Why are total soil respiration measurements higher in urban forests than rural forests

Background/Question/Methods

Long-term monitoring has shown that total soil respiration (the production of carbon dioxide (CO₂) by roots and microorganisms) is higher in a series of urban forests than a series of rural forests in the Baltimore, MD metropolitan area. These differences may be due to differences in: root density, the quantity and quality of soil organic matter, microbial biomass, soil moisture and temperature, vegetation characteristics, distance from trees, leaf litter amount and the abundance of earthworms.

Soil respiration has been measured monthly since 1998 on the Baltimore Experimental Study (BES) plots using an in situ flux chamber method. Gas samples of 9 mL were taken at 0, 10, 20, and 30 min from gas sampling ports in the center of the cylinder top using fine-needle polypropylene syringes. Samples were analyzed for CO₂ by gas chromatography with thermal conductivity detection. Flux calculations were from the linear rate of change in gas concentration, the internal volume of the cylinder and soil surface area.

Soil cores (three per plot) were collected with a bulb corer. These soil samples were taken within 10 to 20 m of the soil respiration collars. Measurement of root and microbial biomass, soil organic matter, soil moisture and microbial respiration were obtained from these cores. Amounts of microbial biomass C and N were measured with the chloroform fumigation-incubation method.

Field collections of earthworms were extracted from a 25 x 25 cm grid in each plot using a mustard solution, which consisted of one cup of Coleman’s Mustard powder mixed with four gallons of water. Each extraction was given 30 minutes. The earthworms were then killed in 75% ethanol, set in 4% formalin for several days and preserved in 75% ethanol.

Differences between urban and rural sites were evaluated using one-way analysis of variance. Relationships between variables were explored with Pearson Product Moment Correlations. SAS statistical software was used for all analyses.

Results/Conclusions

The results show that faunal (earthworm) biomass is higher in urban sites than in rural sites. Higher total soil respiration measurements may thus be due to earthworm respiration as well as earthworm stimulation of root respiration. Earthworms were most abundant in wet soils suggesting that if climate change results in changes in soil moisture, earthworm effects on soil respiration may be altered.