Woods Hole Research Center

A summer intern measures soil respiration

In 1995, Center scientists established six study sites within an eddy covariance tower footprint, to study spatial and temporal variation in soil C dynamics. These six locations range in drainage characteristics from well drained to very poorly drained. Mixed hardwoods (oak and maple) are the dominant vegetation, with the exception of a very poorly drained site, which is located in a swampy area. At these six locations, soil respiration, soil temperature and moisture have been measured.

Methods and Measurements

Soil respiration and soil temperature are measured at weekly intervals in the summer and bi-monthly to monthly during the spring, fall and winter season. Soil respiration is measured using a dynamic chamber system described in more detail below. Soil temperature is measured using type T thermocouple wire at 10cm depth, concurrent with soil respiration measurements. There is also automated soil temperature (type T thermocouple) and soil moisture probes (Campbell Scientific CS615) at 4 depths within the soil profile.

Manual Soil Respiration: Manual measurements of soil respiration, using a vented, flow through, non-steady-state system are made once per week between 08.00 h and 15.00 h. Soil respiration is measured using a Li-Cor 6252 portable Infrared Gas Analyzer mounted on a backpack frame. The calibration of the IRGA is checked before each measurement. The IRGA is connected to a vented white acrylonitrile butadiene styrene (ABS) chamber top (10 cm in height) that is placed over a collar already in the ground. A pump circulates the air at a rate of 0.5 L min-1 from the chamber top to the IRGA and back to the chamber top. The IRGA and the pump are both run from a 12 V battery. The collars, 25 cm in diameter (0.05 m2 surface area), are made from thin-walled PVC tubing were cut into 10 cm lengths and inserted into the ground to a depth of approximately 5 cm. Pressure differences between the chamber headspace and ambient air outside the chamber were below detection limits (0.1 Pa measured by an Infltec micromanometer). The chamber top is left on the collar for 5 min, and the change in CO2 concentration within the chamber is recorded using a Hewlett-Packard HP 200LX palmtop**. A linear regression is preformed on the increasing concentration to determine a flux rate, which is corrected for atmospheric pressure and chamber air temperature.

** Note that both the HP 200LX palmtop and the Licor 6252 IRGA are discontinued products. The new Licor IRGA products have included data collecting devices. If you are using the Licor 6252 (or a Licor 6262), then a computer or PDA that will run a DOS program with an RS232 connection can be used in lieu of the HP200LX palmtop.

Results

• The effects of temperature and soil moisture on soil respiration are confounded in this temperate forest, where soils are saturated with water during the cool winters and dry out during the hot summers. Hence, while CO2 efflux rates in well-drained soils generally increase with increasing temperature, they can also decrease with summer drought

• Wetlands behave differently, because soil CO2 efflux increases as they dry out, due to better aeration.

• Only CO2 production in the litter layer demonstrates interannual variation that covaries with water content and that is related to the occurrence of summer drought.

• The litter layer contributes >40% of the production of CO2 within the soil.

• The interannual variation in litter layer decomposition is sufficiently large that it could contribute significantly to interannual variation in NEE measured by tower-based eddy covariance.

Key Publications

Davidson, E.A., E. Belk, and R.D. Boone. 1998. Soil water content and temperature as independent or confounded factors controlling soil respiration in a temperate mixed hardwood forest. Global Change Biology 4:217-227.

Savage, K.E., and E.A. Davidson. 2001. Interannual variation of soil respiration in two New England forests. Global Biogeochemical Cycles 15:337-350.