Urban soils a possible carbon source via increases in respiration rates due to intensive management practices

Background/Question/Methods

Carbon dioxide (CO₂) is one of the most important greenhouse gases and its atmospheric concentration is predicted to increase over the next decades. The conversion of native forest soils (soil-C sink) to urban centers (i.e. impervious surfaces; soil-C source) is expected to significantly contribute to atmospheric concentrations of CO₂. Approximately 20-30% of urban landscapes are often dedicated as managed turfgrass ecosystems. Intensified soil management practices within urban environments may act as a potential carbon sink and, offset biogenic fluxes. Thus, there is a need to assess the seasonal contribution of urban biogenic CO₂ fluxes (i.e. soil respiration) as part of the total urban Carbon cycle budget in managed and unmanaged soils. The objectives of our investigation were to quantify and compare the seasonal biogenic CO₂ efflux from soil respiration between managed (lawn) and unmanaged (urban forest) urban ecosystems (Cub Hill, Maryland). To assess our objectives, we took hourly direct field measurements of net CO₂ exchange from managed and unmanaged soils using an automated soil CO₂ efflux system beginning January 2015, and extending through June 2016. Data analysis utilized mixed-effects model (analogous to ANOVA with repeated measurements) with nighttime data to compare soil respiration between lawn and urban forests across seasons.

Results/Conclusions

Our initial analysis suggests that soil respiration rates differed significantly (F = 7948, p = 0) between urban forest and lawn across sampling period with mean soil CO₂ efflux rates of 1.92 + 0.02 and 2.67 + 0.03 µmol CO₂ m^-2 s^-1 respectively. Further analysis of individual seasonal effects indicates that lawn soil CO₂ efflux rates were significantly greater when compared to urban forest soils (F = 2434, p = 0). The greatest difference between soil respiration rates occurred during the summer season with mean soil CO₂ efflux rates of 3.77 + 0.03 (urban forest) and 4.95 + 0.03 µmol CO₂ m^-2 s^-1 (lawn) however, spring soil respiration rates were similar between sites 2.37 + 0.04 and 2.55 + 0.03 µmol CO₂ m^-2 s^-1 respectively. Our results clearly suggest that soil respiration rates were significantly greater in lawn soils. The greater CO₂ production in lawn soils is likely linked to management decisions (i.e. increases in irrigation and fertilizer application) in turn increasing soil CO₂ efflux rates. The similarity of soil CO₂ rates during the spring months suggests that additional environmental factors such as canopy cover may be influencing soil respiration rates.