The effects of CO2 and temperature on the soil microbial carbon of urban and rural forests

Background/Question/Methods

           Soil microbial biomass plays an important role in sequestering and storing carbon (C) in the soil. Although urbanization and climate change are increasing temperatures and CO₂ levels in urban forests, little is known about their effects on microbial biomass carbon (MBC) in urban forest soils. The research objective was to investigate the effects of elevated temperature and CO₂ on MBC quantities from urban and rural forest soils. Soil samples were collected from three urban and three rural Baltimore Long-term Ecological Research (BLTER) forest sites in June and October, 2014. The MBC quantities were determined using the chloroform fumigation extraction method.  Statistical analyses were conducted to determine significant differences in MBC due to site and season effects.  A greenhouse pot study was conducted to determine the effects of elevated temperature and CO₂ on MBC from these urban and rural forest sites. Pots were filled with soil collected in June from the forest sites, planted with one hybrid poplar cutting, and exposed to temperature/CO₂ treatments for 49 days. The treatments were ambient and elevated levels of temperature and CO₂ in factorial combination. A factorial analysis was conducted to test for significant differences between sites, temperatures, CO₂ levels, and their interactions.

Results/Conclusions

           In June, MBC in urban and rural soils did not differ significantly.  However, in October MBC was significantly higher in rural (447±20 μg C/g dry soil) than urban soils (388±20 μg C/g dry soil). Soils collected in June were incubated for 49 days and exposed to differing temperature/CO₂ treatments.  Regardless of treatment, MBC was greater in rural soils (415±14 μg C/g dry soil) than urban soils (303±14 μg C/g dry soil).  Elevated CO₂ significantly increased MBC (376±10 μg C/g dry soil) relative to ambient CO₂ (342±10 μg C/g dry soil), whereas elevated temperature and CO₂ x temperature interaction did not significantly affect MBC. 

             Soil microbes are gaining recognition as important factors in global carbon models. This study provides a first accounting of differences in microbial C storage between urban and rural forest soils. Results suggest that future increases in CO₂ will increase long-term soil C stored as MBC in both urban and rural forests in the Baltimore area.  Also, rural forest soils contained larger MBC pools than urban forest soils, which will likely persist under future climate conditions. As microbial function in urban and rural forest soils appears to differ, their distinction in global carbon models may improve predictive accuracy.