Nitrogen and phosphorus availability in oak forest stands exposed to contrasting anthropogenic impacts

We compared soil N and P availability in similar oak forest stands located in either an urban or a rural area. To compare N and P availability in the urban and rural soils, we measured: (1) net N-mineralization potential and extractable inorganic P; (2) plant demand for NH4(+), NO3(-) and PO4(3-) using a root nutrient uptake bioassay; and (3) N and P acquisition by red oak (Quercus rubra L.) seedlings grown in the contrasting soils. Although net N-mineralization potential did not differ between the urban and rural soils, we found a 5-fold increase in net nitrification in urban compared to rural soils. Despite an apparent trend toward lower labile inorganic P in the urban versus the rural soils, the treatment means did not differ statistically. Root nutrient uptake bioassays indicated that demand for NH4(+) was higher (i.e. availability lower) in the urban than in the rural soils in 1995, but did not differ in 1996. A trend toward greater root demand for NO3(-) in the rural than the urban soils was observed but the means did not differ significantly. We also found that demand for PO4(3-) was significantly higher in oak roots collected from the urban compared to the rural soils in both 1995 and 1996. Total seedling N content (mg N in tissue) and leaf N concentration were significantly lower in Q. rubra seedlings grown in urban compared to rural soils, with decreased N content associated with lower leaf, stem and tap root nitrogen. Leaf and stem P contents were also significantly lower in urban-grown Q. rubra seedlings compared to rural-grown seedlings. We found no differences in tissue P concentration between urban and rural seedlings. Together our results suggest that soil P and, to a lesser degree, N availability is lower in the urban than the rural stands and that these differences are associated with anthropogenic impacts. We discuss the potential for differences in litter quality, exotic earthworms and N deposition between the urban and rural soils to lower soil N and P availability in the urban forest stands.