2), p.267-278
• Main Authors: Zhu, Wei-Xing, Carreiro, Margaret M.
• Format: Article
• Language: English
• Subjects: Agronomy. Soil science and plant productions; Biochemistry and biology; Biological and medical sciences; Chemical, physicochemical, biochemical and biological properties; Deciduous forest; Extractable soil N; Fundamental and applied biological sciences. Psychology; Nitrification; Nitrogen mineralization; Physics, chemistry, biochemistry and biology of agricultural and forest soils; Quercus rubra; Soil science; Temporal and spatial variations; Urban–rural gradient
• ISSN: 0038-0717; 1879-3428
• DOI: 10.1016/j.soilbio.2003.09.013

Ecosystem processes such as N transformations have seldom been studied in urban and suburban areas. Here we report the temporal and spatial variations in soil N measured continuously over 16 months in remnant forests dominated by northern red oak ( Quercus rubraL.) along a 130 km urban–rural transect in the New York City metropolitan area. Urban, suburban and rural forests all exhibited clear seasonal patterns in soil N concentrations and transformation rates. Concentrations of extractable inorganic N were highest in early spring, while net N mineralization and nitrification rates were highest in summer. Peak N mineralization and nitrification in urban stands tended to occur a month earlier than in rural stands. Daily net N mineralization rates averaged 4.45 mg N kg −1 soil organic matter (SOM) in urban stands, 3.51 in suburban stands, and 2.49 in rural stands. In urban and suburban forests, between 23.2–73.8% of the annual net N mineralized was nitrified, but in rural forests, net nitrification was mostly below the detection limit. Annual net N mineralization rates, expressed on an areal basis (to a depth of 7.5 cm), averaged 11.6 g m −2 in urban forests, 11.3 g m −2 in suburban sites, and 7.3 g m −2 in rural forests. N returns in oak litter fall were 2.15, 1.32, and 1.81 g m −2 in urban, suburban, and rural stands, respectively. The elevated N transformation rates and nitrate production, in combination with possible pollution constraints on tree growth in urban environments, raises concern that these urban and suburban forests may be approaching an N saturated status.