Key nitrogen cycling processes in pine plantations along a short urban-rural gradient in Nanchang, China

We used pine (Pinus elliottii Engelm.) forests located along a short urban-rural gradient in Nanchang, China to study nitrogen (N) cycling responses to urbanization. Annual average rates of nitrification and net N-mineralization in soils (0-15 cm depth) measured from February 2007 to January 2009 increased from rural (8 and 37 kg ha−1 year−1) to suburban (69 and 79 kg ha−1 year−1) and urban sites (114 and 116 kg ha−1 year−1) (P < 0.05). Soil nitrate and mineral N pools exhibited the same spatial patterns in response to urban location. In comparison to rural sites, urban and suburban sites experienced soil microbial biomass N that increased by 98% and 38%, sucrase activity that increased by 40% and 26%, and urease activity that decreased by 35% and 25%, respectively. Soil microbial biomass C:N and free amino acids varied little along the urban-rural gradient. Foliar N concentrations and N resorption proficiencies were higher in urban (12.3 and 4.8 g kg−1) and suburban (12.3 and 6.2 g kg−1) than in rural (9.9 and 3.6 g kg−1) sites, while N resorption efficiencies (from 58% to 72%) were not statistically different. These results indicate that forests in suburban and especially in urban areas are moving rapidly towards a state of “N saturation” and increased potential N loss most likely attributable to higher N deposition to these sites.