Short- and long-term effects of site factors on net N-mineralization and nitrification rates along an urban-rural gradient

Long- and short-term effects of urban site factors on net N-mineralization and nitrification rates were investigated in oak stands along an urban-rural land-use transect in the New York City metropolitan area. We used reciprocal transplants of undisturbed soil cores between urban and rural forests to determine the relative importance of long-term effects (mor vs. mull soils, quality of soil organic matter, and deposition of N) vs. short-term effects (soil temperature) of urban factors in controlling field N-transformation rates along the gradient. In addition, undisturbed soil cores from surface (A, Oe horizons) and subsurface (B horizon) soil were collected from urban, suburban, and rural stands and allowed to incubate in these respective sites to compare the net effect of all urban factors with transplanted-core results. The transplant experiment revealed that soil type (long-term) affected net N-mineralization and nitrification rates. Urban soils nitrified nearly 6.3 and 5.4 times more than rural soils incubating in urban and rural stands, respectively (p = 0.003 and p = 0.002, respectively). Similarly, in rural stands total accumulation of inorganic N was 87% higher in urban than in rural soils, whereas in urban stands, urban soils mineralized 83% more N than rural soils (p = 0.043 and 0.08, respectively). Comparing soils incubating in their native locations, urban soils incubating in urban stands mineralized more than 2.5 times the amount of N than rural soils incubating in the rural stands (p = 0.019). By contrast, urban soils incubating in urban stands exhibited a 8-fold increase in nitrification over rural soils incubating in rural stands (p = 0.008). As with the transplanted cores, the urban and suburban environments had a positive effect on net rates of N-mineralization and nitrification in both surface and subsurface layers of soil. The surface layer of suburban and urban stands had a 3- and 2.3-fold higher accumulation of net inorganic N than rural stands (ANOVA, p = 0.05). Similarly, in the subsurface layer both urban and suburban stands had 2.6-fold higher net N-mineralization rate than rural stands (ANOVA, p = 0.01). Along this urban-rural gradient, soils in oak stands exhibit higher net nitrification and, to a lesser extent, net N-mineralization rates in urban and suburban stands than in rural stands. Results from the transplant experiment and in situ measurements of surface and subsurface soil indicate that long-term effects (mor vs. mull soils, N deposition) contribute to the higher N-transformation rates in urban and suburban stands. As a result of these effects, urban and suburban stands have the potential for higher losses of N than rural stands.