Microbial mechanisms of carbon and nitrogen acquisition in contrasting urban soils

Urban soils play an essential role in delivering ecosystem services due to soil microbial functions but there is limited evidence of the role of urban soils in the global carbon cycle. Inorganic nitrogen (N) reduces microbial respiration of soil organic matter (SOM) in pristine and managed forest soils but there is less evidence available on the extent to which this occurs in contrasting urban soils. This study examined the ephemeral effect of inorganic N and SOM (woodland versus grassland urban soil) on microbial functions represented by extracellular enzyme activities and microbial respiration of added substrates of contrasting quality. It was hypothesized that inorganic N stimulates extracellular enzyme activities and microbial respiratory responses to the addition of substrates varying in SOM quantity or quality. Results showed significantly higher SOM content, DOC and dissolved phenolic compounds in the woodland compared to grassland soil. In the woodland soil only, N addition increased beta-glucosidase and N-acetyl-glucosaminidase enzyme activities and decreased microbial respiration responses to substrates. This suggests a microbial requirement for C acquisition dependent on N availability that reduced overflow respiration of the microbial community due to the composition of the woodland SOM pool. In conclusion, urban soils that contrast in vegetation types and hence OM content will likely differ mechanistically in response to increased N deposition and climate change altering their potential ability to store soil C in the future.