Urbanization has significantly altered soil properties and processes. Changes in nitrogen cycling associated with land-use change depend on a variety of environmental factors and are likely to be highly variable within and between urban ecosystems. We evaluated the effects of different types of planted vegetation (lawns versus flower beds), fertilizer application, and soil moisture on nitrogen cycling and earthworm populations in a suburban neighborhood in Baltimore, MD, USA. Soil cores were taken from two land cover types (lawn, flowering beds) from fifty and forty year old residential parcels. Within each parcel, samples were taken from the front, back and side of the property. Potential net N mineralization and nitrification were measured in a 10 day laboratory incubation and soil moisture was measured gravimetrically.

Results/Conclusions

Potential N mineralization rate was slightly lower on residential lawns than in flower beds (0.34 and 0.78 mg N/kg of dry soil), but the differences were not statistically significant. There was a positive relationship between soil moisture and N mineralization in lawns, but not in flower beds. There was no relationship between soil pH and soil N mineralization in either lawns or flower beds. There was a high variation within a property ( i.e., between sides of a house, 60%) possibly driven by differences in added fertilizers. A model including vegetation type and soil moisture explained 70% of the variation in N mineralization.

Potential nitrification rates were significantly different between the two planted vegetation types, flower beds and lawns (p=0.02). The average soil nitrification rate was negative in flower beds (-0.12 mg NO₃/kg dry soil) and positive in lawns (0.15 NO₃/kg dry soil), because most likely there were high organic matter additions to the flower beds in mulch or compost that stimulated immobilization. There were negative relationships between soil moisture and nitrification, but the relationship was stronger in lawns than in flower beds. Variation within a property ( i.e., between sides of a house) explained 32% of the total variation in nitrification rate. These results show that there is considerable variability in N cycling within residential parcels that should be accounted for in efforts to assess and management urban N cycles.