Winter soil respiration varies across mountain to urban transitions

Background/Question/Methods

Almost 10% of atmospheric CO₂ is respired through the immense stores of C in soils annually. Seasonal fluctuations are regulated to a large extent by soil organic C quality and quantity, bacterial and fungal populations, moisture, and temperature. Under urbanization, all of the key drivers regulating soil respiration may change. For example, urbanization introduces built infrastructure and impervious surfaces potentially altering soil temperature and moisture, and fertilizer additions and native vegetation removal may influence C sources and microbial diversity. To evaluate the effects of urbanization on soil respiration, we measured CO₂ concentrations and soil moisture and temperature in real-time in urban and montane ecosystems at two depths (5cm and 20cm) in three watersheds across northern Utah, USA. Soil CO₂was monitored with Vaisala CO2 GMP220 sensors. We also evaluated soil C availability as dissolved organic C using a Schimadzu-TOC analyzer, and C quality using fluorescence Excitation-Emission Matrices with a Horiba Aqualog.

Results/Conclusions

Urbanization rather than montane systems demonstrated more variable soil respiration. In winter months, the often snow-bare soils in urban systems experienced freeze-thaw cycles that created pulses of CO₂. However, CO₂ levels in montane soils remained relatively constant under a permanent snowpack. For example, urban soil CO₂ concentrations in the three watersheds varied as much as 1000ppm while soil temperatures dipped multiple times below subzero temperature. The gradient in CO₂ concentrations between the two depths, which reflects the magnitude of CO₂ efflux, was always higher in the montane sites, suggesting that soil efflux was consistently higher in the more constant soil moisture and temperature conditions. DOC tended to decrease from montane to urban ecosystems (RMANOVA, F = 0.52, P = 0.62) with the availability of organic substrates potentially leading to higher soil respiration. Also, we did find a potential link with urbanization and nitrogen fertilization where total nitrogen availability increased 62% from montane to urban systems (RMANOVA, F= 8.5, P = .03). Our findings suggest that urbanization may alter soil temperature and moisture enough to drive microbial processes in the winter.